DOI: 10.1111/1365‐2745.13116 BIOLOGICAL FLOR A OF THE BRITISH ISLES* No. 289 Biological Flora of the British Isles: Aesculus hippocastanum Peter A. Thomas1 | Omar Alhamd1,2 | Grzegorz Iszkuło3,4 | Monika Dering3,5 | Tarek A. Mukassabi6 1 School of Life Sciences, Keele University, Staffordshire, UK Abstract 2 1. This account presents information on all aspects of the biology of Aesculus hip‐ College of Education for Pure Science, University of Mosul, Nineveh, Iraq 3 Institute of Dendrology, Polish Academy of Sciences, Kórnik, Poland 4 Faculty of Biological Sciences, University of Zielona Góra, Zielona Góra, Poland 5 Poznań University of Life Sciences, Department of Forest Management, Poznań, Poland pocastanum L. (horse‐chestnut) that are relevant to understanding its ecological characteristics and behaviour. The main topics are presented within the standard framework of the Biological Flora of the British Isles: distribution, habitat, communi‐ ties, responses to biotic factors, responses to environment, structure and physiol‐ ogy, phenology, floral and seed characters, herbivores and disease, history and conservation. 2. Aesculus hippocastanum is a large deciduous tree native to the Balkan Peninsula. 6 Department of Botany, University of Benghazi, Benghazi, Libya Native populations are small (<10,000 trees total) and apparently in decline, but the tree has been widely planted in gardens and streets across Europe and other Correspondence Peter A. Thomas Email: p.a.thomas@keele.ac.uk temperate areas from the 17th century onwards. It was voted the UK's favourite tree in a 2017 poll. As a British neophyte, it is occasionally naturalised in open wooded habitats. 3. Horse‐chestnut is renowned for the beauty of its large (up to 30 cm long), upright panicles of white flowers, and for the large seeds (up to 42 g each) used in the formerly common children's game of “conkers.” More recently, the triterpene gly‐ cosides, extractable from various plant parts but especially the seeds, have been widely used in medicine. 4. In much of Europe, horse‐chestnut is affected by chestnut bleeding canker (caused by Pseudomonas syringae pv. aesculi), the horse‐chestnut leaf miner Cameraria ohridella and the leaf blotch fungus Guignardia aesculi. The canker is likely to lead to death of <10% individuals, but seeds of plants infested with the leaf miner are 40%–50% smaller, which may affect long‐term establishment in non‐planted areas. KEYWORDS Cameraria ohridella, chestnut bleeding canker, conservation, geographical and altitudinal distribution, germination, herbivory, mycorrhiza, reproductive biology Horse‐chestnut. Hippocastanaceae). Stokes, Esculus hipocastanea (L.) Raf., Hippocastanum aesculus Cav., Aesculus hippocastanum L. (A. asplenifolia Loud., A. castanea Gilib., Sapindaceae (formerly Hippocastanum vulgare Gaertn., Pawia hippocastanum (L.) Kuntze) is a A. heterophylla hort. ex Handl., A. ohiotensis Lindl., A. incisa hort. large deciduous tree with a wide‐spreading, flat‐topped crown up to ex Handl., A. memmingeri C. Koch, A. procera Salisb., A. septenata 20 m across, branches upswept at first, then dipping down to form *Nomenclature of vascular plants follows Stace (2010) and, for non‐British species, Flora Europaea. 992 | © 2019 The Authors. Journal of Ecology © 2019 British Ecological Society low spreading limbs that turn sharply up at the ends. Height up to 25 (‐39) m tall, trunk up to 190 cm diameter. Bark dark grey‐brown, wileyonlinelibrary.com/journal/jec Journal of Ecology. 2019;107:992–1030. Journal of Ecology THOMAS eT Al. | 993 smooth when young, and on mature trees forming long fine scales Horse‐chestnut is best known as a tree planted for ornamenta‐ that become detached at both ends before falling. Twigs stout, pale tion and shade in parks and streets, particularly by the Victorians grey or brown, glabrous with pale lenticels. Buds large, 2.5–5 cm, (Rackham, 1986), since little else can rival the sight of a horse‐chest‐ ovoid, deep red‐brown, resinous and very sticky. Leaves opposite, nut in full flower. Indeed, it was voted the UK's favourite tree in large (≤60 cm wide), with a long (7–20 cm) terete petiole, palmate 2017 in a poll run by the Royal Society of Biology (Royal Society of with five to seven leaflets, each (8)10–20(25) cm long, the terminal Biology, 2017). The British population is an estimated 470,000 trees. one the largest. Leaflets sessile, obovate, long cuneate at base, usu‐ ally acuminate, irregularly crenate–serrate or serrate–dentate, dark green, glabrous above, somewhat tomentose beneath when young, often glabrous at maturity, joined to the petiole by a pulvinus‐like 1 | G EO G R A PH I C A L A N D A LTIT U D I N A L D I S TR I B U TI O N leaflet base. Inflorescence a large (15–30 × 10 cm), terminal, erect, conical Aesculus hippocastanum has been extensively planted throughout to cylindrical panicle, andromonoecious with male flowers at the the British Isles and has continued to expand its range through nat‐ top of the panicle and hermaphrodite flowers below. Flowers zy‐ uralisation into a range of open habitats. It is most frequent in the gomorphic c. 2 cm across; sepals 5 forming a tubular or campan‐ south and east of England, becoming less common in the wetter ulate, toothed calyx; petals 4–5, each c. 1 cm, white with basal areas of west Ireland and northern Scotland (Figure 1) and upland spots at first yellow then pink. In Poland, Weryszko‐Chmielewska, areas of Scotland. Nevertheless, even in these areas it can be found Tietze, and Michońska (2012) found 56% 4‐petalled and 44% 5‐ planted in urban areas and gardens and has even been planted in petalled flowers. Stamens 5–9 of variable length within the same policy woodlands on Hebridean islands such as Rhum (Batten & flower, mostly longer than petals and arched downwards; pollen Pomeroy, 1969) although it is now rare (Pearman, Preston, Rothero, red. Ovary 3‐celled, each cell with two ovules; single style small & Walker, 2008). Hill, Preston, and Roy (2004) list horse‐chestnut as and simple, stigma minute. Male flowers with a small non‐func‐ being present in 2,186 10‐km squares in Great Britain and the Isle tional pistil and an underdeveloped ovary. Nectary a lobed disc. of Man (80%), 557 in Ireland (57%) and 12 in the Channel Islands Only 2–5(8) flowers at the base of the panicle develop fruit. Fruit (86%). 5–8 cm diameter, subglobose, spiny with 1 (rarely 2 or 3) seeds or Aesculus hippocastanum is native to mountains of the Balkans in “conkers.” Seed a large lustrous or glossy, glabrous, smooth, red‐ south‐east Europe (Figure 2). The biggest native populations are in dish‐brown nut, 34–48 × 25–37 mm, ellipsoid, the radicular lobe mainland Greece, in the central Thessaly Mountains and northern visible as a low, broad dark ridge especially in dried nuts; hilum Pindos range and in the southern counties of Evrytania and Fthiotida large, white, circular or elliptic. between 40°20′N, 21°05′E and 38°37′N, 22°26′E (Avtzis, Avtzis, Aesculus contains between 12 and 19 extant species (Harris, Vergos, & Diamandis, 2007; Walas et al., 2018). Tsiroukis (2008) Xiang, & Thomas, 2009; Xiang, Crawford, Wolfe, Tang, & counted a total of 1,464 adult horse‐chestnut trees across all 98 Depamphilis, 1998) with most authors recognising 12 or 13 species known populations in the mountains of Greece; 38% of all individuals (Forest, Drouin, Charest, Brouillet, & Bruneau, 2001; Hardin, 1960; were in the Pindos Mountains. Populations varied from 1 to 153 in‐ Koch, 1857; Zhang, Li, & Lian, 2010). Aesculus hippocastanum is the dividuals, and 63% of populations had <10 trees. Its native range also only European species within Section Aesculus along with A. turbi‐ includes populations in Albania, the Republic of Macedonia, Serbia nata Blume from Japan (Zhang et al., 2010). The genus is otherwise and eastern Bulgaria (Anchev et al., 2009; Evstatieva, 2011; Gussev confined to North America and South and East Asia (Forest et al., & Vulchev, 2015; Peçi, Mullaj, & Dervishi, 2012). These populations 2001; Xiang et al., 1998). are variable in size within and between countries. Albania has <500 Bean (1976) and Bellini and Nin (2005) list a number of varieties, individuals, with populations of <50 individuals and most containing cultivars and forms. Of these, A. hippocastanum 'Baumannii' is par‐ 10–15 individuals (L. Shuka, pers. comm. 2017 cited in Allen & Khela, ticularly common; this is a double‐flowered, fruitless variety (Hoar, 2017). The Macedonian population is probably <100 individuals, and 1927) planted where conker hunting by children has been seen as the Bulgarian populations are probably smaller (Allen & Khela, 2017; a problem (Leathart, 1991). Aesculus hippocastanum 'Pyramidalis' is Peçi et al., 2012). The total extent of its native extant range is esti‐ becoming more common; this reaches 25 m with a conical or nar‐ mated to be 163,642 km2, c. 25% of the Balkan Peninsula (Allen & rowly pyramidal crown when young, becoming more ovate at matu‐ Khela, 2017). rity. Only one hybrid is known (Section 8.2). Horse‐chestnut has, however, been extensively planted since Aesculus hippocastanum is native to the Balkan Peninsula in south‐ the 17th century (Section 10) across central Europe and south into east Europe but has been widely planted in temperate areas from Italy and Serbia (Figure 2). It is also grown widely in urban areas of the 17th century onwards. As a neophyte, it is widely naturalised in Iran, northern India, Asia Minor, United States, Canada (as far north Europe, though only partially so in Britain where it was known from as Edmonton, Alberta) and New Zealand (Kapusta et al., 2007; the wild by 1870 (Preston, Pearman, & Dines, 2002). It is often self‐ Loenhart, 2002; Zhang et al., 2010) and further north in the Faeroe sown in grassy places, copses, thickets, hedges and rough ground Islands, Iceland and Norway (Højgaard, Jóhansen, & Ødum, 1989), throughout lowland Britain and into west and central Europe. still producing fruit at 65°N in Sweden and Norway (Bellini & Nin, 994 | Journal of Ecology THOMAS eT Al. F I G U R E 1 The distribution of Aesculus hippocastanum in the British Isles. Each dot represents at least one record in a 10‐km square of the National Grid. (●) non‐native 1970 onwards; (○) non‐native pre‐1970. Mapped by Colin Harrower, Biological Records Centre, Centre for Ecology and Hydrology, mainly from records collected by members of the Botanical Society of Britain and Ireland, using Dr A. Morton's DMAP software 2005). It was introduced into the United States most probably in is 3.6°C, and the mean July temperature is 14.8°C (Hill et al., 2004). 1746 in Philadelphia (Anon, 1925). Within the same area, the mean annual precipitation is 1,014 mm. Horse‐chestnut is generally found in the lowlands of Britain These figures are, of course, very similar to other trees that have a but reaches 505 m altitude at Ashgill (Cumberland) (Preston et al., distribution extending over much of Britain, such as Fraxinus excel‐ 2002) and 1,300 m in Sweden and Norway (Bellini & Nin, 2005). It sior and Acer pseudoplatanus. occurs at 218–1,485 m in its native range in Greece and Bulgaria, Across its range, horse‐chestnut is a mesophytic tree, adapted and up to c. 1,600 m in Albania (Avtzis et al., 2007; Horvat, Glavac, & to warm‐temperate climates. Walas et al. (2018) modelled its cli‐ Ellenberg, 1974; Leathart, 1991; Peçi et al., 2012; Walas et al., 2018). matic requirements over the whole of its native range in the Balkan In Greece, 35% of individuals and 31% of populations are found be‐ Peninsula and concluded that the main limitations to its distribu‐ tween 900 and 1,000 m, and 72% of adult trees grow between 500 tion were high precipitation in the coldest quarters of the year and and 1,000 m (Tsiroukis, 2008). low precipitation in the warmest. It is also limited by a wide range in annual temperature, preferring those areas without temperature 2 | H A B ITAT extremes. These limitations appear to act primarily through humid‐ 2.1 | Climatic and topographical limitations humidity is low, such as in Mediterranean and urban areas, high soil The mean January temperature within the 10‐km squares in which important in helping horse‐chestnut survive (Walas et al., 2018). horse‐chestnut occurs within Britain, Ireland and the Channel Islands Certainly, the native populations in Greece rely on high air humidity ity, which was seen as the main factor limiting distribution. Where moisture and low stomatal conductance (Section 6.5) appear to be Journal of Ecology THOMAS eT Al. | 995 F I G U R E 2 Distribution of Aesculus hippocastanum across Europe (small dots) and its native range (larger dots). Frequency of occurrence is from field observations as reported by the National Forest Inventories. From: Ravazzi and Caudullo (2016), reproduced courtesy of the European Union [Colour figure can be viewed at wileyonlinelibrary.com] resulting from the presence of open water, and undoubtedly high the Ellenberg value for moisture (F) is 5, indicating a moist site, simi‐ soil moisture. Tsiroukis (2008) found 62% of populations in ravines lar to the requirement by many deciduous trees. Moist, CaCO3‐rich with constantly flowing water, 12% in humid forests, 11% in ravines and fertile soils produce a strong, flat root system in horse‐chestnut, with intermittent flow and only 9% in dry ravines and 6% on arid but poorer soils result in horse‐chestnut growing a shallow but much slopes by roads. It is likely that the recalcitrant nature of the seeds more extensive root system (Karliński et al., 2014). Simon and Lena (Section 8.4), and their intolerance of desiccation, is the main factor (2016) and Poljanšek and Lena (2016) found that radial growth of limiting horse‐chestnut naturally to moist sites (Horvat et al., 1974), street trees in Slovenia was positively associated with soil moisture, and explains why seedlings, past this vulnerable stage, can be suc‐ underlining the importance of moisture. cessfully transplanted into a wide range of climatic conditions from woodlands to harsh, dry urban areas. In its native range, horse‐chestnut is usually restricted to moist but well‐drained stony soil (often in moist depressions). In Bulgaria There appear to be few topographical limitations as horse‐chestnut and Albania, horse‐chestnut grows on calcareous soils derived grows on flat urban sites and is found on steep slopes and in ravines from limestone (Allen & Khela, 2017). These medium‐deep soils are within its native range (Thalmann, Freise, Heitland, & Bacher, 2003; slightly acidic on the surface and more alkaline at depth (Gussev Tsiroukis, 2008). In Bulgaria, it grows up onto steep slopes and scree & Vulchev, 2015). Horse‐chestnut will, however, cope with being (Horvat et al., 1974; Peçi et al., 2012; Walas et al., 2018) that reach 25– planted on a range of soils from nutrient‐poor sand to heavy clay, 60° (Gussev & Vulchev, 2015). In Albania, it is found on limestone rocky from acid to alkaline although an optimum pH of 6.6–7.2 is recom‐ slopes of valleys and canyons (Allen & Khela, 2017). The importance of mended (Puchalski & Prusinkiewicz, 1975). The Ellenberg value for aspect is unknown but is likely to be related to the need for moisture. reaction (R) in Britain is 7, the commonest requirement by plants in Britain, indicating an association with weakly acidic to weakly basic 2.2 | Substratum conditions and never on very acidic soils (Hill et al., 2004). The optimum soils for horse‐chestnut are generally deep, siliceous, free‐draining and rather fertile soils (Fitter & Peat, 1994; Tsiroukis, 3 | CO M M U N ITI E S 2008). The Ellenberg value for nitrogen (N) corrected for Britain is 7 (Hill et al., 2004), which is slightly higher than associated with In Britain, horse‐chestnut is frequently planted in urban and rural Fraxinus excelsior, Acer platanoides and Tilia cordata (all 6). Similarly, settings such as in estates, parks and gardens (Whitney & Adams, 996 | Journal of Ecology THOMAS eT Al. 1980), churchyards, urban streets and village greens. It is also found 4 | R E S P O N S E TO B I OTI C FAC TO R S in a large number of planted woodlands resulting “from the Victorian forester's habit of trying everything” (Rackham, 2003). Horse‐chest‐ The saplings of horse‐chestnut are semi‐shade‐tolerant with an nut is often self‐sown in open habitats, such as unmanaged scrub, Ellenberg value for light (L) of 5 corrected for Britain (Hill et al., waste ground or rough grassland. As such, it is rarely found in semi‐ 2004) and so are tolerant of some competition for light. Although natural vegetation types in Britain. It does occasionally regenerate most adult trees are planted in the open or grow in open stands in locally in woodland but is seldom naturalised in native woodland its native range, it is able to persist in denser woodland communities (Preston et al., 2002; Rackham, 2003). The exception is as an infre‐ (Section 3). Indeed, Hirons and Sjöman (2018) list horse‐chestnut as quent member of W12 Fagus sylvatica–Mercurialis perennis woodland being suitable for shaded late‐successional conditions. This supports (Rodwell, 1991) where it has become naturalised in some places and evidence that horse‐chestnut will sometimes invade established locally abundant. woodlands in Britain although it is likely that this only happens in In mainland Europe, it is planted in a wide range of urban and fairly open conditions. In eastern Europe, horse‐chestnut is consid‐ garden areas and is naturalised in grassy places, copses, thickets ered to be a competitive species that invades native woodlands, par‐ and hedges and rough ground through west and central Europe ticularly where disturbed, growing as isolated trees or small patches (Łukasiewicz, 2003). (Chmura, 2004; Křivánek, Py̌sek, & Jarošík, 2006). In its native range in Greece, it is found in the conifer and Horse‐chestnut has a dense crown and casts a deep shade. mixed broadleaf forest ecoregion (WWF, 2013). On dry and City trees in south‐east Hungary transmitted just 7 ± 10% (SD) rocky south‐facing slopes, it is found with evergreen oaks (WWF, of full sunlight in mid‐summer, increasing to 25 ± 14% at the end 2013), which is common through the Middle East and into Asia. of September when half leafless due to the start of autumn ab‐ In moist mountainous valleys, horse‐chestnut is found with Abies scission. At this point in autumn, there was more shade below cephalonica, Juglans regia, Ostrya carpinifolia, Platanus orientalis horse‐chestnut than cast by Styphnolobium japonicum (L.) Schott and Alnus incana (Leathart, 1991) and in cooler places with Abies (=Sophora japonica) (15 ± 8%) and Tilia cordata (12 ± 13%) that borisii‐regis (which forms 52% of the total tree population in na‐ were losing leaves, but less than Celtis occidentalis that was not tive stands), Fraxinus ornus (41%), Fagus sylvatica (37%), Platanus yet shedding leaves (Takács, Kiss, Gulyás, Tanács, & Kántor, 2016), orientalis (12%), Salix alba (11%) and Juniperus communis (10%) suggesting that horse‐chestnut maintains its deep shade compara‐ (Tsiroukis, 2008). Further north on the north‐east face of Mount tively late into autumn, increasing its competitiveness. Certainly in Ossa in central Greece, horse‐chestnut grows with Tilia platy‐ most urban areas around Europe, little will grow beneath a group of phyllos in the formation Aesculus hippocastanum–Tilia platyphyllos horse‐chestnut trees. (Raus, 1980). Horse‐chestnut can tolerate atmospheric pollution and persists In the Preslavska Mountain of the East Balkan Range in Bulgaria, in polluted inner cities in Europe. Leaves collected in polluted areas horse‐chestnut is present in deciduous forests that are a relict veg‐ of mainland Europe can appear small, curved and brittle, though the etation type typical of the northern Mediterranean that was more widespread in the past (Gussev & Vulchev, 2015). The stands form three associations within the formation Aesculeta hippocastani: Aesculus hippocastanum–subnudum, Aesculus hippocastanum–Carpinus betulus and more locally Aesculus hippocastanum–Aegopodium podagraria (Gussev & Vulchev, 2015). Horse‐chestnut is usually dominant in all these, although 10%–20% degree of pollution causing this is not stated (Godzik & Sassen, 1978). Pollution can alter the epidermis of leaves, particularly the adaxial surface, leading to it becoming porous or cracked with “abnormal” stomata (Godzik & Halbwachs, 1986; Godzik & Sassen, 1978). Horse‐ chestnut is listed as sensitive to chlorine and moderately sensitive to hydrogen chloride by Khan and Abbasi (2000) but tolerant to sulphur of the crown may be composed of other species, particularly Acer dioxide and hydrogen sulphide (Velagić‐Habul, Lazarev, & Custović, platanoides, A. pseudoplatanus, Fagus sylvatica subsp. moesiaca, 1991). Salt tolerance is considered in Section 5.3. Juglans regia, Sorbus torminalis, Tilia tomentosa and Ulmus glabra. Due to its thin bark, horse‐chestnut is sensitive to forest fires The horse‐chestnuts can be up to 100 years old with a maximum (Ravazzi & Caudullo, 2016). However, it will freely resprout from cut DBH of 80–90 cm. Seed production is limited, and reproduction is stumps and coppices well, although it is not often used in coppices primarily by root suckers. Cornus mas, C. sanguinea, Corylus avellana, since the poles grow slowly in comparison with other species such as Crataegus monogyna and Staphylea pinnata show high constancy Fraxinus excelsior and they are mechanically weak (Özden & Ennos, among the shrubs. The herbaceous layer is sparse and species‐poor 2018). and in the spring is typified by Anemone ranunculoides, Aremonia Urban trees are tolerant of heavy pruning undertaken to main‐ agrimonoides, Cardamine bulbifera, Corydalis bulbosa, C. solida, tain their aesthetic shape and to maintain access and sight lines Erythronium dens‐canis, Euphorbia amygdaloides, Isopyrum thalictroi‐ (Cutler & Richardson, 1989), but large wounds can be problematic. des, Mercurialis perennis, Milium effusum, Scilla bifolia, Symphytum Wounds over 15–20 cm in diameter heal poorly, and in Lithuania, tuberosum and Viola reichenbachiana. Horse‐chestnut can also be c. 80% of such wounds have been seen to be infected by wood and found associated with Fagus sylvatica in the order Fagion sylvaticae pith rot after 2–4 years (Snieskiene, Stankeviciene, Zeimavicius, & (Peçi et al., 2012). Balezentiene, 2011). Journal of Ecology THOMAS eT Al. 5 | R E S P O N S E TO E N V I RO N M E NT | 997 and, due to the large, compound leaves, horse‐chestnut has a low number of leaves per length of stem (Özden & Ennos, 2018). 5.1 | Gregariousness In planted areas throughout Europe, horse‐chestnut is variably gre‐ garious, obviously depending on the whims of each planting scheme. However, in many areas, groups or lines of horse‐chestnuts are typi‐ cally planted for visual impact or for shade. In native populations in Greece, horse‐chestnut trees can grow close together, just a few me‐ tres apart, but density is variable. Walas et al. (2018) noted that the density of small trees (<1 m tall) varied from 31–33 to 1,017 trees/ ha, intermediate‐sized trees (1.1–10.0 m) from 53 to 339 trees/ha and the tallest trees (≥10.1 m) from 16–20 to 565 trees/ha. There appeared to be no link between density and mean annual tempera‐ ture or annual precipitation, but the highest densities were found at the lowest elevations (705–915 m) compared to lowest densities at 1,089–1,463 m. However, the availability of soil moisture (Section 2.1), topographical barriers and slope steepness, affecting the dis‐ persal of seeds, must all also play their part in determining tree den‐ sity in naturally regenerated populations. Although horse‐chestnut grows well when planted in urban areas, growth is restricted in paved area. A study of 231 mature A. hippocastanum trees growing in Munich, Germany (mean height 16.1 m, mean DBH 63.6 cm, mean crown width 5.5 m) were growing in non‐paved areas with a mean of 15.08 m2. A positive linear rela‐ tionship was found between non‐paved area in proportion to the crown projection area and basal area increment, presumed to be due to the limited water infiltration into paved areas (Dahlhausen, Biber, Rötzer, Uhl, & Pretzsch, 2016). Mean annual ring width was 1.61 mm (range 0.32–7.91 mm), the lowest mean of all species compared in cities around the world (mean ring width of other species ranged between 1.63 and 5.30 mm). As such, it was the slowest growing tree, reaching a DBH of c. 80 cm at 160 years old with a biomass of 3.5 t per tree (Dahlhausen et al., 2016). They also found a positive relationship between stem diameter and height and crown radius. Performance in urban areas can be improved by injecting sucrose solution (50 g/L) into the soil around the roots. Percival, Fraser, and Barnes (2004) found this to increase fine root dry mass (>4 mm diameter) from 0.24 to 1.17 g/m3 5 months after treatment. This 5.2 | Performance in various habitats Growth in Britain is rapid when young, with trees gaining 60–90 cm in height annually and with growth rates of 30 cm per year sustained for at least 60 years (Leathart, 1991), particularly in the east and south‐east of England. In the less optimal conditions on acidic and nutrient‐poor soils of pine stands in north‐east Germany, the maxi‐ mum height growth of horse‐chestnut was 7 cm per year, the lowest of all 13 deciduous trees tested. In comparison, Acer pseudoplatanus grew 0.29 m per year and Sorbus aucuparia 0.65 m per year (Zerbe & Kreyer, 2007). As horse‐chestnut matures, height growth slows, but annual increment is maintained such that in the city of Duisburg, Germany, Scholz, Hof, and Schmitt (2018) recorded horse‐chestnut reaching 25 m tall at a DBH of 111 cm while Betula pendula at the same height had a diameter of <64 cm. Across native populations in Greece, Walas et al. (2018) found that saplings (≤1 m height) formed >50% of populations. They con‐ cluded that the abundance of seedlings and saplings showed that natural populations of horse‐chestnut are capable of performing well and maintaining themselves under favourable management (Section 11). For optimum growth, horse‐chestnut requires shelter from high winds that would otherwise damage the large leaves (Bellini & Nin, 2005) and snap off branches, especially when combined with heavy, rain‐soaked foliage (Mitchell, 1997). In extreme cases, this dam‐ age produces a characteristic standing trunk with few remaining branches. Horse‐chestnut is unusual in that its branches are com‐ paratively stiff so branches fall with a clean break rather than buck‐ ling (Özden & Ennos, 2018). The branches are stiffer than its coppice shoots, whereas for Acer pseudoplatanus and Fraxinus excelsior, it is the other way round; this may make the branches less flexible and more prone to snapping. Branches with fewer leaf nodes are stiffer, was also seen to work for Betula pendula and Quercus robur but not Prunus avium. Paulić, Drvodelić, Mikac, Gregurović, and Oršanić (2015) found that horse‐chestnuts in urban environments of Croatia displayed a positive correlation of radial stem growth with average spring precipitation and a negative correlation with maximum spring air temperatures. Similarly, Wilczyński and Podlaski (2007), working with horse‐chestnuts growing in a Fraxino‐Alnetum community in south‐central Poland and using tree ring width data from 1932 to 2003 from 15 trees without Cameraria ohridella infections (see Section 9.1.1), found a positive correlation of radial growth with air temperature of the previous winter (December to March) and of summer (August) in the growing season, and with precipitation of the previous winter. However, excessive precipitation in August, which raised the already high water‐table, had a negative effect on radial growth. Warm year‐round temperatures in a continental cli‐ mate would go some way to mimicking temperatures in its native habitats, although the negative effect of high spring temperatures is likely due to drought and low humidity. Similarly, horse‐chest‐ nut favours moist habitats (Section 2.2), so high rainfall short of causing flooding of roots would be beneficial. In native Greek pop‐ ulations, leaves with the smallest leaflets (8.6 ± 1.35 cm2; SE, n un‐ stated) were found in the most northerly population with the lowest mean annual temperature (7.3°C). However, the precise effect of environmental parameters on performance is not always easy to disentangle. In Greece, leaves were significantly larger and longer in the Karitsa population than in the more northerly Perivoli pop‐ ulation (69.22 ± 4.04 cm2, 50.10 ± 2.26 cm2, and 15.16 ± 0.48 cm, 13.48 ± 0.34 cm, respectively); Karitsa is at lower altitude (Karitsa 705 m, Perivoli 915 m) and had a higher mean annual temperature (12.4°C, 10.5°C, respectively) but lower precipitation (553 mm, 830 mm, respectively) (Walas et al., 2018). 998 | Journal of Ecology THOMAS eT Al. 5.3 | Effect of frost, drought, etc horse‐chestnut does not tolerate long‐term maritime exposure; Hill After the juvenile growth phase, horse‐chestnut is relatively hardy tolerance (S) as 0, indicating an absence from saline sites and a short and generally tolerates low winter temperatures well (Bellini & life span in coastal sites. et al. (2004) list the Ellenberg value (corrected for Britain) for salt Nin, 2005). However, Wilczyński and Podlaski (2007) argued that Direct application of saturated salt solutions to horse‐chest‐ long cold continental winters negatively affect the tree in the nut buds delayed bud break for up to 8 days (Paludan‐Müller, Saxe, subsequent growing season since it “weakened the trees” and de‐ Pedersen, & Randrup, 2002). Street trees in Poznań, Poland, ex‐ layed the start of the growing season. But they also point out that posed to de‐icing salt had foliar Cl concentrations of 4.9 mg/g dry horse‐chestnut “survived through many frosty and long winters mass (Oleksyn, Kloeppel, Łukasiewicz, Karolewski, & Reich, 2007), in the 20th century” and so cold is not seen as a lethal problem. 43% higher than the toxic level of 3.5 mg/g dry mass in sensitive Snow may help survival in continental climates, such as in Poland, trees (Marschner, 1995). In street trees in Berne, Switzerland, Fuhrer by protecting roots and root collar from frosts (Wilczyński & and Erismann (1980) recorded foliar Cl of 8–14 mg/g dry mass in Podlaski, 2007). horse‐chestnut, associated with 25% leaf area necrosis. Large temperature fluctuations, particularly at the end of Eckstein, Liese, and Ploessl (1978) found a significant reduction winter and into spring, have been seen to cause stem fissures in annual ring width of horse‐chestnuts growing 0.5 m from a road in horse‐chestnuts in open urban areas of central and eastern edge in Freiburg, Germany, after salt was first used in the mid‐1960s. Europe. These fissures are invaded by the fungus Schizophyllum Growth was so reduced that between 1970 and 1973, annual rings commune Fr. (Basidiomycota, Agaricales) which is saprotrophic but were virtually non‐detectable, and by 1974, the trees were dead. can become parasitic causing white surface rot (Snieskiene et al., Control trees away from salty roads were unaffected. Petersen and 2011). Eckstein (1988) recorded a similar decline following the use of salt The wood anatomy of horse‐chestnut suggests a moderate in Hamburg. Trees declining from high salt levels showed a similar amount of drought tolerance. Jansen, Choat, and Pletsers (2009) wood anatomy to drought‐stressed trees; more but smaller xylem measured the maximum diameter of pit membranes between ves‐ vessels, and smaller wood rays and fibres, which were replaced by sels as 179 nm, with an air‐seeding threshold of 1.62 ± 0.36 MPa, in parenchyma making the stems weaker and less effective in water a range of 0.95 MPa in Betula pendula to 2.8 MPa in Laurus nobilis. transport (Eckstein, Liese, & Parameswaran, 1976; Petersen & However, A. hippocastanum is often considered unsuitable for dry Eckstein, 1988). urban areas due to its moderate sensitivity to drought (Hirons & Sjöman, 2018; Roloff, Korn, & Gillner, 2009) and the hybrid A. car‐ nea (Section 8.2) is preferred in central European parks and gardens as it is more drought‐tolerant. Simon and Lena (2016) report that street trees in Ljubljana, Slovenia, that were water‐stressed due to 6 | S TRU C T U R E A N D PH YS I O LO G Y 6.1 | Morphology low May–July precipitation underwent delayed cambial activity from Aesculus hippocastanum is a large tree, reaching 39 m height. The the start of May to end of June, whereas in “healthy” trees, it was current largest tree in Britain is 36 m height and 187 cm DBH in middle of April to middle of July. Juvenile horse‐chestnuts are more Kettering, Northamptonshire. An exceptional tree in Andover, sensitive to water stress than adults, due to their more restricted Hampshire, has been recorded with a trunk of 304 cm DBH (The root spread, resulting in yellowing and falling of the leaves (Bellini & Tree Register, 2018). In urban areas, it can commonly reach between Nin, 2005). The resin on the “sticky buds” is thought to help increase 16 and 20 m but can be up to 25 m in height (Cutler & Richardson, resistance to drought. 1989). Branching is initially monopodial, extending shoot length Grosse and Schröder (1985) looked at gas transport through from the apical buds. However, upon sexual maturity, flowers are stems of leafless 6‐month‐old trees as an indication of their ability to borne at the apex of shoots, so subsequent growth is sympodial cope with flooding. They found that gas transport in horse‐chestnut from lateral buds. was 136% higher in the light compared to the dark due to tempera‐ The wood is diffuse‐porous, with spiral grain (Pyszyński, 1977), ture differentials, leading to increased gas exchange between root close‐grained and white. It has a low density (0.5 g/cm³), lower than and shoot of flooded trees during the day. This compares to <25% many conifers, and the wood lacks strength and durability. The nitro‐ increase in Carpinus betulus, Acer pseudoplatanus, Fagus sylvatica and gen content of wood at 0.32% is higher than many broadleaf species Fraxinus excelsior, and a 314% increase in Alnus glutinosa. This sug‐ (Robinson, Tudor, & Cooper, 2011), likely decreasing its resistance gests that horse‐chestnut can cope moderately well with temporary to decay. As with most trees, there is an overall increase in vessel flooding or anoxic soils (Hirons & Sjöman, 2018). diameter and length from the small branches (28 and 208 μm, re‐ Horse‐chestnut is generally tolerant of saline soils and urban spectively) down to roots c. 15 mm in diameter (57 and 439 μm, re‐ salt spray (Chaney, 1991; Šerá, 2017) although some authors have spectively) but vessel density becomes less, reducing from 400 per described it as being sensitive to salt spray (Dobson, 1991). Horse‐ mm2 in small branches to 53 per mm2 in roots (Poole, 1994). Tyloses chestnut is more tolerant to saline soil and spray than Fagus sylvat‐ (outgrowths of parenchyma cells into the vessels of heartwood) are ica and Tilia cordata and to a lesser extent Acer pseudoplatanus. But not formed (Barnett, Cooper, & Bonner, 1993). The bark has a small Journal of Ecology THOMAS eT Al. | 999 proportion of lenticels containing embedded waxes (3%), compared the tropics, but are more pronounced in Aesculus than in other tem‐ to up to 35% in other species tested (Groh, Hübner, & Lendzian, perate species (Boldt & Rank, 2010). Weryszko‐Chmielewska and 2002). The ratio of water vapour loss through lenticels from which Haratym (2012) observed that horse‐chestnut leaves in Poland had waxes were extracted using chloroform compared to control len‐ a few glandular trichomes on the adaxial leaf surface with a mean ticels was 1.7, suggesting that the waxes may reduce water loss. length of 84 μm and head diameter of 61 μm while the abaxial sur‐ However, Groh et al. (2002) concluded that this did not affect water face had non‐glandular trichomes 116–436 μm long. loss through the bark to a large degree. Roots can be shallow to moderately deep, depending upon the soil type, and, as befitting a large tree, can spread a considerable 6.2 | Mycorrhiza distance beyond the crown. Cutler and Richardson (1989) record Arbuscular mycorrhizal fungi are present on the roots of horse‐ the maximum distance that a building has been damaged by horse‐ chestnut (Harley & Harley, 1987) but, unlike many woody plants, chestnut roots on clay soils as 23 m with 90% of cases being within there is no evidence of ectomycorrhizas. Bainard, Klironomos, and 15 m of the tree. This is similar to Acer pseudoplatanus and Ulmus spp. Gordon (2011) found that c. 40% of roots tips of horse‐chestnut and not far short of Quercus spp. (maximum distance 30 m, 90% of were mycorrhizal in rural trees and c. 17% in urban areas of south‐ cases within 18 m). Fine root (<0.8 mm diameter) biomass in Poland ern Ontario (estimated from a figure), similar to many other ar‐ has been recorded at 223–474 g/m2 soil, fine root volume at 615– buscular mycorrhizal trees. However, Karliński et al. (2014) found 1,225 cm3/m2 of soil and number of fine root tips at 553,000–1.5 no difference between rural and urban sites in Poland, colonisa‐ 2 million/m (Karliński et al., 2014). The large terminal leaf bud consists of seven to eight pairs of tion ranging from 54% to 73% of root tips. The differences may be due to Bainard et al. (2011) sampling 20‐ to 35‐year‐old trees cataphylls enclosing the whole of next year's shoot, usually made up in May–June while Karliński et al. (2014) sampled c. 100‐year‐old of three to four pairs of foliage leaves, and often one to two pairs of trees in November, since age and/or establishment does appear to scale primordia of the following year's terminal bud (Foster, 1929b). affect mycorrhizal colonisation. Ferrini and Fini (2012) inoculated As the bud opens, there is a gradual transition from the basal cat‐ horse‐chestnut trees in Milan with a mixture of arbuscular fungi aphylls to cataphylls with a small green lamina through to normal growing in heavily compacted soil. One year after inoculation, the leaves at the top (Foster, 1929a, 1929b). Cataphylls contain chlo‐ frequency of arbuscular roots (51%–59%) was not significantly dif‐ rophyll and can photosynthesise (Solymosi, Bóka, & Böddi, 2006). ferent between inoculated and control newly planted trees 6–8 cm Specific leaf area in western Romania has been seen to change from diameter, but in mature trees (38–51 cm diameter), colonisation 389 cm2/g in April to 250 cm2/g in September as leaves become pro‐ was significantly higher in inoculated trees (76%) compared to con‐ gressively thicker as they complete development (Ianovici, Latiş, & trol trees (63%). However, in both age groups, shoot growth in the Rădac, 2017). This is matched by the mean ash content rising from third growing season was significantly longer in mycorrhizal trees 7.9% to 10.7%. than in controls (mature trees: mycorrhizal 8.8 cm, control 5.7 cm; Horse‐chestnut leaves are hypostomatous with stomata only young trees: mycorrhizal 13.7 cm; control 12.1 cm). This sup‐ on the abaxial (underside) leaf surface (Meidner & Mansfield, 1968; ports the suggestion that mycorrhizal inoculation in urban trees Weryszko‐Chmielewska & Haratym, 2012). Stomatal density has is worthwhile. been seen to vary across Europe from 118 to 298 per mm2. Some of Karliński et al. (2014) found 1%–9% of root tips colonised by fun‐ this variation is due to geographical location. Cetin, Sevik, and Yigit gal endophytes, low compared to that found in other broadleaved (2018) gave a mean density of 198 per mm2 across Turkey with indi‐ trees (Mandyam & Jumpponen, 2005). vidual stomata 24.29 × 16.23 μm in size. However, they varied from 298 per mm2, 16.55 × 9.54 μm in the temperate ecoregion (Central Anatolia) to 177 per mm2, 37.28 × 27.02 μm in the Mediterranean 6.3 | Perennation: reproduction ecoregion, to 118 per mm2, 19.03 × 11.30 μm in the high precipita‐ Phanerophyte. Reproduction is primarily by seeds, but occasionally, tion of the Black Sea ecoregion. Position within the crown is also it will spread vegetatively by new shoots growing from adventitious important; in North Bosnia and Herzegovina, Oljača, Govedar, and buds on the roots of established trees up to 4–5 m from the trunk Hrkić (2009) recorded stomatal density of 293–372 per mm2 at edge (Czekalski, 2005). Hill et al. (2004) classified horse‐chestnut as not of the crown in full light, and 169–230 per mm2 on an inner, shaded spreading clonally, but Howard (1945) noted that branches that part of the crown. Similarly, Boldt and Rank (2010) found density to touch the ground may root and produce new shoots. He describes vary from 146 per mm2 at the base of the crown to 321 per mm2 in such a tree at Hawkhurst Moor, Kent with a height of 27.4 m and a the middle and 322 per mm2 at the top. Boldt and Rank (2010) also combined crown 27.6 m in diameter. Theoretically, such a tree can found that stomata can vary in size within a leaf, with 3%–10% of the keep spreading laterally by the production of new semi‐autonomous stomata being “giant”‐sized, reaching 25 to >40 μm long, mixed with stems as the central stem(s) die. Layering has been confirmed in those c. 20 μm long. The proportion of giant stomata varied between natural populations especially on rocky and steep slopes; for exam‐ the base of the crown (9.6%), the middle (4.7%) and top (7.5%). These ple, genetic analysis of 114 trees found 94 genotypes (Walas et al., large stomata are common in many woody species, particularly in 2018; M. Dering and G. Iszkuło, pers. comm., September 7, 2018). 1000 | Journal of Ecology THOMAS eT Al. Horse‐chestnut will also produce new stems on cut stumps and so with large seeds (Krahulcová, Trávníček, Krahulec, & Rejmánek, will coppice (Czekalski, 2005). 2017). Horse‐chestnut can be grafted onto 1‐ or 2‐year‐old seedlings using dormant apical scions in mid‐winter. Budding is also successful in late summer using medium size buds from the middle of a branch 6.5 | Physiological data (McMillan‐Browse, 1971). It can also be propagated using semi‐hard‐ Horse‐chestnut grows best in sunny, sheltered locations and is clas‐ wood cuttings (Chapman & Hoover, 1982), though this is uncommon. sified as shade‐intolerant (Fitter & Peat, 1994), but it can tolerate Horse‐chestnut has been used as a rootstock in the grafting of other partial shade (Jagodziński, Łukasiewicz, & Turzańska, 2003), espe‐ Aesculus species planted in Britain, such as A. octandra and A. carnea cially as saplings. Young stems contain chlorophyll and are capable of (Leathart, 1991). photosynthesising (Skribanek, Apatini, Inaoka, & Böddi, 2000); the Tissue culture has been successfully used to produce horse‐ protochlorophyllide content (a precursor of chlorophyll) of horse‐ chestnut plants from a variety of somatic and gametic sources in‐ chestnut twigs was comparatively high at c. 10 μg/g fresh mass, cluding microspores (Radojević, 1978; Radojević, Marinkovic, & compared to 3–5 μg/g in Acer campestre. In full sunlight, net pho‐ Jevremovic, 2000), anther filaments (Capuana, 2016; Jörgensen, tosynthetic rate has been measured in north‐east Italy at 5–9 μmol 1989; Kiss, Heszky, Kiss, & Gyulai, 1992), embryos (Profumo, [CO2] m−2 s−1, stomatal conductance at c. 105 mmol m−2 s−1, transpi‐ Gastaldo, Bevilacqua, & Carli, 1991; Troch, Werbrouck, Geelen, & ration rate at c. 2.4 mmol m−2 s−1 and leaf hydraulic conductance at Van Labeke, 2009), and leaf and stem explants (Dameri, Caffaro, 4.5 kg s−1 m−2 MPa−1 (estimated from figures in Nardini, Raimondo, Gastaldo, & Profumo, 1986; Gastaldo, Carli, & Profumo, 1994; Scimone, & Salleo, 2004; Raimondo, Ghirardelli, Nardini, & Salleo, Šedivá, Vlašínová, & Mertelík, 2013). Embryogenic tissues can also 2003). Stomatal conductance in full sunlight in Gothenburg, Sweden, be cryopreserved for long‐term storage (Jekkel, Gyulai, Kiss, Kiss, & was measured by Konarska et al. (2016) at c. 80–90 mmol m−2 s−1, a Heszky, 1998; Lambardi, De Carlo, & Capuana, 2005; Wesley‐Smith, similar figure to other deciduous trees tested such as Betula pendula, Walters, Pammenter, & Berjak, 2001), overcoming the storage prob‐ Fagus sylvatica and Tilia × europaea, but lower than Quercus robur lems of recalcitrant seeds (Section 8.4) (Pence, 1990). Anther and (c. 200–210 mmol m−2 s−1—estimated from figures). Horse‐chestnut microspore cultures have been used to produce haploid plants in should not therefore be at a disadvantage in low humidity conditions horse‐chestnut (Ćalić‐Dragosavac, Stevović, & Zdravković‐Korać, compared to many native trees. 2010). Up to 10%–12% of the embryos produced by these methods Drought tolerance of horse‐chestnut can be improved by spray‐ were found to be albino, particularly when grown under short days ing trees with the triazole (fungicide) derivatives paclobutrazol, of 8 hr light, but the proportion could be reduced by the addition of penconazole, epoxiconazole and propiconazole. Percival and Noviss abscisic acid to the cultures (Ćalić et al., 2013). Procedures for ge‐ (2008) treated 4‐year‐old saplings transplanted into pots that netically modifying A. hippocastanum embryos using the bacterium 2 weeks later were exposed to a 3‐week drought. Spraying reduced Agrobacterium rhizogenes have been devised by Zdravković‐Korać, visible leaf necrosis by 33%–83% compared to drought‐treated Muhovski, Druart, Ćalić, and Radojević (2004). but unsprayed controls, and electrolyte leakage (a measure of cell Flowers are first produced at 10–15 years old. Horse‐chestnut membrane damage) by 36%–64%, depending upon the fungicide normally lives for a maximum of 150–200 years (Maurizio & Grafl, used. Chlorophyll fluorescence ratio (Fv/Fm) increased 59%–121% 1969) but can survive for 300 and exceptionally 500 years (Fitter & and light‐induced CO2 fixation increased by 16%–137%. Leaves of Peat, 1994; Leathart, 1991; Mitchell, 1997). treated trees also had higher concentrations of total carotenoids (29%–2,891%), chlorophylls (53%–288%) and proline (42%–109%) 6.4 | Chromosomes and higher superoxide dismutase (23%–118%) and catalase (24%– 2n = 40 (Bennett, Smith, & Heslop‐Harrison, 1982; Hardin, 1960). also showed faster recovery after drought in the above characteris‐ The hybrid, A. carnea (Section 8.2), is a tetraploid (2n = 80) according tics. The triazole myclobutanil had no effect. to Hoar (1927) and Hardin (1957). A backcross between A. carnea and A. hippocastanum is reported with 2n = 60 (Upcott, 1936). 133%) activities than drought‐treated control trees. Treated trees Horse‐chestnut planted in poor soils in urban environments re‐ sponds well to nutrient addition. Oleksyn et al. (2007) investigated Skovsted (1929) noted that the chromosomes of A. hippocasta‐ street trees in Poznań, Poland, with a mean height of 13.6 m and num were relatively small (c. 0.5 μm, estimated from a figure) while 0.53 m DBH. Trees were given a mulch of organic matter (chipped those of A. pavia were larger (c. 1 μm), and that A. carnea had a mix tree waste stored for 1 year) and nutrient applications over 3 years of both sizes. However, this appears to be an artefact of sample of 17 g N m−2 month−1 for 2 years, then 7 g N and 16 g S m−2 month−1 preparation since subsequent measurements have found the chro‐ for one further year. The treatments increased foliar N by 36% from mosomes in all three taxa to be similar at 1–2 μm long (Pogan, Wcislo, 1.7% to 2.7% compared to control trees. Total phenolic concen‐ & Jankun, 1980; Upcott, 1936). tration in treated trees was 43% lower than in control trees after Aesculus hippocastanum has a comparatively small nuclear ge‐ 3 years of treatment (mean 256 μmol/g in control and 193 μg/g in nome size for a woody angiosperm at 1.22 ± 0.010 pg (2C), but treated) suggesting lower investment in secondary defence com‐ such a small genome size is not unusual among woody species pounds in trees with better nutrition. Net photosynthesis per unit Journal of Ecology THOMAS eT Al. area and unit mass both increased by 21%–30% in treated sites. 2 Leaves became bigger (3.3 vs 1.8 g/leaf; 386 vs 248 cm /leaf) and | 1001 Pirożnikow, Zambrzycka, & Swiecicka, 2016). Leaves have epicutic‐ ular wax containing large amounts of triterpenols and triterpenol thicker (specific leaf area 119 vs 141 cm2/g). Leaves also remained on esters including β‐amyrin, a‐amyrin, lupeol, friedelanol and frie‐ the tree longer (175 vs 130 days) and treated trees produced more delanone (Gülz, Müller, & Herrmann, 1992). Despite the abundant seeds (>100 seeds vs <12 seeds/tree), but this was still half that of wax, Papierowska et al. (2018) found that, based on the angle of trees growing in the better soil of the nearby botanic garden. contact of water droplets, the adaxial surface of horse‐chestnut is “wettable” and the abaxial “highly wettable.” This is similar to that 6.6 | Biochemical data found in Acer pseudoplatanus and Betula pendula but in contrast to 14 of the 19 European deciduous species tested where the abaxial Various parts of Aesculus hippocastanum contain high levels of trit‐ surface was the least wettable. Although high wettability may re‐ erpene glycosides or saponins, including aescigenin, hippocaesculin sult in a film of water reducing gas exchange, the authors suggest and barringtogenol (Konoshima & Lee, 1986), the mix collectively that it also allows water droplets to spread out and quickly evap‐ called aesculin, aescin or escin; these have medical uses (Section orate and so leaves quickly dry, reducing the time that pathogens 10.1). Horse‐chestnut also contains tannins, carotenoids (includ‐ have a moist surface to invade, and so lower the need for inter‐ ing aesculaxanthin, lutein and citraurin), fatty acids (including lau‐ nal chemical defences. This may be linked to the low precipitation ric, myristic, palmitic, stearic, archaic and oleic acids), at least 10 of its native climate where gas exchange problems would be less coumarin derivatives (including esculetin) and at least 15 flavo‐ frequent. noids, mainly glycosides of quercetin, leucocyanidin, procyanidin Seeds contain 30%–60% starch, 6%–11% protein, 4%–8% lip‐ and kaempferol (Birtić & Kranner, 2006; Coruh & Ozdogan, 2014; ids and 8%–26% saponins (Baraldi et al., 2007; Čukanović et al., Czeczuga, 1986; Deli, Matus, & Tóth, 2000; Dudek‐Makuch & 2011; Duke & Ayensu, 1985; Lemajić, Savin, Ivanić, & Lalić, 1985), Matławska, 2011; Kapusta et al., 2007; Kędzierski, Kukula‐Kocha, but these are variable between populations. Seeds from southern Widelski, & Głowniak, 2016; Kim et al., 2017; Morimoto, Nonaka, Bulgaria contained 81 ± 3 g/kg (SD) of oils, including relatively high & Nishioka, 1987; Turkekul, Colpan, Baykul, Ozdemir, & Erdogan, levels of unsaponifiable compounds (57 ± 1 g/kg), sterols (12 ± 2 g/ 2018; Yoshikawa, Murakami, Yamahara, & Matsuda, 1998; Zhang kg), phospholipids (3 ± 0.1 g/kg) and tocopherols (627 ± 15 mg/kg) et al., 2010) and polyprenols including undecaprenol, tridecapre‐ (Zlatanov, Antova, Angelova‐Romova, & Teneva, 2012). The starch nol and particularly dodecaprenol and castoprenol (Khidyrova & has a low amylose content (Hricovíniová & Babor, 1992). Seeds also Shakhidoyatov, 2002; Wellburn, Stevenson, Hemming, & Morton, contain sugars; thirteen compounds have been identified including 1967). The highest concentration of most chemicals is in the seeds, glucose, sucrose, fructose, amylosaccharide, galactosylsucrose and particularly the cotyledons but they are also found in the fruit, bark, fructosylsucrose (Hricovíniová & Babor, 1991; Kahl, Roszkowski, & leaves and buds (Bombardelli, Morazzoni, & Griffini, 1996; Otajagić, Zurowska, 1969; Kamerling & Vliegenthart, 1972). Pinjić, Ćavar, Vidic, & Maksimović, 2012) and in embryonic callus Pollen grains of horse‐chestnut contain moderate amounts of tissue (Profumo, Caviglia, Gastaldo, & Dameri, 1991). Abudayeh, Al antioxidants, although total phenols (3,375 mg 100/g), flavanols Azzam, Naddaf, Karpiuk, and Kislichenko (2015) investigated seeds (624 mg 100/g) and anthocyanins (183 mg 100/g) were found to from Poland and found lower levels of saponins in the seed coat be lower than in other woody species tested (Robinia pseudoacacia, (0.19–0.32 g/kg) than the seed's endosperm (34.9–52.05 g/kg). The Malus domestica and Pyrus communis) by Leja, Mareczek, Wyżgolik, levels decreased by >30% in the endosperm and >40% in the skin Klepacz‐Baniak, and Czekońska (2007). when stored air‐dried for 2 years. Atmospheric levels of heavy metals are reflected in their con‐ The sterols and a number of monoterpene phenols (e.g., car‐ centration on and in leaves and bark (Table 1). Levels in Turkey were vacrol) in bark, quercetin and kaempferol in seeds, and at least 17 comparatively low compared to levels in the soil (Pb, 0.81–6.75; Cd, phenolic compounds in leaves (Hübner, Wray, & Nahrstedt, 1999; 0.002–0.006; Zn, 2.20–4.60; Cu, 0.52–1.12 μg/g dry weight; Yilmaz, Oszmiański, Kalisz, & Aneta, 2014) are known to have antifeedant Sakcali, Yarci, Aksoy, & Ozturk, 2006). Levels in Turkey tend to be properties against insects (Eriksson, Månsson, Sjödin, & Schlyter, lower than in Bulgaria and Serbia (Table 1); similar figures for Serbia 2008). The highest concentration of coumarins is in the bark, par‐ are also found in Tomašević et al. (2004) and Deljanin et al. (2016). ticularly during summer, lower in spring and autumn (Matysik, Washing leaves reduced levels of Pb, Ca, Zn and Cu, indicating that Glowniak, Soczewiński, & Garbacka, 1994), emphasising their role these are primarily surface particles (Yilmaz et al., 2006). Levels of as antifeedants. Pb, Zn and Cu were very high in New Zealand, reported in Kim and The buds of horse‐chestnut are renowned for being “sticky.” Fergusson (1994) before lead‐free petrol became legally compulsory. The abundant resin causing the stickiness contains relatively small In the same leaded petrol era in Scotland, Guha and Mitchell (1966) amounts of flavonoids (13.0%) and larger amounts of triterpenoids found that Pb decreased towards the top of the crown consistent (43.4%) but, most distinctively, high level of C14–C22 aliphatic 3‐ with a petrol origin. Aničić, Spasić, Tomašević, Rajšić, and Tasić hydroxyacids (20.1%). However, the resin had lower antimicrobial (2011) found that horse‐chestnut street trees in Serbia accumulated activity against gram‐positive bacteria than resins of other decid‐ more heavy metals than did Tilia spp. and so are considered a bet‐ uous trees, such as Betula spp and Pinus sylvestris (Isidorov, Bakier, ter species for the assessment of Pb and Cu atmospheric pollution. 1002 | Journal of Ecology TA B L E 1 THOMAS eT Al. Levels of heavy metals (μg/g dry mass) in various tissues of Aesculus hippocastanum in different geographical locations Turkey1 Bulgaria2 Serbia3 Element Washed leaves Unwashed leaves Bark Unwashed leaves Washed leaves Pb 0.02–0.05 0.02–0.12 0.06–0.63 2.75 0.8–21.5 Cd 0.001–0.002 0.002–0.068 0.005–0.006 0.24 Zn 0.39–0.59 0.37–0.53 0.41–0.66 Cu 0.26–0.39 0.32–0.47 0.35–1.03 Cr New Zealand4 294 0.197 15.2–36.2 299 8.2 5.5–87.5 129 0.25 0.3–1.80 Ni 0.41–2.38 Sr 30–81 As 0.11–0.35 V 0.05 U 0.012 0.4–1.78 1 Yilmaz et al. (2006); 2Petrova, Yurukova, and Velcheva (2012); 3Aničić et al. (2011), Kocić, Spasić, Urošević, and Tomašević (2014), Pavlović et al. (2017); Kim and Fergusson (1994). 4 Some studies have shown that levels of heavy metals in leaves in‐ Quercus robur (Fu, Campioli, Van Oijen, Deckmyn, & Janssens, crease through the growing season (e.g., Kim & Fergusson, 1994) 2012). Sparks, Jeffree, and Jeffree (2000) found the mean leaf‐ while others have shown that they decrease (e.g., Šućur et al., 2010), ing date in Britain using a 20‐year record was 10 April (earliest depending upon the relative uptake from the soil and atmosphere, −10 days; latest +15 days). In Poznań Botanical Garden, Poland, the and seasonal changes in atmospheric pollution. mean leafing date was 5 days later (Sparks, Górska‐Zajączkowska, Uptake of radionuclides from the soil is generally very low; the Wójtowicz, & Tryjanowski, 2011). The spring flushing of buds in highest recorded for horse‐chestnut was 40 K, with a 1.3 soil:leaves horse‐chestnut is primarily controlled by February and March transfer factor, giving a concentration of 487 Bq/kg, similar to temperatures in Britain, and March to May temperatures in Poland that seen in Tilia spp. (Todorović, Popović, Ajtić, & Nikolić, 2013). (Menzel et al., 2008; Tryjanowski, Panek, & Sparks, 2006) but it However, 1 year after the Chernobyl reactor accident on 26 April is also said to have a photoperiod requirement (Basler & Körner, 1986, horse‐chestnut leaves contained the highest levels of 40 K 2012) that develops just before leaf flushing (Laube et al., 2014). of any of the land plants tested (670 ± 15 Bq/kg dry mass; SD); Zohner and Renner (2015) found that budburst was delayed by 2 years after the accident, levels had declined to 325 Bq/kg of 4 days when branches on mature trees were given 8‐hr days rather 40 K along with 37 Bq/kg of 212 Pb and 11 Bq/kg of 208 Tl (Heinrich, than 16‐hr days. This compares to a 41‐day delay in Fagus sylvatica Müller, Oswald, & Gries, 1989). Indeed, horse‐chestnut leaves have and no delay in Picea abies. Zohner and Renner (2015) concluded proved useful in tracking Pb isotopes as leaded petrol was phased that the delay in budburst in horse‐chestnut under short days is out (Tomašević et al., 2013). Yoshihara et al. (2014) have shown that simply a consequence of slower growth as a result of lower light Cs‐137 resulting from the Fukushima reactor accident in Japan can availability rather than a photoperiod requirement itself. There be tracked in leaves of a variety of trees, including horse‐chestnut appears to be no elevational change in photoperiod requirement growing nearby. (Basler & Körner, 2012). Flowering in Britain usually lasts between late April and the mid‐ dle of May (Tryjanowski et al., 2006). Jeffree (1960) identified mean 7 | PH E N O LO G Y start of flowering as 9 May ±5 days using a 35‐year dataset, and Sparks et al. (2000) 8 May using a 58‐year dataset with the earliest The cambium of horse‐chestnut begins cytoplasmic activity in mid‐ −20 days and the latest +16 days. In western Poland, mean first flow‐ February; the first cell divisions on the phloem are produced in early ering date over 20 years was 6 May (Sparks et al., 2011). Pollen pro‐ April, and the first xylem elements are formed in the middle of April duction (taken to indicate longevity of flowering) from five trees in (Barnett, 1992). Radial growth continues until August in Poland Lublin, Poland, between 2002 and 2009 lasted for 18–42 days with (Wilczyński & Podlaski, 2007). Jagiełło et al. (2017) identified that in a mean of 28 days (Weryszko‐Chmielewska et al., 2012). Anthesis is saplings c. 85% of the whole‐year stem volume increment occurred at a maximum between 13 and 18°C air temperature, compared to before the end of July. 10.2–18°C in Crataegus monogyna, and is much reduced during rain The winter chilling requirement for buds to flush (321 degree‐ days above 0°C) is similar or lower than many deciduous trees (Percival, 1955). Maximum pollen release was 6–22 May in Poland (Weryszko‐Chmielewska et al., 2012). native to Britain (Laube et al., 2014). Leaf buds open in mid‐April A second flowering was seen in horse‐chestnut in September (Hutchings, Lawrence, & Brunt, 2012), 2–3 weeks earlier than 2000 in Munich and Frankfurt, Germany (Heitland & Freise, 2001), Journal of Ecology THOMAS eT Al. and has been noticed sporadically in Slovenia (Menzel et al., 2008). | 1003 In Germany, a 1°C increase in temperature resulted in advances In Bulgaria, a second flowering has traditionally been seen as a pre‐ of horse‐chestnut flowering date by a mean of 2.6 days, partic‐ diction of a severe winter (Nedelcheva & Dogan, 2011). Second ularly noticeable in areas that normally flowered early anyway flowering is probably a response to a hot, dry summer but is sug‐ (Menzel et al., 2005), and in Poland, flowering has been getting gested to also be in response to damage caused by the leaf miner earlier at the comparatively small amount of 0.07 days per year Cameraria ohridella and the fungus Guignardia aesculi (Menzel et al., (Jabłońska, Kwiatkowska‐Falińska, Czernecki, & Walawender, 2015). 2008). Interestingly, there has been no apparent advancement in the start Saponins and flavonoid content reach their highest levels in seeds in August, 13–16 weeks after the beginning of flowering of fruit ripening despite leaf colouring beginning earlier (Menzel, Estrella, & Fabian, 2001). (Kędzierski et al., 2016). Seeds fall from mid‐September (Farrant & The damage to leaves caused by the horse‐chestnut leaf miner Walters, 1998; Sparks et al., 2011), especially during autumn gales. In Cameraria ohridella has reduced the length of the growing season of western Romania, horse‐chestnut keeps its leaves for 130–175 days horse‐chestnut in Slovenia by 12 days/decade since 2000 (Menzel (Ianovici et al., 2017). Leaf colouring usually begins at the end of et al., 2008) and this is undoubtedly also occurring elsewhere in September (Hutchings et al., 2012; Sparks et al., 2011) but a warm Europe. This shortening is tending to counteract the effects of cli‐ May (in Germany) and June (in Slovenia) and a dry September leads mate change lengthening the growing season. Similarly, Jochner to earlier leaf colouring. Conversely, a warm September delays co‐ et al. (2015) stated that increased ozone pollution in cities is delaying louring (Estrella & Menzel, 2006; Menzel et al., 2008). Leaves begin leaf flushing and flower opening of horse‐chestnut (although NOx to fall in the first half of October, and leaf fall generally lasts 28 days are not) but data are not given. so trees are bare of leaves by the beginning of November (Hutchings et al., 2012; Sparks et al., 2011). 8 | FLO R A L A N D S E E D C H A R AC TE R S 7.1 | Climate change 8.1 | Floral biology Modelling based on the natural range of horse‐chestnut suggests The large panicle has male flowers at the top and hermaphrodite that the distribution and abundance of horse‐chestnut should not flowers below. In Polish samples, 73% of flowers were male and 27% change significantly in the Balkan Peninsula under current climate of flowers hermaphrodite (Weryszko‐Chmielewska & Chwil, 2017). change scenarios (Walas et al., 2018). However, horse‐chestnut However, some flowers at the base of the panicle can be functionally is sensitive to spring warming, and this is leading to rapid changes female (Ćalić‐Dragosavac, Zdravković‐Korać, Miljković, & Radojević, in spring phenology (Menzel, Estrella, & Testka, 2005; Tryjanowski 2009; Maurizio & Grafl, 1969). In Serbia, Ocokoljić, Vilotić, and et al., 2006; Walther et al., 2002). Some of this is due to local changes; Šijačić‐Nikolić (2013) found 50% male, 28% hermaphrodite and 22% for example, bud burst of horse‐chestnut has become earlier in cen‐ female flowers. tral Geneva since 1808 at the rate of 0.23 days per year, attributed Aesculus hippocastanum is andromonoecious, amphimictic and to the heat island effect of the growing city (Defila & Clot, 2001). normally cross‐pollinated (Fitter & Peat, 1994). Large bees such as Nevertheless, climate change does appear to be having an effect. Bombus spp. (Hymenoptera, Apidae) tend to work their way upwards Chen et al. (2018) looked at five deciduous European trees, including on a panicle from female, to hermaphrodite, to male flowers, helping horse‐chestnut, across Europe and found that at low altitude (<10 m), to reduce self‐pollination (Kevan, 1990). Hermaphrodite flowers are leaf opening advanced between 1951 and 2013 by c. 2.4 days per protogynous, but the whole inflorescence is protandrous (Fitter & decade (estimated from a figure). Conversely, at high altitude (800– Peat, 1994). 1,000 m) before 1980 spring became later (+2.7 ± 0.6 days per dec‐ Horse‐chestnut is primarily pollinated by insects and is often re‐ ade, SD), and then advanced again c. 2.7 days per decade (estimated garded as an important bee plant as the flowers provide abundant from a figure). But the rate of change appears to be slowing. Fu nectar and pollen for insects (Maurizio & Grafl, 1969) and glandu‐ et al. (2015) looked at the number of days’ advance of leaf unfolding lar trichomes on the sepals and ovary produce olfactory attractants per °C of warming across Europe and found that horse‐chestnut ad‐ (Chwil, Weryszko‐Chmielewska, Sulborska, & Michońska, 2013). vancement had changed from 4.2 ± 1.5 (SD) days/°C in 1980–1994 Synge (1947) lists horse‐chestnut as an important source of pollen to 2.1 ± 1.5 days/°C in 1999–2013, a reduction of 2.1 days/°C, the early in the year, before Tilia spp. flower. Pollinators are honeybees most of the seven species tested. This was attributed at least in part Apis mellifera and bumblebees (Free, 1963; Maurizio & Grafl, 1969; to the reduced chilling the trees are getting due to shorter and milder Percival, 1955; Weberling, 1989), but flowers are also visited by hov‐ winters, despite chilling requirements becoming shorter. The winter erflies (Kugler, 1970), solitary bees such as Osmia spp. (Raw, 1974) chilling requirement of horse‐chestnut has declined across central and some mining bees in the genus Andrena (Hymenoptera, Apidae) Europe from c. 68 days in 1980–1994 to c. 62 days in 1990–2013 (Chambers, 1968). (taken from a figure); chilling was calculated as chilling days when Nectar is found in both hermaphrodite and male flowers daily temperature was between 0°C and 5°C from 1 November to and is released as the buds open. A flower secretes a mean of the average date of leaf unfolding (Fu et al., 2015). 2.64 ± 0.94 mg (SD) of nectar in Poland (Weryszko‐Chmielewska 1004 | Journal of Ecology THOMAS eT Al. & Chwil, 2017) or c. 1.2 μl in Belgium, compared to c. 0.8 μl in there to be between 3,600 and 5,000 pollen grains per anther, de‐ Tilia × europaea and c. 1.7 μl in T. cordata, estimated from a fig‐ pending on genotype, with viability determined by staining with ure (Somme et al., 2016). The nectar contains a comparatively low fluorescein diacetate to be 56%–68%, and by germination on basic amount of sugar, c. 25%–32%, similar to Tilia tomentosa and A. car‐ medium to be 50%–66%. By contrast, Kugler (1970) calculated nea, but low compared to >60% in Robinia pseudoacacia and Tilia 26,000 pollen grains per anther, 181,000 per flower and thus 42 mil‐ × europaea. Sucrose makes up 90% of the sugars, which is highly lion pollen grains from a single inflorescence. attractive to bees (Percival, 1961; Somme et al., 2016; Weryszko‐ Chmielewska & Chwil, 2017). Pollination is primarily by insects, but due to the large number of pollen grains produced, wind pollination is considered a viable An inflorescence produces a total of 1 mg of pollen at the rate of supplement. Certainly air‐borne pollen has been detected in many 0.5 mg/day (Percival, 1955), high compared to the other tree species European countries, averaging 8–69 pollen grains/m3 of air during tested (e.g., totals of 0.3 mg in Ilex aquifolium and 0.8 mg in Crataegus the flowering season (Biçakci, Benlioglu, & Erdogan, 1999; Popp monogyna). Stamens are normally bent downwards but become erect et al., 1992; Weryszko‐Chmielewska et al., 2012). Studies across when shedding pollen, presumably as a mechanism for aiding pol‐ Europe have shown that horse‐chestnut pollen in urban areas ac‐ len removal by insects. Percival (1955) classified horse‐chestnut as counts for 0.13%–1.54% of total air‐borne pollen (Melgar et al., a “chiefly morning” flowerer, presenting pollen for honeybees from 2012; Peternel, Čulig, Mitić, Vukušić, & Šostar, 2003; Rizzi‐Longo, 5 a.m. to 6 p.m. with the peak period at 5 a.m. when 20% of the day's Pizzulin‐Sauli, Stravisi, & Ganis, 2010; Stefanic, Rasic, Merdic, & pollen was presented; 63% of its total pollen is presented by midday. Colacovic, 2007), high enough densities to cause an allergic reaction Anthers in a flower dehisce over one to several days, similar to other in children in Vienna (Popp et al., 1992) and presumably high enough insect‐pollinated trees such as Prunus spp. and Crataegus monogyna. to supplement insect pollination. Horse‐chestnut pollen has 39.5 ± 7.0 (SD) μg/mg of polypeptide, 331.7 ± 27.1 μg/mg amino acid content and 4.93–5.07 μg/mg sterol content, similar to the eight other hardwood trees commonly grown 8.2 | Hybrids in parks in Belgium (Somme et al., 2016). The red connective pro‐ Aesculus hippocastanum is known to hybridise with the four North trusions at each end of the anthers secrete droplets which contain American species of Section Pavia when they are grown together— lipids and thus may also act as food bodies (Weryszko‐Chmielewska A. pavia L., A. glabra Willd., A. flava Sol. (=A. octandra Marsh.) and & Chwil, 2017). A. sylvatica L. (Hardin, 1957). The only hybrid commonly found in Bee deaths have been reported when fed horse‐chestnut pol‐ Europe is the red horse‐chestnut, A. carnea Willd. (=A. rubicunda len and nectar (Maurizio, 1945), which may be due to the high sa‐ Lodd., A. rubicunda Loisel., A. intermedia Andre.), a hybrid of A. hip‐ ponin content (Section 6.6). The cause may also possibly be due to pocastanum and A. pavia (note that as a fully fertile hybrid, the spe‐ the presence of mannose or nicotine (Somme et al., 2016) although cies name is not preceded by “x” by convention). This hybrid is fertile Detzel and Wink (1993) have found honeybees to tolerate low con‐ and breeds true (linked to being a tetraploid). It is often grafted onto centrations of alkaloids including nicotine. Bees have been seen to A. hippocastanum for vigour. It was first grown in Britain around prefer flowers with lower saponin content (Maurizio, 1945; Schulz‐ 1818 (Leathart, 1991) and was recorded in the wild by 1955 (Preston Langner, 1967). et al., 2002) and is now occasionally self‐sown in Surrey, West Kent Pollinators are attracted to flowers by yellow floral guide spots on and North Hampshire (Stace, 2010). A cultivar, A. carnea 'Briotii', the petals. Once the flower is pollinated, these turn to red and nec‐ produced in France in 1958, has brighter red flowers and glossier, tar (Lunau, 1996; Willmer, 2011) and scent productions are greatly more attractive leaves than the original hybrid, and is widely planted reduced or stopped (Lex, 1954). The red spots are unattractive to (Leathart, 1991). Irzykowska, Werner, Bocianowski, Karolewski, and insects (Kugler, 1936) and are presumed to be a mechanism for not Frużyńska‐Jóźwiak (2013) found, perhaps unsurprisingly, that ge‐ wasting the bee's efforts on flowers that are already pollinated. It is netic diversity was higher in A. hippocastanum (mean genetic simi‐ suggested that this colour change occurs, rather than petals falling larity of 0.55) than in A. carnea (0.98). The majority of the genetic once pollinated, to maintain the large visual signal of a tree to attract variance (73.0%) was contributed by the differentiation between pollinators across large distances in mountainous habitats with dis‐ A. hippocastanum and A. carnea, whereas 27.0% was partitioned persed populations (Thomas, 2014). Both ends of the anthers have within species. Hardin (1960) also lists a triploid backcross between red appendages, and the upper part of the style and the stigma are A. carnea and A. hippocastanum named A. × plantierensis. similarly red‐coloured. These markings may also act of pollinator guides (Weryszko‐Chmielewska et al., 2012). Pollen of horse‐chestnut is round‐to‐oval and very distinctive 8.3 | Seed production and dispersal with coarse spines (Pozhidaev, 1995). Size ranges from 14 to 30 μm There are typically 2–5(8) fruits per panicle, each containing one diameter, varying with bud size and position in the inflorescence (rarely two or three) seeds (Thalmann et al., 2003). But the number (Ćalić, Zdravković‐Korać, Pemac, & Radojević, 2003‐2004; Ćalić‐ of panicles is very variable, giving a seed production of from 2–3 to Dragosavac et al., 2009; Radojević, 1989). In material collected from 25 kg of fresh seeds per tree (Bellini & Nin, 2005), which equates 125‐year‐old trees in Serbia, Ćalić and Radojević (2017) estimated to approximately 125 to 1,600 seeds per tree. Horse‐chestnut has Journal of Ecology THOMAS eT Al. | 1005 shown masting, with large seed crops produced every 2 years in embryo increased from 0.5 to 4.0 mg during development. Other natural populations in Greece (Tsiroukis, 2008). This was seen to be work by Pammenter and Berjak (1999) showed that respiration in synchronised throughout its distribution range in Greece. developing horse‐chestnut seeds remained high (3.0–5.0 nmol O2 g Mean fruit mass has been measured at 42.14 g in natural pop‐ dw−1 s−1) until the seed started drying indicating that they were still ulations in Greece (Tsiroukis, 2008). Seeds are large, each typically developing. Seeds from further north certainly have a lower mass 13–20 g fresh mass (Daws et al., 2004; Bonner & Karrfalt, 2008), al‐ (Section 8.3) and are likely less developed when shed than seeds though are largest (15.3–22.6 g) in street trees in Serbia (Ocokoljić grown in warmer conditions. This is supported by c. 70% of Scottish & Stojanović, 2009; Ocokoljić et al., 2013), but smaller (mean 9.9– seeds being found to be empty or underdeveloped and non‐viable 14.5 g) in natural populations in Greece (Daws et al., 2004; Tsiroukis, (Daws et al., 2004). Indeed, British seed has been seen to increase 2008) and smallest (1.2 g) at the northern end of its planted range in in dry mass right up to seed fall, and also maintain high seed mois‐ Scotland (Daws et al., 2004). ture content which was linked to a decrease in desiccation tolerance Seeds are primarily dispersed by gravity (barochory), with seeds and germinability (Tompsett & Pritchard, 1993). Fresh seed moisture falling from the fruits more or less under the crown of the mother content at time of seed fall has been measured at 59.7 ± 0.2% (SE, tree. Little is known about distances moved by horse‐chestnut but n = 150) in Greece and 69.0 ± 1.9% in Scotland, with a solute poten‐ seeds of the closely related A. turbinata were found to disperse a tial −3.0 ± 0.2 MPa in Greece and −2.1 ± 0.1 MPa in Scotland (Daws mean of 12.2–44.7 m from the parent trees during a 3‐year study in et al., 2004). Japan, with a maximum distance of 41.5–114.5 m (Hoshizaki, Suzuki, Proteins found in the cytosol of the seed cells are mainly & Nakashizuka, 1999). It is likely that dispersal distances are similar water‐soluble albumins which, being hydrophilic, may help prevent in A. hippocastanum. Aesculus turbinata seeds are known to be dis‐ the seed dehydrating over winter, and may also confer protection persed by rodents (Hoshizaki et al., 1999) and this is likely to happen, against cold stress (Azarkovich & Bolyakina, 2016). Seeds can be to some extent at least, in horse‐chestnut. Seedlings have been seen stored when hydrated at 16°C, with more than one‐third of seeds far away from mature trees in Greece with rodent movement the remaining germinable after 3 years (Pritchard, Tompsett, & Manger, most likely cause (M. Dering and G. Iszkuło, pers. comm., September 1996). Nevertheless, seeds are sensitive to desiccation and short‐ 7, 2018). Ridley (1930) records that even the removal of one cotyle‐ lived when dried (recalcitrant); as they dry to 32%–40% moisture, don by rats does not prevent at least initial stages to germinate such they develop dormancy and lose viability upon further drying as radical elongation. In Britain, other vectors undoubtedly include (Tompsett & Pritchard, 1998). As with Quercus spp. and Castanea grey squirrels (Sciurus carolinensis Gmelin), corvids (Briggs, 1989) and sativa, horse‐chestnut viability typically declines to 50% germi‐ children collecting and ultimately discarding conkers. There is also a nation over 10–24 weeks (Gosling, 2007). Higher temperatures suggestion that seeds of horse‐chestnut are secondarily dispersed during development and lower moisture content at seed fall result by water, particularly during snow‐melt, primarily based on the con‐ in greater desiccation tolerance and shallower dormancy (Farrant sistent occurrence of horse‐chestnut along mountain streams and & Walters, 1998; Obroucheva & Lityagina, 2007; Pritchard et al., rivulets in the native Greek populations (Briggs, 1989; Tsiroukis, 1999; Tompsett & Pritchard, 1993, 1998) and helps account for vari‐ 2008). However, this is probably a result of dispersal by gravity and ation in dormancy geographically and between years. Median water the needs of germination and early growth rather than a facet of potential resulting in seed death was −5.1 ± 0.65 MPa (SE, n = 150) secondary dispersal. for Scottish seeds and −16.2 ± 0.83 MPa for Greek seeds (Daws et al., 2004). The physiological and morphological changes that 8.4 | Viability of seeds: germination occur in seeds as they develop, pass through dormancy and ger‐ Seeds grown in Britain and sown onto agar immediately after fall‐ Vedenicheva, and Vasyuk (2003), Obroucheva, Lityagina, Novikova, ing will germinate at between 26 and 36°C (in the dark at constant and Sin'kevich (2012); Obroucheva, Sinkevich, and Lityagina (2016); temperature) within 1 month of sowing reaching up to 80%–90% and Obroucheva, Sinkevich, Lityagina, and Novikova (2017). minate are described further by Musatenko, Generalova, Martyn, germination (Pritchard, Steadman, Nash, & Jones, 1999; Tompsett The embryo‐based physiological dormancy caused by dry‐ & Pritchard, 1993), aided by the presence of heat‐shock proteins in ing can be broken by stratification at −3 to 6°C (4°C optimum) the embryo (Azarkovich & Gumilevskaya, 2012). But since British for 8–21 weeks, longer for more northern seed (Azarkovich & autumn temperatures are lower than this, germination in the open Gumilevskaya, 2012; Obroucheva & Lityagina, 2007; Pritchard is unlikely (Daws et al., 2004). However, Greek seeds can germi‐ et al., 1999; Steadman & Pritchard, 2004; Takos et al., 2008; nate in the field at 15–19°C, and thus, germination may occur in Tompsett & Pritchard, 1993). Stratification increases germination the autumn coinciding with autumn rain. This may give seedlings at temperatures from 6 to 36°C (Pritchard et al., 1999; Steadman an advantage in allowing establishment and growth before sum‐ & Pritchard, 2004). Germination at 2–6°C in the dark at constant mer drought the following year (Daws et al., 2004). Variation in the temperature is possible but takes up to 4 months, and total ger‐ temperature needed for germination appears to be linked to seed mination is not increased by stratification (Pritchard et al., 1999). (and particularly embryo) development. Farrant and Walters (1998) However, the minimum temperature at which germination will noted that in seeds collected in Colorado, USA, the dry mass of the occur was found to be reduced at a mean rate of 0.18°C/day during 1006 | Journal of Ecology THOMAS eT Al. stratification, with the reduction being fastest during stratification horse‐chestnut are given in Table 2. Pollen is used as food by hov‐ at 2°C and slowest at 16°C (Steadman & Pritchard, 2004). erflies (Diptera, Syrphidae) (Kugler, 1970), and it is known that Apis Dormancy can also be overcome by partial drying from 50% to mellifera L. (Hymenoptera, Apidae) harvests resins from horse‐chest‐ 32%–40% moisture content (Tompsett & Pritchard, 1998). The ef‐ nuts, but whether this is from the “sticky buds” or elsewhere is not fect of drying appeared to be interchangeable with stratification, stated (Wilson, 2014). Rotheray et al. (2001) visited 300 Scottish and it is likely that both aid the seed maturation process since the woodlands, recording 31 species of saproxylic Diptera on horse‐ effect of partial drying was only seen in the relatively immature shed chestnut, ranking the tree 11th out of the 22 tree species investi‐ seeds of Scottish and English origin. Without stratification, seeds gated, similar to Ulmus glabra (35) and Populus tremula (36), compared can also be induced to germinate by soaking in water from 48 hr fol‐ to 74 species on Betula pubescens. The Diptera on horse‐chestnut lowed by cutting away one‐third of the seed at the hilum without included the red‐listed species Systenus bipartitus (Loew) (Diptera, removing the seed coat (Bellini & Nin, 2005). Dolichopodidae) found only on horse‐chestnut and Phaonia exoleta (Meigen) (Diptera, Muscidae) also found on Acer pseudoplatanus and 8.5 | Seedling morphology Fagus sylvatica. The larvae of a number of rare saproxylic hoverflies (Diptera, Syrphidae) were found in Cambridgeshire on horse‐chest‐ Germination is hypogeal and usually is complete after 3–4 weeks nut in rot holes, including Myathropa florea (L.), Callicera spinolae (Bellini & Nin, 2005). Seedling morphology is shown in Figure 3. Rondani and Mallota cimbiciformis (Fallén), and in sap runs, includ‐ ing Brachyopa insensilis (Collin), B. scutellaris Robineau‐Desvoidy and 9 | H E R B I VO RY A N D D I S E A S E B. bicolor (Fallén) by Damant (2005). He suggested that these had 9.1 | Animal feeders or parasites entomologists because it is an introduced tree. not been found before since horse‐chestnut has been neglected by Anoplophora chinensis (Forster) (Coleoptera, Cerambycidae) will ovi‐ Deer and wild boar (Sus scrofa L.) are known to eat horse‐chest‐ posit on horse‐chestnut trunks but adult beetles do not feed on its twigs nut seeds (Bean, 1976; Bratton, 1974), but there are few other re‐ (Peverieri & Roversi, 2010); by contrast, horse‐chestnut is very suscep‐ cords of browsing or grazing. Insects that have been recorded on tible to damage by larvae and adults of A. glabripennis (Motschulsky) F I G U R E 3 Seedlings of Aesculus hippocastanum at (a) 1 week, (b) 2 weeks, (c) 4 weeks and (d) 8 weeks after germination. Drawings by Omar Alhamd Journal of Ecology THOMAS eT Al. | 1007 TA B L E 2 Invertebrates recorded from Aesculus hippocastanum in Britain. Nomenclature follows that of the Database of Insects and their Food Plants (DBIF, 2018) Species/classification Ecological notes Source Acari Eriophyidae Tegonotus carinatus (Nalepa) Larvae and adults; leaves; Aesculus only 1 Vasates hippocastani (Fockeu) Galling on leaves; Aesculus only 1 Eotetranychus pruni (Oudemans) Leaves; variety of deciduous trees 1 E. tiliarum (Hermann) Leaves; variety of deciduous trees 1 Also on Acer 1 Edwardsiana avellanae (Edwards, J.) Leaves; Also on Ulmus, Acer, Corylus 1 E. hippocastani (Edwards, J.) Variety of deciduous trees 1 E. lethierryi (Edwards, J.) Leaves; variety of deciduous trees 1 Tetranychidae Hemiptera Aphididae Periphyllus testudinaceus (Fernie) Cicadellidae Fagocyba cruenta (Herrich‐Schaeffer) Leaves; wide variety of deciduous trees 1 Alebra wahlbergi (Boheman) Leaves; wide variety of deciduous trees 1 Ossiannilssonola callosa (Then) Leaves; also on Alnus, Crataegus, Fagus, Tilia and Acer 1 Stems; wide variety of deciduous trees 1 Coccidae Eulecanium tiliae (L.) Parthenolecanium corni (Bouche) Very wide range of trees 1 P. rufulum (Cockerell) Also on Quercus, Castanea, Carpinus and shrubs 1 Pulvinaria betulae (L.) Variety of deciduous woody plants 1 P. regalis (Canard) Leaves, branches, trunk; variety of trees 1 Very wide range of trees and shrubs 1 Wide range of deciduous trees 1 Stem miner; wide range of trees and shrubs 1 Diaspididae Lepidosaphes ulmi (L.) Pseudococcidae Phenacoccus aceris (Signoret) Lepidoptera Cossidae Zeuzera pyrina (L.) Geometridae Alsophila aescularia (Denis & Schiffermuller) Leaves; variety of deciduous trees and shrubs 1 Campaea margaritata (L.) Larvae; variety of trees and shrubs 1 Ennomos fuscantaria (Haworth) Larvae; Fraxinus, Ligustrum 1 Lycia hirtaria (Clerck) Leaves; wide variety of trees and shrubs 1 Ourapteryx sambucaria (L.) Larvae; variety of deciduous trees and shrubs 1 Horse‐chestnut leaf miner 1 Leaves; wide range of trees 1 Gracillariidae Cameraria ohridella Deschka & Dimić Lymantriidae Lymantria monacha (L.) Noctuidae Lithophane hepatica (Clerck) Leaves; variety of deciduous trees and shrubs 1 Phalera bucephala (L.) Leaves; wide variety of trees 1 Tortricidae (Continues) 1008 | Journal of Ecology TA B L E 2 THOMAS eT Al. (Continued) Species/classification Ecological notes Source Cacoecimorpha pronubana (Hubner) Larvae; wide range of herbaceous plants and trees 1 Bark; Betula, Quercus 1 Leaf rolling; variety of trees 1 Mesites tardii (Curtis) Dead bark and wood; variety of trees and shrubs 1 Rhyncolus lignarius (Marsham) Wood; also on other woody plants 1 Dead wood; also on Populus, Fagus and Abies 1 Wood, roots; variety of deciduous trees 1 Yponomeutidae Argyresthia glaucinella Zeller Coleoptera Attelabidae Attelabus nitens (Scopoli) Curculionidae Eucnemidae Hylis olexai (Palm) Lucanidae Lucanus cervus (L.) Melandryidae Abdera quadrifasciata (Curtis) Dead wood; Fagus, Quercus, Carpinus Diptera Aulacigastridae Aulacogaster leucopeza (Meigen) Larvae in sap exudate 2 Larvae in sap exudate 2 Ceratopogonidae Dasyhelea flavifrons (Guérin) Culicidae Anopheles plumbeus Stephens In wet tree hollow 2 Finlaya geniculata (Olivier) In wet tree hollow 2 Orthopodomyia pulchripalpis (Rondani) In wet tree hollow 2 Systenus pallipes (von Roser) In wood pulp 2 S. scholtzi Loew In wood pulp 2 In wood pulp 2 Phaonia cincta Zett. In wood pulp 2 P. exoleta (Meigen) In wood pulp 2 Larvae in sap exudate 2 Dolichopodidae Limoniidae Rhipidia ctenophora Loew Muscidae Mycetobiidae Mycetobia pallipes Meigen 1. DBIF (2018); 2, Keilin (1927). (Ravazzi & Caudullo, 2016). Cebeci and Acer (2007) list insect pests described in 1985 by Deschka and Dimić (1986). It has since spread found on horse‐chestnut in Turkey, Milevoj (2004) in Slovenia, and rapidly through central and western Europe and into the Ukraine, Majzlan and Fedor (2003) in Slovakia. Details of fifteen spider species Belarus and western Russia (Avtzis & Avtzis, 2002; Gilbert et al., (Arachnida, Araneae) forming part of the food web on horse‐chestnut 2005; Gussev & Vulchev, 2015; Pirc, Dreo, & Jurc, 2018; Thalmann in Latvia are given by Petrova, Voitkane, Jankevica, and Cera (2013). et al., 2003; Tomiczek & Krehan, 1998; Weryszko‐Chmielewska & Haratym, 2011, 2012) at a rate of approximately 50–100 km per 9.1.1 | Horse‐chestnut leaf miner year (Šefrová & Laštůvka, 2001) or c. 3 km per generation (Gilbert, Grégoire, Freise, & Heitland, 2004). It was first seen in Britain in The horse‐chestnut leaf miner Cameraria ohridella Deschka & Dimić Wimbledon in 2002, and by the end of 2010, it was in most parts (Lepidoptera, Gracillariidae) is a leaf‐mining micro‐moth that was of England and Wales. It was originally thought to be of exotic ori‐ first observed in the late 1970s near Lake Ohrid, Macedonia, and gin (Grabenweger et al., 2005) but has been shown to be native on Journal of Ecology THOMAS eT Al. horse‐chestnut in the Balkans (Kenis et al., 2006; Kenis et al., 2005; | 1009 and at lower altitudes (<1,000 m), leaves had similar numbers of Lees et al., 2011; Valade et al., 2009) and that just a few lineages mines to “artificial” sites where horse‐chestnut has been intro‐ with limited genetic diversity moved to urban areas in the second duced (Walas et al., 2018). This, together with herbarium speci‐ half of the 20th century (Valade et al., 2009). Grabenweger et al. mens of branches with leaves having very few mines per leaf (Lees (2005) found C. ohridella to be abundant in natural stands of horse‐ et al., 2011), suggests that native horse‐chestnut populations chestnut in Greece and Bulgaria, confirmed by Walas et al. (2018) have been secondarily infected by a more invasive mitochondrial for some populations in Greece. Cameraria ohridella shows a prefer‐ DNA haplotype that has been observed in most artificial stands ence for A. hippocastanum and the closely related A. turbinata; lar‐ in Europe. val mortality is high on A. carnea (Freise, Heitland, & Sturm, 2003), Populations of the leaf miner readily establish in planted popula‐ and other Aesculus species are rarely predated (D'Costa, Koricheva, tions of horse‐chestnut (<2% of hatching larvae fail to establish a leaf Straw, & Simmonds, 2013; D'Costa, Simmonds, Straw, Castagneyrol, mine; Gilbert et al., 2004) and leaf damage can rise dramatically in & Koricheva, 2014; Ferracini, Curir, Dolci, Lanzotti, & Alma, 2010; the first 3 years of infestation (Pocock & Evans, 2014), reaching 200 Freise, Heitland, & Sturm, 2004; Straw & Tilbury, 2006) undoubtedly mines per leaf and removing up to 75% of total leaf area in southern due to the high saponin levels in the leaves of these other species England (Straw & Williams, 2013) and even causing complete defoli‐ (Ferracini et al., 2010). The moths will occasionally mine Acer pseu‐ ation (Thalmann et al., 2003) by shortening the life span of the leaves doplatanus and A. platanoides when horse‐chestnut is unavailable up to 30%, which fall from the tree beginning in July (von Skuhravý, or as opportunistic infestations near heavily infested horse‐chest‐ 1998). nuts (Krehan, 1995; Péré, Augustin, Turlings, & Kenis, 2010; Straw & Tilbury, 2006). A number of studies have shown that there is no loss of assimila‐ tion before early June in Italy and late June in Britain (Nardini et al., Female moths lay their eggs on the adaxial leaf surface (Weryszko‐ 2004; Percival, Barrow, Noviss, Keary, & Pennington, 2011) during Chmielewska & Haratym, 2012), and the larvae burrow in and feed which loss of leaf area is low (6%–11% leaf area loss in late July in on the palisade mesophyll leaving dead, dried lines of epidermis on Britain). Raimondo et al. (2003) found that photosynthesis in the both sides of the leaf (Weryszko‐Chmielewska & Haratym, 2011). green parts of mined leaves was as high as in intact leaves so subse‐ Most often three, but sometimes up to five, generations are pro‐ quent loss of assimilation was due to changes in leaf area only. Over duced per year (Šefrová & Laštůvka, 2001) with the last generation the whole of the growing season, the loss in assimilation is usually overwintering as pupae in the leaf litter, probably along with a few no more than 30%–37% even in years of heavy infestation (Nardini from previous generations, to produce the first generation the fol‐ et al., 2004; Percival et al., 2011; Straw & Williams, 2013). This is lowing year (Hněvsová, Kodrík, & Weyda, 2011; Samek, 2003; von because leaf area loss in the early part of the season is mainly in Skuhravý, 1998). The first generation feeds mostly in the lower the lower part of the crown where leaves are more shaded and less part of the crown, while subsequent generations feed mainly in the productive (Straw & Williams, 2013). Moreover, loss of leaf area pro‐ upper part of the crown (Krehan, 1995). Thus, Nardini et al. (2004) gressively develops through the summer and is maximal when the observed in north‐east Italy that in early May 2%–3% leaf area was photosynthetic efficiency of the leaves has decreased and they are lost at 2 and 6 m above ground; by the end of June, leaf area loss was contributing less to total seasonal assimilation (Nardini et al., 2004). 30% at 2 m, 18% at 6 m, 10% at 10 m and 8% at 14 m; and by the Loss of assimilation reduces the growth of a tree. However, in the end of August, it was 85% at 2 m, 75% at 6 m, 65% at 10 m, 55% at short‐term, infestation with the leaf miner leads to greater annual 14 m. Spatial analysis in the Czech Republic showed that damage be‐ wood production that is probably a reaction to maintain hydraulic tween sites was not related to the infestation of neighbouring areas, conductivity. Salleo et al. (2003) in north‐east Italy measured mean indicating that the distribution of C. ohridella is random (Kopačka ring widths of 1.35 mm in the 4 years before infestation, and of & Zemek, 2017). However, Gilbert, Svatoš, Lehmann, and Bacher 2.20 mm in the 4 years after, resulting in 62% more wood production (2003) found a positive correlation in Bern, Switzerland, between after attack. Wood grown in attacked trees had its water‐conducting infestation level on a tree and the number of other horse‐chestnut cross‐sectional area increased to 32% compared to 25% in control trees within 800 m distance, and in Brussels the proportion of green trees sprayed with insecticide. This extra conductivity was due to areas within 100 m and the number of other horse‐chestnut trees a higher proportion of larger vessels 30.1–40.0 μm diameter com‐ within 2,000 m. pared to the controls (18 and 9%, respectively) and the widest vessels In native Greek populations, infestation rates are apparently (>40 μm) which were only present in infested trees, forming c. 5% of low with a mean of 0.08 mines per leaflet recorded by Lees et al. the total or 40 vessels/mm2. In both infested and control trees, water (2011) and <3 mines per leaflet, covering <5.3% of leaf area re‐ was taken up through the outer two rings, so the higher xylem con‐ corded by Walas et al. (2018) although the latter data were col‐ ductivity would have helped compensate for defoliation in the lower lected in June–July before maximum infestation. The average part of the crown and overall shorter leaf life span by increasing water number of mines was negatively correlated with altitude, but and nutrient supply to less damaged leaves higher in the crown. there was no significant correlation between temperature, pre‐ One growing season after treating mature horse‐chestnuts cipitation and the number of mines (Walas et al., 2018). At the with insecticide to reduce leaf miner attack they had 33% higher highest sites investigated (1,239–1,463 m), mines were absent, root carbohydrate concentrations and 1,719% higher twig starch 1010 | Journal of Ecology THOMAS eT Al. content than untreated controls (Percival et al., 2011). In the long‐ Insecticide implants in trees, such as Acecap, have proved less use‐ term, however, infestation reduces tree vigour and growth (Bednarz ful (Krehan, 1997). Glue bands around the trunk to trap adult moths & Scheffler, 2008; Percival et al., 2011; Straw & Williams, 2013). In have proved effective if applied every year (Percival, 2016), but Kórnik, Poland, Jagiełło et al. (2018) found that trees injected with pheromone traps have not been found to reduce damage (Sukovata, insecticide (imidacloprid) were taller 10 years after treatment than Czokajlo, Kolk, Ślusarski, & Jabłoński, 2011). untreated controls (15 vs. 13 m, estimated from figure) and had Biocontrol is unlikely to be widely effective since natural rates higher mean DBH (28 vs. 22 cm) and higher basal area increment of parasitism of the leaf miner larvae are typically <6% of a pop‐ (125 vs. 80 cm2). But there is no compelling evidence that damage by ulation (Freise et al., 2004; Pocock & Evans, 2014; von Skuhravý, C. ohridella leads to long‐term health problems or tree death (Straw 1998) but can be up to 20% (Stojanović & Marković, 2004; Volter & & Bellett‐Travers, 2004). Kenis, 2006) especially in sun‐exposed trees (Tarwacki, Bystrowski, Flowering can be reduced by the leaf miner primarily by trees pro‐ & Celmer‐Warda, 2012). In Italy, the larvae were predated by the ducing a smaller number of functionally female flowers (Weryszko‐ ant Crematogaster scutellaris (Olivier) (Hymenoptera, Formicidae) Chmielewska et al., 2012; Franiel, Wożnica, & Orlik, 2014), although (Radeghieri, 2004). However, the adult moths can detect infection of there is no evidence that the number of seeds is reduced (Thalmann leaves by Erysiphe flexuosa and Guignardia aesculi (Section 9.3) under et al., 2003). However, the seeds from infested trees are 40%–50% laboratory conditions, which results in lower egg laying (Johne, smaller (Jagiełło et al., 2017; Nardini et al., 2004; Takos et al., 2008; Weissbecker, & Schütz, 2008) and may be useful for biocontrol in Thalmann et al., 2003), in Italy typically with a mass of 6 g compared the field. to 10–12 g in trees with well‐developed foliage (Salleo et al., 2003). The most effective control measure for urban trees is likely to be This effect is still clear in partially defoliated trees; Thalmann et al. removing fallen leaves from the ground where the pupae overwin‐ (2003) found that heavily infested trees in Munich (>75% leaf area ter (Kukuła‐Młynarczyk & Hurej, 2007; Pavan, Barro, Bernardinelli, lost) had smaller fruits (c. 6 g) and seeds (c.4.5 g) than did lightly in‐ Gambon, & Zandigiacomo, 2003). Adults will, however, disperse to‐ fested trees (<25%) with fruits of >9 g and seeds of >7 g. wards trees from areas where it is difficult to remove litter (Augustin Percival et al. (2011) found in South England that seed germina‐ et al., 2009; Kehrli & Bacher, 2003; Straw & Bellett‐Travers, 2004) tion was 47.6% higher in seeds from insecticide‐treated trees com‐ and local populations can rebuild (Baraniak, Walczak, Tryjanowski, pared to leaf miner infested trees, presumably because the seeds & Zduniak, 2004; Gilbert et al., 2005). Moving collected litter also were larger. However, in native populations in Greece, Takos et al. carries the risk of introducing the miner to new areas, so it is unlikely (2008) found the opposite that germination was significantly but to be a complete solution. The collected litter can be composted; marginally higher in seeds from infected trees (97.0%) compared experiments burying infected litter under 15 cm of uninfected foli‐ with insecticide‐treated trees (92.3%) and was 1 week quicker. This age or 10 cm of soil both reduced the emergence of C. ohridella by is likely due to seeds on infested trees having a longer post‐ripening 96% (Kehrli & Bacher, 2004). Composting litter with sewage sludge period after the trees are defoliated and so are more mature (Section producing temperatures >50°C for 7 days should lead to eradication 8.4) and ready to germinate upon falling. of the miner (Łowiński & Dach, 2006). Litter can be removed anytime Seedling survival in the first 2–3 years does not appear to be af‐ before spring with the same effect (Kehrli & Bacher, 2003). fected by the leaf miner (Raimondo, Trifilò, Salleo, & Nardini, 2005; Takos et al., 2008). Raimondo et al. (2005) noted that the leaf ex‐ pansion of 3‐year‐old seedlings was complete before the leaves 9.2 | Plant parasites were mined, so growth of infested seedlings was similar to that of The wood is readily decomposed and is noted for its ability to controls. However, Takos et al. (2008) saw that non‐infested seed‐ host spalting fungi, particularly Scytalidium cuboideum (Sacc., & lings grew taller (c. 27 cm) over the first 2 years, compared to in‐ Ellis) Sigler & Kang (=Arthrographis cuboidea (Sacc., & Ellis) Sigler; fested seedlings (c. 18 cm) and were larger (35.3 g total dry mass Ascomycota, Incertae sedis) and Ophiostoma piceae (Münch) Syd., non‐infested, 19.5 g infested). Raimondo et al. (2005) attributed & P. Syd. (Ascomycota, Ophiostomatales) (Robinson et al., 2011). slower growth in infested seedlings to lower leaf water conduc‐ A number of saprophytic fungi are known on horse‐chestnut, in‐ tance both in mined and in green areas of attacked leaves (control cluding Kretzschmaria deusta (Hoffm.) P.M.D. Martin (=Ustulina de‐ 160 mmol m−2 s−1, green parts of infected leaves 130 mmol m−2 s−1, usta (Fr .) Petrak; Ascomycota, Xylariales), Ganoderma australe (Fr.) mined areas 60 mmol m−2 s−1) and higher hydraulic resistance. Pat. (=G. adspersum (Schulz.) Donk; Basidiomycota, Polyporales), The larval stages can be controlled by spraying or injecting trees G. gibbosum (Blume & T. Nees) Pat. and G. resinaceum Boud. (Greig, with chitin synthesis inhibitors, such as diflubenzuron or triflumuron 2012; Guglielmo, Bergemann, Gonthier, Nicolotti, & Garbelotto, (Dzięgielewska & Kaup, 2008; Krehan, 1997; Nejmanová et al., 2007; Pearce, 1991). Schizophyllum commune Fr. (Basidiomycota, 2006). Insecticides such as imidacloprid, abamectin, acetamiprid Agaricales) is saprophytic on horse‐chestnut but can become para‐ and clothianidin have also proved effective (Burkhard et al., 2015; sitic causing white surface rot (Snieskiene et al., 2011). Increment Ferracini & Alma, 2008; Kobza, Juhásová, Adamčíková, & Onrušková, cores taken from horse‐chestnut resulted in extensive vertical dis‐ 2011; Kosibowicz & Skrzecz, 2010) as has injection of seed extracts coloration 10 years later with a mean distance 39 cm, compared to of the neem tree, Azadirachta indica A. Juss. (Pavela & Bárnet, 2005). Tilia platyphyllos 21 cm, T. cordata 16 cm and Betula pendula 155 cm Journal of Ecology THOMAS eT Al. | 1011 (Dujesiefken, Rhaesa, Eckstein, & Stobbe, 1999). Horse‐chestnut trees (Sweet & Barbara, 1979). The Strawberry latent ringspot virus is generally less good at compartmentalising decay after pruning (Group 4, Picornavirales) was detected in one of six trees with a leaf than, for example, Tilia spp. (Dujesiefken, Stobbe, & Eckstein, 1998). vein yellows disease (Sweet & Barbara, 1979). Horse‐chestnut leaves are sensitive to a number of powdery Fungi associated with horse‐chestnut, excluding those on soil or mildews such as the North American Erysiphe flexuosa (Peck) Barun litter below the trees, or those found solely on dead wood, are given & Takamatsu (Ascomycota, Erysiphales), introduced to Europe at in Table 4. Colletotrichum acutatum Simmonds and C. gloeosporioides the turn of the century (Ale‐Agha, Braun, Feige, & Jage, 2000; Kiss, (Ascomycota, Glomerellales) have been found on horse‐chestnut Vajna, & Fischl, 2004; Stankeviciene, Snieskiene, & Lugauskas, leaves in Norway (Talgø et al., 2012). 2010; Tozlu & Demirci, 2010; Zimmermannová‐Pastirčáková & Pastirčák, 2002) including Britain (Ing & Spooner, 2002). Erisiphe flexuosa also affects A. carnea (Irzykowska et al., 2013; Werner, 9.3.1 | Phytophthora Irzykowska, & Karolewski, 2012). It causes small, white patches on Since 1969, a number of Phytophthora spp. (Oomycota, Peronosporales) the leaves that then expand to cover both leaf surfaces. In Poznań, have been isolated from dead and dying roots and stems of horse‐ Poland, infection has been found to reach up to 50% of leaf area chestnut in England including P. megasperma var. megasperma (Irzykowska et al., 2013). Both young and old leaves are usually in‐ Drechsler, P. citricola Saw., P. cactorum (Leb., & Cohn.) Schroet. P. cin‐ fected. It causes more damage on vigorous trees and pruned trees namomi Rands and possibly P. cambivora Petri (Brasier & Strouts, 1976; with large sprouts (Snieskiene et al., 2011), but in many cases, in‐ British Mycological Society, 2018; Strouts & Winter, 2000). These are fected trees were found next to uninfected ones (Irzykowska et al., known to cause small‐scale bleeding cankers on horse‐chestnuts and 2013). Trees that are more resistant to Cameraria ohridella (Section other tree species, such as Tilia spp. Since 2001/2002, stem bleed‐ 9.1.1) are also more resistant to E. flexuosa (Werner et al., 2012). ing on A. hippocastanum has become more prevalent in Britain and Guignardia leaf blotch is found in Europe, North America and horse‐chestnut leaves have been seen to be moderately susceptible South Korea (Pastirčáková, Pastirčák, Celar, & Shin, 2009), includ‐ to P. ramorum (similar to Quercus spp., Castanea sativa and Taxus bac‐ ing Britain (Hudson, 1987), and affects various Aesculus species. cata) but more susceptible to P. kernoviae Brasier than most other The casual fungus, Guignardia aesculi (Peck) Stewart (=Phyllosticta woody plants (Brasier & Jung, 2006; Denman, Kirk, Whybrow, Orton, paviae Desm.; Ascomycota, Incertae sedis: conidial anamorph & Webber, 2006). Phytophthora obscura Grünw. & Werres, P. cactorum, Phyllosticta sphaeropsoidea Ellis & Everh., spermatial synanamorph P. citricola and P. syringae (Kleb.) Kleb. have been identified under dis‐ Leptodothiorella aesculicola Höhn.; Pastirčáková et al., 2009), pro‐ eased horse‐chestnuts with bleeding canker in Germany (Grünwald, duces reddish or dull brown necrotic areas with bright yellow bor‐ Werres, Goss, Taylor, & Fieland, 2012; von Werres, Richter, & Veser, ders in horse‐chestnut leaves. These “blotches” are usually at the 1995). tips or edges of leaves. The yellow border helps distinguish this from the leaf miner Cameraria ohridella (Section 9.1.1). Horse‐chestnut is moderately susceptible to Guignardia (Ćalić et al., 2013) but it seems to cause little significant damage. 9.3.2 | Chestnut bleeding canker Bleeding cankers on horse‐chestnut bark have become more com‐ Horse‐chestnut can carry many epiphytes. Of the 13 tree spe‐ mon this century, partly due to Phytophthora spp., as described cies investigated in central Switzerland by Ruoss (1999), horse‐ above. However, since 2001/2002, bleeding cankers have become chestnut carried the most lichen species (114), compared to Acer significantly more common throughout Britain, Ireland and western pseudoplatanus (93) and Fraxinus excelsior (52). Papp, Alegro, Šegota, mainland Europe (McEvoy et al., 2016). In the majority of cankers Šapić, and Vukelić (2013) list 11 bryophytes found on horse‐chest‐ sampled, the gram‐negative bacterium Pseudomonas syringae pv. aes‐ nut in Croatia, and Seaward and Letrouit‐Galinou (1991) list seven culi (Pae) has been identified as the cause (Webber et al., 2008). In species found on Paris trees. In Britain, this is somewhat lower; 2007, over 70% of horse‐chestnut trees surveyed in England, 42% Bates, Proctor, Preston, Hodgetts, and Perry (1997) found <1 spe‐ in Scotland and 36% in Wales had symptoms of bleeding canker cies of bryophyte per tetrad on horse‐chestnut along a transect (Forestry Commission, 2008). Similar rates of infection have been across southern England, and was ranked 20th out of a list of 52 seen in the Netherlands, with more urban trees infected than rural host taxa, although this result is undoubtedly partly due to the low trees (Webber et al., 2008). density of horse‐chestnuts in this area compared to native species. The disease is most prevalent in cool, wet climates of north‐west Horse‐chestnut is a rare host of the hemiparasite Viscum album. Europe (Kennelly, Cazorla, de Vicente, Ramos, & Sundin, 2007), and Slime moulds (Amoebozoa, Myxomycetes) associated with horse‐ the pathogen is thought to originate from the Himalayas where it in‐ chestnut are given in Table 3. fects leaves of A. indica. Isolates of Pae in Britain were found to be genetically virtually identical to each other and to isolates from the 9.3 | Plant diseases Netherlands and Belgium, so the epidemic in north‐west Europe is The apple mosaic virus (Group 4, Bromoviridae) has been observed into western Europe, possibly from India (Green et al., 2010; McEvoy to cause a severe yellow mosaic disease on leaves of horse‐chestnut et al., 2016). likely descended from a single, recent introduction of the pathogen 1012 | Journal of Ecology THOMAS eT Al. Arcyria denudata (L.) Wettst. Live bark A. pomiformis (Leers) Rostaf. Live bark Badhamia utricularis (Bull.) Berk. Dead bark Calomyxa metallica (Berk.) Nieuwl. Live bark TA B L E 3 Slime moulds (Amoebozoa, Myxomycetes) associated with Aesculus hippocastanum. Nomenclature follows the Fungal Records Database of Britain and Ireland (British Mycological Society, 2018) Ceratiomyxa fruticulosa (O.F. Müll.) T. Macbr. Comatricha nigra (Pers.) J. Schröt. Fallen branches Craterium minutum (Leers) Fr. Fallen leaves Didymium squamulosum (Alb., & Schwein.) Fr. Fallen twigs and leaves Echinostelium brooksii K.D. Whitney Live bark E. colliculosum K.D. Whitney & H.W. Keller Live bark E. fragile Nann.‐Bremek. Live bark Enerthenema papillatum (Pers.) Rostaf. Live bark, fallen branches Fuligo septica (L.) F.H. Wigg. Hemitrichia minor G. Lister Bark Licea bryophila Nann.‐Bremek. Live bark L. denudescens H.W. Keller & T.E. Brooks Live bark L. kleistobolus G.W. Martin Live bark L. marginata Nann.‐Bremek. Live bark L. minima Fr. Live bark L. operculata (Wingate) G.W. Martin Live bark L. parasitica (Zukal) G.W. Martin Live bark L. scyphoides T.E. Brooks & H.W. Keller Live bark Lycogala epidendrum (J.C. Buxb. ex L.) Fr. Fallen, rotting wood L. terrestre Fr. Stumps, fallen wood Macbrideola cornea (G. Lister & Cran) Alexop. Live bark Metatrichia floriformis (Schwein.) Nann.‐Bremek. Fallen wood Mucilago crustacea P. Micheli ex F.H. Wigg. Live and dead bark Paradiacheopsis cribrata Nann.‐Bremek. Live bark P. fimbriata (G. Lister & Cran) Hertel ex Nann.‐Bremek. Live bark P. solitaria (Nann.‐Bremek.) Nann.‐Bremek. Live bark Perichaena chrysosperma (Curr.) Lister Live bark P. depressa Lib. Fallen branches Physarum cinereum (Batsch) Pers. Dead bark Protostelium mycophagum L.S. Olive & Stoian. Live bark Reticularia lycoperdon Bull. Fallen wood Stemonitis ferruginea Ehrenb. Trichia persimilis P. Karst. Fallen branches T. scabra Rostaf. Fallen branches Tubulifera arachnoidea Jacq. Fallen trunk Symptoms are rust‐coloured or blackened liquid oozing from There may also be some local spread along phloem fibres lead‐ necrotic lesions (“bleeding cankers”) in the bark of the trunk up to ing to elongated necrotic areas (Bultreys, Gheysen, & Planchon, small diameter branches (Green, Laue, Fossdal, A'Hara, & Cottrell, 2008). Horse‐chestnut produces a number of antifungal and anti‐ 2009; Green, Laue, Steele, & Nowell, 2014; Webber et al., 2008). bacterial proteins (Ah‐AMP1 and β‐1,3‐glucanase and peroxidase) The infection penetrates the cambium and phloem, but there is as a plant defence (Fant, Vranken, & Borremans, 1999; Osborn little evidence of it penetrating the wood (Steele, Laue, MacAskill, et al., 1995). However, once within the host the bacterial cells Hendry, & Green, 2010) or of systemic spread through the vascu‐ persist within a mucoid gel which may help isolate them from lar tissue. It spreads primarily by lateral invasion of parenchymal outside stressors or toxins (e.g., Keith & Bender, 1999) and host cells, producing local infections that spread by 70–1,000 μm/day. defences. Journal of Ecology THOMAS eT Al. TA B L E 4 Fungi (by Order) directly associated with Aesculus hippocastanum not including those found on soil or litter below the trees, or those found solely on dead wood. Details of these can be found in the Fungal Records Database of Britain and Ireland (British Mycological Society, 2018). Nomenclature follows this database. Fungi that can be lichenised were identified from the British Isles List of Lichens and Lichenicolous Fungi (Natural History Museum, 2018) Species/classification Ecological notes Oomycota Bark Species/classification Botryosphaeriales Necrotic spots on leaves Capnodiales Ramularia endophylla Verkley & U. Braun Fallen leaves Septoria hippocastani Berk., & Broome Living and fallen leaves Diaporthales Pezicula cinnamomea (DC.) Sacc. Bark Pezizella discreta (P. Karst.) Dennis Petioles Pseudohelotium pineti (Batsch) Fuckel Fruits Rutstroemia sp. P. Karst. Old fruits Sclerotinia sp. Fuckel Fruits Hypocreales Leaves Volutella ciliata (Alb., & Schwein.) Fr. Live bark Camposporium pellucidum (Grove) S. Hughes Fruits Chalara aurea (Corda) S. Hughes Fruits C. cylindrosperma (Corda) S. Hughes Fruits C. rhynchophialis Nag Raj & W.B. Kendr. Fruits Haplariopsis fagicola Oudem. Fruits Polyscytalum fecundissimum Riess Fallen leaves Torula herbarum (Pers.) Link Fruits Necrotic spots on fading leaves Lecanorales Ophiognomonia setacea (Pers.) Sogonov Petioles, dead leaves Ostropales Erysiphales Leaves Ramalina fraxinea (L.) Ach. Lichen, bark Dimerella pineti (Schrad.) Vezda Lichen, live bark Phlyctis argena (Ach.) Flot. Lichen, live bark Pleosporales Eurotiales Penicillium sp. Link Fruits Fusidium griseum Ditmar Diaporthe coneglanensis Sacc., & Speg. Erysiphe flexuosa (Peck) U. Braun & S. Takam. Ecological notes Incertae sedis Ascomycota Guignardia aesculi (Peck) V.B. Stewart 1013 (Continued) Cylindrodendrum album Bonord. Peronosporales Globisporangium intermedium (de Bary) Uzuhashi, Tojo & Kakish. TA B L E 4 | Old fruits Helotiales Anguillospora longissima (Sacc., & P. Syd.) Ingold Bud scales Clavariopsis aquatica De Wild. Bud scales Peltigera membranacea (Ach.) Nyl. Lichen, live bark Botryotinia fuckeliana (de Bary) Whetzel Fallen fruits Botrytis cinerea Pers. Live leaves B. fascicularis (Corda) Sacc. Fruits Pertusaria leioplaca DC. Lichen, bark Calycellina lachnobrachya (Desm.) Baral Fruits P. pertusa (Weigel) Tuck. Lichen, bark Crocicreas subhyalinum (Rehm) S.E. Carp. Petioles, fruits Teloschitales Gibberella baccata (Wallr.) Sacc. Fruits Old fruits Amandinea punctata (Hoffm.) Coppins & Scheid. Lichen, live bark Hyaloscypha fuckelii var. fuckelii Nannf. Calicium viride Pers. Lichen, bark Pertusariales Hymenoscyphus albidus (Gillet) W. Phillips Decaying petioles H. albopunctus (Peck) Kuntze Dead leaves H. calyculus (Sowerby) W. Phillips Old fruits H. caudatus (P. Karst.) Dennis Fallen leaves, decaying petioles Massjukiella polycarpa (Hoffm.) S.Y. Kondr., Fedorenko, S. Stenroos, Kärnefelt, Elix, J.S. Hur & A. Thel Live bark Xanthoria parietina (L.) Th. Fr. Live bark, dead wood Xylariales H. fructigenus (Bull.) Fr. Old fruits Lachnum niveum (R. Hedw.) P. Karst. Falling leaves L. virgineum (Batsch) P. Karst. Falling leaves Lanzia echinophila (Bull.) Korf Old fruits Leptodontidium trabinellum (P. Karst.) Baral, Platas & R. Galán Fruits Niptera subbiatorina Rehm Fruits Annulohypoxylon multiforme (Fr.) Y.M. Ju, J.D. Rogers & H.M. Hsieh Bark, dead wood Basidiomycota Agaricales Armillaria gallica Marxm., & Romagn. (Continues) Roots, base of live trunk (Continues) 1014 | Journal of Ecology TA B L E 4 THOMAS eT Al. The disease can result in foliar discoloration and crown dieback (Continued) Species/classification Ecological notes A. mellea (Vahl) P. Kumm. Roots, base of live trunk A. ostoyae (Romagn.) Herink Stump Arrhenia acerosa (Fr.) Kühner Fallen petioles Atheliales when lesions girdle branches (Green et al., 2010), but possible re‐ sistance to the disease has been observed. For example, Pánková, Krejzar, Mertelík, and Kloudová (2015) found in the Czech Republic that 2–3 years after infection with Pae there was a natural resistance with 40% of trees appearing to be resistant, 40% tolerant (not appre‐ ciably affected by infection with small lesions) and 20% susceptible. Resistant and tolerant trees maintain healthy crowns, and disease Athelia epiphylla Pers. Leaves progression is slow or stops, and may even show signs of recovery Clitocybe candicans (Pers.) P. Kumm. Fallen, rotting fruit with new callus development around the edge of the cankers. Trees Fistulina hepatica (Schaeff.) With. Live trunk are known to have survived for a decade or more with half of their Flammulina velutipes (Curtis) Singer Live trunk, dead wood bark area affected. A study in southern England found that Pae was Hemimycena lactea (Pers.) Singer Live trunk, dead wood 27% of A. carnea, and surviving trees showed a decrease in growth Marasmius epiphyllus (Pers.) Fr. Fallen leaves sity by 4%–5% (Straw & Williams, 2013). The biggest threat appears Mycena olida Bres. Live trunk to be the cankers merging around the main trunk and girdling the Pleurotus dryinus (Pers.) P. Kumm. Live trunk, dead wood tree leading to death (Steele et al., 2010). Secondary agents such as P. ostreatus (Jacq.) P. Kumm. Live trunk, dead wood timate cause of death. Resupinatus applicatus (Batsch) Gray Fallen leaves over long distances at high altitude. Pae can survive for at least a R. trichotis (Pers.) Singer Fallen leaves year in soil even without host debris and can survive when stored Volvariella bombycina (Schaeff.) Singer Live trunk, dead wood in a nutrient solution for a year at −80°C, so is very robust (Laue, Atractiellales Phleogena faginea (Fr.) Link Phytophthora may also gain access to a diseased tree and be the ul‐ The spores of Pae are spread mainly in wind‐blown rain, possibly Steele, & Green, 2014). Although it is a bark pathogen, it has also can be artificially infected with it, but it is not known whether leaves play a role in spreading the disease (Mullett & Webber, 2013). The Wood of live trunk bacterial cells enter branches directly through leaf scars or branch axils and lenticels (Laue et al., 2014; Steele et al., 2010). Polyporales Abortiporus biennis (Bull.) Singer rate by 22% between 2003 and 2012, and a decline in crown den‐ been found within leaves of horse‐chestnut in Britain, and leaves Live trunk Boletales Serpula himantioides (Fr.) P. Karst. responsible for the death or removal of 11% of A. hippocastanum and Pae has been seen to affect trees of all ages. Koskella, Meaden, Live roots, trunk and dead wood Crowther, Leimu, and Metcalf (2017) found that both the leaf miner Fomes fomentarius (L.) J.J. Kickx Live trunk Cameraria ohridella and Pae are associated with taller, larger trees but Ganoderma applanatum (Pers.) Pat. Wood on live trunks Pae is also more prevalent on young fast‐growing horse‐chestnuts G. australe (Fr.) Pat. Live trunk while the leaf miner is most common on taller trees. This is undoubt‐ G. lucidum (Curtis) P. Karst. Live trunk G. resinaceum Boud. Live trunk, roots Laetiporus sulphureus (Bull.) Murrill Live trunk, dead wood Meripilus giganteus (Pers.) P. Karst. Live trunk, dead wood Perenniporia fraxinea (Bull.) Ryvarden Live trunk, dead wood defences as a result of leaf miner defoliation (Percival & Banks, 2014). Polyporus squamosus (Huds.) Fr. Live trunk, dead wood and silicon phosphites that act to prevent, and to a lesser extent re‐ Postia ptychogaster (F. Ludw.) Vesterh. Live trunk year‐old saplings at 39°C for 48 hr kills all Pae in wounds (de Keijzer, Rigidoporus ulmarius (Sowerby) Imazeki Live trunk, dead wood van den Broek, Ketelaar, & van Lammeren, 2012). Trametes ochracea (Pers.) Gilb., & Ryvarden Live branch, dead wood T. versicolor (L.) Pilát Live trunk, dead wood edly a spatial preference rather than a direct interaction between the two organisms at a landscape level. Leaf miner presence does not appear to be spatially linked to Pae symptoms, and it is unlikely that the leaf miner is a vector of Pae (Koskella et al., 2017). Within an individual tree, however, Pae canker size was positively correlated with leaf miner infestation, probably due to the suppression of tree Pae infections have been successfully treated with potassium duce the impact of, infection (Percival & Banks, 2015). Heating 4‐ 10 | H I S TO RY Based on molecular phylogenetic reconstruction of the genus and fossil evidence, Aesculus evolved in eastern Asia at the Cretaceous/ Journal of Ecology THOMAS eT Al. | 1015 Tertiary boundary c. 65 MYA (Xiang et al., 1998). From there, two the fashion of planting horse‐chestnut in avenues, most famously major lineages spread into Europe and North and Central America at Bushy Park, Hampton Court, and Loudon (1838) listed many via the Bering Land Bridge as an element of the boreotropical specimens that were then 80–100 years old. Horse‐chestnut flora (Hardin, 1960; Harris et al., 2009; Manchester, 2001; Xiang was introduced into the United States around 1828 but has been et al., 1998). Forest et al. (2001) suggested an American origin for largely surpassed by A. carnea (Leathart, 1991). John Gerard ap‐ Aesculus with a single migration to Eurasia, but this now appears preciated the tree, saying “[t]he Horse‐chestnut groweth likewise unlikely. The closely related Japanese A. turbinata and A. hip‐ to be a very great tree, spreading his great and large armes or pocastanum split apart 15.5 ± 1.93 MYA or earlier in the middle branches far abroad, by which meanes it maketh a very good coole Miocene (Xiang et al., 1998). Most of the Eurasian lineage was lost shadow” (Gerard, 1633). during the Miocene and Pliocene, but fossil evidence indicates that A. hippocastanum was widespread throughout Europe during the Miocene–Pliocene, when warm climatic conditions were op‐ 10.1 | Uses timal for this species (Mijarra, Manzaneque, & Morla, 2008) and The horse‐chestnut flower is the symbol of the city of Kiev in the it was distributed from North Africa and the Iberian Peninsula to Ukraine, and it is traditionally planted in Bavarian beer gardens northern Europe. (Loenhart, 2002). Further cultural connections include the Anne Pleistocene pollen records are mostly confined to the Frank Tree in Amsterdam which she mentions in her diary and Mediterranean Basin, from Barcelona through to Turkey and which sadly snapped in high wind in August 2010 (Gray‐Block, the Caucasus Mountains in the east (Mijarra et al., 2008). Large 2010). It is locally planted as a forestry tree in, for example, the amounts of Aesculus pollen (up to 15%) have been recorded in Czech Republic (Křivánek et al., 2006). The biggest use of horse‐ early Pleistocene sediments from Leffe, Italy (Ravazzi, 2003). chestnut, however, is in medicine, reflected in the voluminous Aesculus hippocastanum persisted in the Quaternary refugia of the medical literature in comparison with a comparatively small eco‐ Balkans, Italian and Iberian Peninsulas (Postigo‐Mijarra, Gómez‐ logical literature. Manzaneque, & Morla, 2008; Tsiroukis, 2008). By the end of the The common name may have come from the use of seeds Middle Pleistocene, A. hippocastanum was restricted to its current to treat horses for overexertion, colic and coughs by the Turks native distribution in the Balkan Peninsula (Grove & Rackham, and Ancient Greeks (Bombardelli et al., 1996; Vokou, Katradi, & 2001; Prada, Velloza, Toorop, & Pritchard, 2011; Xiang et al., 1998). Kokkini, 1993). Extracts of seeds, bark and leaves have long been Gobet et al. (2017) noted an increase in fruit and fodder tree pol‐ used in traditional and folk medicine (Tiffany et al., 2002). The len, including Aesculus, along with crop and weed pollen in Ukraine triterpenoid saponins extracted from the seed have been used around 6,500–6,000 BP suggesting the involvement of Aesculus in as a treatment for rheumatism, coughs, rectal complaints includ‐ Neolithic agriculture. ing haemorrhoids, bladder and gastrointestinal disorders, fever When first introduced into modern cultivation, its native origins (the first written account in 1720) and leg cramps (Anon, 2009; were unknown. Linnaeus (1753) suggested that it was native to the Küçükkurt et al., 2010; Sirtori, 2001; Zhang et al., 2010). In Bosnia northern regions of Asia, near the Himalayas, and North India was and Herzegovina, Redžić (2007) records that horse‐chestnut long regarded as its original home (Bean, 1976) and as late as 1837, “fruits” are still carried by people who suffer from rheumatism Loudon (1838) suggested North America. In 1795, John Hawkins and sciatica. almost certainly found natural stands in Greece, but these were Currently, extracts from horse‐chestnut seeds are widely used only confirmed by Theodor von Heldreich in 1879 (Lack, 2000, to treat peripheral vascular disorders including chronic venous in‐ 2002). sufficiency, haemorrhoids and post‐operative oedema (Dickson, As recently as 1945, Howard (1945) thought that horse‐chest‐ Gallagher, McIntyre, Suter, & Tan, 2004; Dudek‐Makuch & nut might have been introduced via Iran, northern India or Tibet. Studzińska‐Sroka, 2015; Facino, Carini, Stefani, Aldini, & Saibene, However, it is now believed that horse‐chestnut was introduced 1995; Gurel et al., 2013; Pittler & Ernst, 1998; Ruffini, Belcaro, to various parts of Europe by the Romans (Bradshaw, 2004) Cesarone, & Dugall, 2004; Suter, Bommer, & Rechner, 2006; and that European diplomats came across the horse‐chestnut Underland, Sæterdal, & Nilsen, 2012), as a preventative of dental in Constantinople and seeds were sent to Prague in 1557 (Lack, plaque and periodontitis in toothpaste (Aravind, Lakshmi, & Arun, 2000). These seeds were reportedly non‐viable, and it almost 2012; Kim et al., 2017), and to counter male infertility by improv‐ certainly reached western Europe from seedlings sent to Vienna, ing sperm quality (Fang et al., 2010). Saponin extracts have been again from Constantinople, in 1576 (Bean, 1976; Leathart, 1991). used to prevent colon cancer in rats (Szabadosova et al., 2013) It was first grown in central Europe, primarily Vienna, in the 16th and may also reduce growth of tumours in a number of cancers century before being spread throughout Europe (Lack, 2000). It in humans (Cheong et al., 2018; Geran, Greenberg, McDonald, reached France in 1615, and many avenues were lined with horse‐ Schumacher, & Abbott, 1972; Turkekul et al., 2018). Rat and mice chestnut trees (Loenhart, 2002). It was growing in Tradescant's models have been used to show that seed extracts also relieve di‐ Lambeth garden in 1633 and so probably arrived in England at the abetic nephropathy and thromboses (Ahmad et al., 2018; Elmas, same time as in France (Leathart, 1991). Evelyn (1664) mentioned Erbas, & Yigitturk, 2016), reduce ethanol absorption (Yoshikawa 1016 | Journal of Ecology THOMAS eT Al. et al., 1994), reduce cholesterol in mice fed a high‐fat diet (Avcı, The fruit shells of horse‐chestnut have shown promise as a bio‐ Küçükkurt, Akkol, & Yeşilada, 2010) and protect against bacte‐ sorbent of chromium and copper from aqueous solutions (Parlayıcı rial endotoxemic injuries in mice livers (Jiang et al., 2011). Seed & Pehlivan, 2015; Parus, 2018). Nanoparticles of ZnO, 50–100 nm extracts also have anti‐inflammatory and anti‐oedematous prop‐ diameter, have also been produced from the fruit shells (Çolak, erties (Dumitriu, Olariu, Nita, Zglimbe, & Rosoiu, 2013; Matsuda Karaköse, & Duman, 2017). Leaves heated to 450°C and so par‐ et al., 1997; Sirtori, 2001; Vasiliauskas, Leonavičienė, Vaitkienė, tially carbonised have been used in absorbing ions from sewage Bradūnaitė, & Lukšienė, 2010; Wilkinson & Brown, 1999) and so (Sapronova, Sverguzova, Sulim, Svyatchenko, & Chebotaeva, 2018). have proved effective at clearing skin conditions as an antiwrin‐ The wood is soft and comparatively weak and has found few kle treatment and in reducing skin ageing (Fujimura et al., 2007; uses but they include kitchen utensils and dishes, brush backs, toys, Masaki, Sakaki, Atsumi, & Sakurai, 1995) and may help against cel‐ prosthetic limbs and occasionally veneers (Bean, 1976; Mitchell, lulite and hair loss (Bellini & Nin, 2005). Extracts of leaves and the 1997). The wood does not burn particularly well, but its charcoal bark of young branches have also proved effective as an antioxi‐ has been used in making gunpowder (Leathart, 1991; von Maltitz, dant and anti‐inflammatory drug (Braga et al., 2012; Margină et al., 2003). In southern Europe, the wood has been used for fruit‐storing 2015) and one of the most effective plant extracts in inhibiting shelves as the porous nature absorbs moisture preserving the fruit Candida albicans (Tambur et al., 2018). (Howard, 1945). Perhaps not surprisingly, seeds have proved poisonous. Ingestion Horse‐chestnut gives good shelter as a street tree. Leuzinger, of seeds can cause anaphylaxis (Jaspersen‐Schib, Theus, Guirguis‐ Vogt, and Körner (2010) recorded that horse‐chestnut crown Oeschger, Gossweiler, & Meier‐Abt, 1996; Vega et al., 2012) and temperatures in Basel, Switzerland, were 1°C below ambient, there is a record of a 4‐year‐old boy who died after eating raw compared to 4°C above ambient in Acer platanoides. This has the horse‐chestnuts (Lampe & Fagerstrom, 1968). Edem, Kahyaoğlu, and effect of reducing the apparent temperature below the crown by Çakar (2016) also describe a person who developed pericarditis after 7.5–10.0°C (Kántor, Chen, & Gáld, 2018; Streiling & Matzarakis, consuming “3 boxes of horse‐chestnut paste” (no details given) over 2003). In Freiburg, Germany, NO x and O 3 levels were reduced the previous 1.5 months. Saponins are poorly absorbed in the gut by 45% (down to 19 μg/m 3 ) and 55% (down to 37 μg/m 3 ), re‐ and are largely destroyed by heating, so roasting horse‐chestnuts spectively, below horse‐chestnut trees (Streiling & Matzarakis, instead of sweet chestnuts (Castanea sativa) is unlikely to cause great 2003). harm. Saponins and tannins can be removed by boiling or leaching thin slices in running water for 205 days (Weiner, 1980), then grind‐ ing them to make a flour to be mixed with wheat or rye, or used as 11 | CO N S E RVATI O N pig food. The boiling and leaching remove many of the minerals and vitamins, leaving starch which is fairly edible (Mabey, 1972). Ground Despite the recent pests and diseases experienced by horse‐chest‐ seeds have been used as a coffee substitute (Loenhart, 2002) and nut (Sections 9.1 and 9.3), its distribution and abundance in Britain in combination with wheat flour as a strong glue in bookbinding do not appear to be declining, and it may even be expanding its range (Bainbridge, 1984). It is reported that giving horse‐chestnut seeds slightly. Preston et al. (2002) show a small but probably insignificant to cows in moderation improves the yield and flavour of the milk increase (+1.08) in distribution between 1930–1960 and 1987–1999, (Loenhart, 2002). and Braithwaite, Ellis, and Preston (2006) show a similar small in‐ The best known use of whole seeds is in playing the sadly de‐ crease in British distribution (+38 tetrads) between 1987–1988 clining children's game of conkers or “conquerors” (Bean, 1976). and 2003–2004, similar to Prunus padus and Ligustrum ovalifolium. Traditionally, conkers have also been used to repel spiders but this Preston et al. (2002) note that horse‐chestnut was better recorded was convincingly disproved by Roselyon Primary School, Cornwall, in south‐west England, Wales and Ireland between 1962 and 2002, using choice experiments (Anon, 2010). Bainbridge (1984) records which may account for some of the increase. that children were used to collect conkers in WWI as a source of The native range of horse‐chestnut (Section 1) is estimated to tannin for leather, and a bleaching agent for flax, hemp, silk and wool. currently be 163,642 km2, c. 25% of the Balkan Peninsula (Allen & Tannin and dye can also be obtained from the bark and fruit shells Khela, 2017), with perhaps as many as 10,000 mature trees (Allen (Zemanek, Zemanek, Harmata, Madeja, & Klepacki, 2009). Saponins & Khela, 2017). However, despite its abundance as a planted orna‐ can be extracted in hot water and used as a substitute for soap. An mental tree, it appears to be declining in its native range, despite infusion of horse‐chestnut seeds can be used to bring worms to the ability to regenerate freely (Section 5.2). The Greek popula‐ the soil surface (Loenhart, 2002). Bainbridge (1984) also records tions have satisfactory regeneration in only 6% of the areas and accounts of conkers being used in the manufacture of TNT and no regeneration in 62% (Avtzis et al., 2007; Tsiroukis, 2008). It also fermentation to produce acetone. More recently, starch from the appears to be declining in Albania, where the population is smaller seeds has been used to make biodegradable thermoplastic (Castaño than c. 500 individuals, and in Macedonia where the population is et al., 2014) and the ground fruit shells used as a filler in polymers to probably <100 individuals (Allen & Khela, 2017; Peçi et al., 2012). make them more biodegradable (Barczewski, Matykiewicz, Krygier, Bulgarian populations are also small and limited in area (Evstatieva, Andrzejewski, & Skórczewska, 2018). 2011). THOMAS eT Al. The main threats to many of the native populations of horse‐ chestnut are undoubtedly the leaf miner moth Cameraria ohridella which impairs reproduction (Section 9.1.1) and its limited ability to disperse to new areas (Section 8.3). Other threats include defor‐ estation and forestry, firewood collection, forest fires, increasing water demand for irrigation, mining, overgrazing, tourism develop‐ ment and pollution, and population fragmentation affecting spread and microclimate (Allen & Khela, 2017; Evstatieva, 2011; Gussev & Vulchev, 2015; Laraus, 2004). Mountain tourism, ski facilities and road construction are also degrading large mountain forest ecosystems within its native range although the Pindos Mountains still host significant old‐growth forest stands on inaccessible high mountain slopes and canyons (WWF, 2013). Collecting seeds for herbal medicine and larger‐scale pharmaceutical use is also taking its toll on population regeneration (Tsiroukis, 2008; WWF, 2013). The potential effects of climate change on horse‐chestnut have been largely unstudied (Section 7.1), but it is considered that al‐ though horse‐chestnut is regarded as sensitive to environmental changes (Łukasiewicz, 2003; Łukasiewicz & Oleksyn, 2007), the effects of current scenarios on population size and distribution will be minor (Walas et al., 2018). As a consequence of the decline, horse‐chestnut in its native range is classified as Vulnerable or Near Threatened using IUCN criteria (Allen & Khela, 2017) and is considered as Endangered or Critically Endangered in Bulgaria and Albania (Evstatieva, 2011; Gussev & Vulchev, 2015). Horse‐chestnut is known to occur in protected areas in Albania, Greece and Bulgaria, including national parks/reserves and Natura 2000 sites (Allen & Khela, 2017), but this includes relatively few of the natural populations, particularly in Greece (Avtzis et al., 2007). Moreover, in Greece horse‐chestnut is in‐ cluded in the national list of protected species of the Presidential Decree 67/1981 (Walas et al., 2018), but is not in the Red List of Greece (Phoitos, Konstantinidis, & Kamari, 2009). There are thus concerns about the long‐term future of the small number of native populations. AC K N OW L E D G E M E N T S The Iraqi Ministry of Higher Education and Scientific Research is thanked for the support of Omar Alhamd. Harvard University is gratefully acknowledged for the provision of a Fellowship in Harvard Forest to Peter Thomas and access to the resources that this brings. We are very grateful to Sarah Green, Marian Giertych, Radosław Jagiełło and Jerzy Zieliński for their unstinting sharing of information/help. REFERENCES Abudayeh, Z. H. M., Al Azzam, K. M., Naddaf, A., Karpiuk, U. V., & Kislichenko, V. S. (2015). 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