Lichens and higher plants on stone: a review - AseanBiodiversity.info

Abstract 

<strong>Lichens</strong> <strong>and</strong> <strong>higher</strong> <strong>plants</strong> on stone: a review 

Marcello Lisci a , Michela Monte b , Ettore Pacini a;∗;1 

a Department of Environmental Sciences, Botany Division, Via Mattioli 4, 53100 Siena, Italy 

b National Research Council, Centre for Conservation of Works of Art, Via Monte d’Oro 28, 00186 Rome, Italy 

This review article deals with the site <strong>and</strong> colonization mode of <strong>plants</strong> <strong>and</strong> lichens on the wall of historical buildings, freezes <strong>and</strong> 

statues. Higher <strong>plants</strong> <strong>and</strong> lichen colonization is essentially conditioned by the adaptability of the species <strong>and</strong> the e ciency of their mode 

of reproduction. A list of the more common <strong>plants</strong> <strong>and</strong> lichens living on historical building is given as well as the type of damage. Nine 

niche types are distinguished for <strong>plants</strong>. The chemical <strong>and</strong> mechanical e ects of plant colonization of walls, <strong>and</strong> the methods of conserving 

the aesthetic <strong>and</strong> functional integrity of the walls, are also presented <strong>and</strong> discussed. 

Keywords: Biodegradation; Wall <strong>plants</strong>; <strong>Lichens</strong>; Water availability; Colonization; Exposure; Substrate; Damage 

1. Introduction 

One of the principal stages in the evolution of civilization 

has been the construction of buildings <strong>and</strong> their decoration 

with sculptures. The types of stone <strong>and</strong> clay found on the site 

of towns determined the types of material used for building 

<strong>and</strong> the development of the arts of construction <strong>and</strong> sculpture. 

In a town like Siena, in which the buildings of artistic 

interest are mainly Medieval or Renaissance, the main materials 

come from sites within a radius of 30 km (Giamello 

et al., 1992). In ancient Rome, because of its political 

<strong>and</strong> economic importance, some of the materials used for 

buildings in the imperial epoch were transported from great 

distances (Adam, 1988). On archaeological ruins <strong>and</strong> 

even relatively recent walls <strong>and</strong> buildings (Plate 2a), various 

types of <strong>plants</strong> <strong>and</strong> related organisms have become 

established in di erent historical periods (Warsheid <strong>and</strong> 

Braams, 2000). Bacteria <strong>and</strong> fungi, for instance, cause 

alterations (e.g. loss of cohesion, stains) the biological 

matrix of which cannot be identi ed by direct observation. 

Algae <strong>and</strong> lichens, on the other h<strong>and</strong>, manifest as 

green or brightly colored lms <strong>and</strong> crusts, <strong>and</strong> are indicative 

of certain environmental conditions. Mosses are 

green <strong>and</strong> evident in the rainy seasons <strong>and</strong> vascular <strong>plants</strong> 

∗ Corresponding 

232-860. 

author. Tel.: +39-577-232-863; fax: +39-577- 

E-mail address: pacini@unisi.it (E. Pacini). 

1 Illustrator of the gures. 

0964-8305/02/$ - see front matter ? 2002 Elsevier Science Ltd. All rights reserved. 

PII: S 0964-8305(02)00071-9 

withroots, stem <strong>and</strong> leaves may also colonize stone monuments 

(Fig. 1). Plants attract attention <strong>and</strong> give ruins a picturesque 

appearance, <strong>and</strong> have often been depicted by artists 

(Pacini, 1994). The <strong>plants</strong> that grow on buildings <strong>and</strong> walls 

are generally found in the surrounding environment. Centranthus 

ruber grows on the edge of the crater of Vesuvius 

(Naples) <strong>and</strong> a few hundred meters lower down in the ruins 

of Pompeii. 

On allochthonous substrates, on the other h<strong>and</strong>, an interesting 

lichen ora that includes species rare or absent in the 

area, may become established. When the growth of lichens 

does not severely prejudice the conservation of a building, 

in our opinion it should be regarded as enriching the cultural 

value of the monument, <strong>and</strong> adding historical <strong>and</strong> artistic 

interest. 

The chemical composition <strong>and</strong> texture of the materials 

determine the resistance of a building to atmospheric agents 

<strong>and</strong> colonization by forms of life. Certain marbles found at 

ancient Ostia (Rome) are still in perfect condition <strong>and</strong> do 

not host lichens or other <strong>plants</strong>. On the other h<strong>and</strong>, where 

restoration has been performed, with the use of modern materials, 

they are completely covered by lichens (Plate 1a). 

Plants are never indi erent to their substrate: sooner 

or later, they all cause damage, whether due to the acid 

metabolites they produce, or because their roots penetrate the 

building material or develop in spaces between stones. Our 

experience on this topic regards archaeological areas in 

Latium (Nimis et al., 1987; Monte, 1991b) <strong>and</strong> Sardinia 

(Tretiachet al., 1991) as far as lichens are concerned, <strong>and</strong>

Fig. 1. Block diagram of the fate of a building in relation to human <strong>and</strong> 

environmental intervention. 

several towns in the south of France, Italy <strong>and</strong> Tunisia as far 

as vascular <strong>plants</strong> are concerned (Lisci <strong>and</strong> Pacini, 1993a, 

b). Since these sites range around the Mediterranean, they 

re ect the situation of much of the Mediterranean Basin 

<strong>and</strong> the monuments of forty centuries of history. 

Since lichens <strong>and</strong> vascular <strong>plants</strong> establish in di erent 

ways, cause di erent types of damage <strong>and</strong> require di erent 

methods of control, they are dealt with separately in the 

pages that follow. 

2. <strong>Lichens</strong> 

2.1. Types of lichens: structure <strong>and</strong> distribution 

<strong>Lichens</strong> are fungi (Ascomycetes or seldom Basidiomycetes) 

that live in symbiosis with photosynthesizing 

organisms (cyanobacteria or green algae) (Hale, 1974). 

The partners of the symbiosis, the fungus (mycobiont) <strong>and</strong> 

the alga (phycobiont), give rise to a simple structure known 

as a thallus. The lichen thallus grows where the alga <strong>and</strong> 

fungus, alone, could not survive. This is why lichens colonize 

a large variety of substrates: trees (epiphytes), the 

ground (terricolous lichens), stone (epilithic lichens) <strong>and</strong> 

even glass. 

On stone objects, the pH of the substrate performs the 

rst selection of lichen ora. Calcicolous species develop 

on neutral <strong>and</strong> alkaline substrates <strong>and</strong> silicicolous species on 

acid substrates. The forms of growth of thalli that colonize 

stone (Fig. 2) may be: 

• crust-like: the thallus adheres closely to the surface <strong>and</strong> 

penetrates it, forming a kind of crust; 

• foliose: the thallus penetrates the substrate only with its 

rhizines (thread-like anchorage devices) <strong>and</strong> can easily 

be removed from the surface of the stone; 

• fruticose: the thallus develops in three dimensions, taking 

on a hanging, rami ed form, generally anchored to the 

substrate by a type of button; 

• endolithic: these only occur on calcareous stone: the thallus 

is completely immersed in the substrate <strong>and</strong> is di cult 

to distinguish, being white in color. It is generally noticed 

when it reproduces because the fruiting bodies emerge 

from the stone, leaving behind small pits. 

Lichen reproduction occurs by germination of spores or 

multiplication of the cells of the soredia <strong>and</strong> isidia. Spores 

arising from fruiting bodies (Fig. 3c) are formed by sexual 

reproduction of the fungal partner only. Dispersed in the 

environment, they reconstitute a symbiosis if they encounter 

a suitable phycobiont. The soredia, balls of fungal laments 

around an algal cell (Fig. 3a), <strong>and</strong> isidia, appendages of the 

upper cortex of the thallus that envelop several algae (Fig. 

3b), are structures of vegetative reproduction in which both 

partners of the symbiosis take part. 

The reproductive structures are dispersed in the atmosphere 

by wind or transported by birds <strong>and</strong> insects. If they 

fall into cracks, pores or cavities that retain water, new 

growthmay arise (Garty, 1992). 

It is not a coincidence that di erent types of lichens 

grow on monuments. Eachspecies can only live in a de - 

nite range of pH, humidity, luminosity <strong>and</strong> nitrogen supply; 

some species can only live in a very narrow range of values 

(stenoic) whereas other species grow over a wider interval 

of conditions (euroic). 

Wirth (1980) devised indices of pH, hygrophytism, 

nitrophytism <strong>and</strong> photophytism on the basis of detailed 

knowledge of the ecology of the various species <strong>and</strong> their 

relationships. These indices can be used to characterize 

the pH range of substrates, their humidity, solar radiation 

<strong>and</strong> eutrophication on the basis of the species identi ed 

(Monte, 1991a). This information is useful for establishing 

existing gradients of humidity values <strong>and</strong> solar radiation so 

that alterations can be interpreted (Monte, 1991b; Corball, 

et al., 2001). 

<strong>Lichens</strong> behave as natural sensors of atmospheric pollution 

because the symbiosis between fungus <strong>and</strong> alga has its 

weak points. Highpollution, particularly by sulfur dioxide, 

damages the lichen thallus, rst leading to retarded growth 

<strong>and</strong> then to death. The number of species tends to decrease 

drastically from the periphery to the center of urbanized areas, 

with a decrease in the number of species <strong>and</strong> of the 

surface area colonized (Seaward, 1976).

Plate 1. (a) Statue in archaeological site in ancient Ostia. The lichens only grow where restoration has been performed. (b) Orvieto Cathedral with 

lichens growing along paths of water run-o <strong>and</strong> obscuring the black <strong>and</strong> white b<strong>and</strong>s. (c) Column of north portal of Orvieto Cathedral. The lichen 

Dirina massiliensis causes pitting that is visible when the thalli drop o . (d) Typical Sardinian “nuraghe”. Exposure plays a fundamental role in lichen 

colonization. Note also <strong>plants</strong> growing between stones. 

2.2. <strong>Lichens</strong> as agents of damage 

In the past, the role of lichens in the break-down of 

stone was muchdiscussed. They were regarded as primary 

colonizers <strong>and</strong> believed to precede any other form of life. 

They were thought capable of colonizing surfaces that 

had not undergone any previous alteration, whether 

chemical or physical.

Fig. 2. Di erent forms of lichen growth on stone: (a) crust-like lichen; 

(b) foliose lichen; (c) fruticose lichen; (d) endolithic lichen. 

According to Savoye <strong>and</strong> Lallemant (1980), however, 

lichens establish on a substrate after it has been partly transformed 

by substances in the air <strong>and</strong> then by bacteria. This 

hypothesis seems veri ed by the fact that lichens grow more 

readily on archaeological ruins on which mould facilitates 

bacterial growth <strong>and</strong> on stone surfaces that have become 

porous. 

Although the role of primary colonizer seems doubtful, 

lichens, especially the crust-like <strong>and</strong> endolithic forms, nevertheless 

transform stone substrates, causing de-cohesion 

<strong>and</strong> biocorrosion. Certain alterations were demonstrated by 

Gehrmann et al. (1988) to be typical of crust-like lichens. 

Mechanical damage is due, in the rst place, to penetration 

of the substrate by the hyphae (Fig. 2). The depth to which 

the thallus penetrates depends on the lichen species <strong>and</strong> the 

nature of the substrate. A study by Syers (1964) showed that 

penetration may range from a minimum of 0:3 mm to a maximum 

of 16 mm. Loss of cohesion may also occur as a result 

of the expansion <strong>and</strong> contraction of the thallus as it undergoes 

uctuations in water supply (Fry, 1924; 1927). <strong>Lichens</strong> 

may also damage a substrate by means of acid substances 

suchas carbon dioxide, lichenic acids <strong>and</strong> oxalic acid. Carbon 

dioxide, produced by respiration of the thallus, dissolves 

calcareous rocks in the presence of moisture, leading to the 

formation of soluble bicarbonates that may be washed away 

or cause encrustation. This type of alteration is characteristic 

of endolithic lichens (Syers <strong>and</strong> Isk<strong>and</strong>er, 1973). A large 

variety of substances produced by the lichen symbiosis are 

grouped under the name of lichenic acids. Shatz (1963) <strong>and</strong> 

later Isk<strong>and</strong>er <strong>and</strong> Syers (1972) showed that some of these 

substances are chelating agents <strong>and</strong> in aqueous suspensions 

of basalt, granite <strong>and</strong> biotite, form metal complexes of low 

solubility. Ascaso et al. (1976) detected mineral complexes 

that were not present on the naked rock surface, at the 

lichen-substrate interface. Fragments of thallus incubated in 

suspensions of the same pulverized rock formed minerals 

similar to those found at the interface. Further con rmation 

of the capacity of lichenic acids to produce alteration was 

provided by Galvan et al. (1981) <strong>and</strong> this enabled a series 

of minerals associated with the presence of lichens to be 

identi ed. 

It has long been known that lichens produce oxalic acid 

(Bracconot, 1825). Unlike lichenic acids, oxalic acid is not 

an exclusive product of lichens. It is also found in fungi as 

a nal product (Chiari et al., 1989) <strong>and</strong> is common in all 

living organisms as an intermediate of the Krebs cycle. It 

reacts withsubstrate minerals to form various oxalates, according 

to the cations available <strong>and</strong> the state of hydration 

of the rock (Corball et al., 2001). Calcium oxalate is by 

far the most common of these; usually it crystallizes as a 

monohydrate (whewellite), but may also occur in the dihydrated 

form (weddelite) (Jones et al., 1980). Due to their low 

solubility, oxalates tend to accumulate inside the thallus (Enderman 

et al., 1977), or at the lichen-substrate interface (Ascaso 

et al., 1976), sometimes forming a crystalline layer on 

the upper surface of the thallus (Jackson, 1981). Del Monte 

<strong>and</strong> Sabbioni (1987) suggested that oxalate patinas on ancient 

monuments (Guidobaldi et al., 1984) are the result of 

the metabolic activity of lichens of the past that have disappeared 

from towns due to pollution. This hypothesis caused 

much controversy <strong>and</strong> stimulated research which, far from 

settling the dispute about the origin of the patinas, nevertheless 

revealed that they can protect the underlying stone by 

virtue of their low solubility (Aless<strong>and</strong>rini, 1989). 

2.3. Recognition of biodeterioration of a material 

As for <strong>plants</strong>, the identi cation of lichens is based on 

the diagnostic characters of the di erent species. The main 

lichen characters used are form of growth, color, thallus 

size, surface structures (isidia, soredia <strong>and</strong> so forth), fruiting 

bodies, number <strong>and</strong> form of spores, <strong>and</strong> speci c substances 

in the thallus. Identi cation is generally performed in 

situ. When microscopic observation is necessary, fragments 

of thallus <strong>and</strong> fruiting bodies are removed with a scalpel 

without damaging the rock surface. In the laboratory, special 

reagents are used that give a color reaction with speci c 

lichen chemicals. 

To study the distribution of lichens over vast areas, phytosociological 

surveys can be performed according to the 

method of Braun-Blanquet (1964). Each survey consists of 

a list of species identi ed in a de ned area. Eachspecies is 

assigned a substrate cover value in a scale of several classes. 

The type of substrate, its inclination <strong>and</strong> aspect are noted for 

eachsurvey. The data is classi ed <strong>and</strong> ordered to identify 

clusters <strong>and</strong> relationships between groups by the method of 

Wildi <strong>and</strong> Orloci (1988). The ecological characterization of 

the various groups can be performed by associating the indices 

of pH, nitrophytism, hygrophytism <strong>and</strong> photophytism 

proposed by Wirth(1980). 

Deterioration can be observed at the rock-lichen interface 

by combined methods such as the study of translucent or

Fig. 3. Di erent structures of vegetative (a,b) <strong>and</strong> sexual (c) reproduction of lichens: (a) soredium; (b) isidium; (c) spores. 

thin sections, X-ray di raction <strong>and</strong> scanning electron microscope 

(SEM) observation using a microprobe. Salvadori <strong>and</strong> 

Lazzarini (1991) demonstrated oxalates by polarization microscope 

observation of thin sections. Modenesi <strong>and</strong> Lajolo 

(1988) proposed various preparation techniques for samples 

of marble colonized by Aspicilia contorta for polarization 

microscope <strong>and</strong> SEM observation; these methods involved 

removing calcium carbonate by placing the samples in 

dilute solutions of hydrochloric or acetic acid. 

2.4. Organisms involved <strong>and</strong> examples of biological attack 

Like other species of animals <strong>and</strong> <strong>plants</strong>, lichens have a 

geographic distribution, the range of which depends on the 

adaptability of eachsymbiotic organism. The species involved 

therefore vary from place to place in relation to climate 

<strong>and</strong> other environmental factors. Here we give some 

examples of lichens growing on monuments, their e ect 

on the substrate <strong>and</strong> the environmental characteristics that 

in uence colonization. 

Orvieto Cathedral is extensively colonized by lichens 

around the tympanum of the main facade (Nimis <strong>and</strong> 

Monte, 1988). One reason for lichen growth is enrichment 

of the substrate by the excrement of birds that roost <strong>higher</strong> 

up. Nitrates are washed over the stone by rain. On the north 

side, moreover, moister conditions due to condensation 

have favored lichen colonization of vast areas, even by nonnitrophiles. 

Unaesthetic color changes on the dark basalt 

b<strong>and</strong>s of the cathedral are due to the growth of pale-colored 

lichens (Haematomma ochroleucum var. porphyrium <strong>and</strong> 

Tephromela atra) (Plate 1b). Seaward (1988) showed the 

di erent roles played by various lichen species in the deterioration 

of stone monuments in Rome. The sporadic growth 

of Lecanora muralis has caused mechanical decay of the 

substrate. Species suchas Lecanora dispersa, C<strong>and</strong>elariella 

vitellina <strong>and</strong> Lecidea fuscoatra that also have signi cant 

cover, do not cause evident modi cation of the substrate. 

Nimis <strong>and</strong> Zappa (1988) studied the colonization of 

stone by endolithic calcicolous lichens such as Bagliet- 

toa parmigerella, Bagliettoa parmigera <strong>and</strong> Caloplaca 

ochracea <strong>and</strong> showed that they cause dissolution. 

In a comparative study of lichen ora growing on the 

buildings of Salento, Seaward et al. (1989) observed that the 

species were di erent from those found in the surrounding 

natural environment. In this case, factors associated with 

human activities were more important than natural ones in 

determining lichen vegetation. 

Nimis et al. (1987) in Latium <strong>and</strong> Tretiachet al. (1991) 

in Sardinia identi ed about 300 di erent species on archaeological 

remains. On close, microcrystalline calcareous rock 

exposed to the sun, the main species were Caloplaca aurantia, 

Caloplaca avescens, Aspicilia calcarea, Aspicilia 

radiosa <strong>and</strong> Verrucaria nigrescens. On porous calcareous 

rock exposed to the north, in shaded positions or near the 

soil, the main species were Bagliettoa parmigera, Placintium 

nigrum <strong>and</strong> Caloplaca ochracea. On vertical brick 

walls of the Capitolium (ancient Ostia) exposed to the 

north, Dirina massiliensis, Dirina stenhammari, Lecidella 

carpathica, Roccella phycopsis prevailed. The cover of 

the latter was greater several meters above ground level 

where the substrate is exposed to moist winds from the sea. 

Dirina massiliensis was also found on the marble portal 

of the Cathedral of Orvieto, where it is causing corrosion 

(Plate 1c). Seaward <strong>and</strong> Giacobini (1989) have con rmed 

the corrosive e ect of Dirina massiliensis growing on frescoes 

of Palazzo Farnese at Caprarola. On granite, trachyte, 

basalt <strong>and</strong> tu “nuraghi” of NW Sardinia (Plate 1d), the 

distribution <strong>and</strong> reproductive <strong>and</strong> propagation strategies of 

the species are in uenced by solar radiation <strong>and</strong> humidity 

(Monte, 1993). Fig. 4 summarizes the disposition of different 

lichen communities on a typical “nuraghe” in relation 

to exposure. The characteristic species of each community 

are indicated. 

Deruelle et al. (1979) examined lichens on the Basilica 

of Notre-Dame de l’Epine, in the country 8 km from 

Chalons-sur-Marne. On the west wall, unsightly populations 

of green <strong>and</strong> orange lichens had become established. 

Identi cation of the nitrophiles C<strong>and</strong>elariella medians,

Fig. 4. Distribution of 12 lichen communities on acid rock of a Sardinian “nuraghe”. A: upper part of the nuraghe; B: lower part of the nuraghe. 

Phaeophyscia orbicularis, Xanthoria c<strong>and</strong>elaria <strong>and</strong> Caloplaca 

citrina showed that nutrient enrichment of the stone 

substrate had caused lichen growth. The nitrates were attributed 

to fertilizers dusted on crops in the surrounding elds. 

On the tower of the Cathedral of Seville, la Giralda, calcicolous 

species (almost all crust-like <strong>and</strong> epilithic) have 

been identi ed. Their growthis associated withbird excrement. 

These species usually live on deteriorated calcareous 

substrates (Saiz-Jimenez, 1981). 

On the pavement mosaics of the Romanesque town of 

Italica (Seville), the tesserae <strong>and</strong> mortar were attacked by 

Aspicilia radiosa, Aspicilia ho mannii <strong>and</strong> several species 

of the genus Caloplaca (Garcia Rowe <strong>and</strong> Saiz-Jimenez, 

1988). 

2.5. Methods of prevention <strong>and</strong> control 

The opportunity of removing lichens from stone substrates 

must be evaluated case by case, since the damage caused 

by eachspecies is di erent. The rst step is to identify <strong>and</strong> 

perform an ecological characterization of the species so as to 

be able to choose a speci c agent to eliminate the lichen <strong>and</strong> 

the cause of its growth. Foliose <strong>and</strong> fruticose lichens have 

very di erent sensitivities to chemical agents (Giacobini et 

al., 1979). If the factors favoring lichen growth are known, 

preventive measures can be suggested to halt or slow down 

processes of colonization. 

Before removing nitrophilic species, which are relatively 

fast-growing <strong>and</strong> cause substantial damage, it is advisable 

to nd a way of avoiding nitrate enrichment of the stone 

surfaces. This can be achieved by controlling the bird populations 

of towns, the use of fertilizers in nearby elds, 

<strong>and</strong> water ow over the surfaces. Water run-o is generally 

avoided by sheltering or re-directing the water (Nimis <strong>and</strong> 

Monte, 1988). 

For surfaces colonized by endolithic lichens, Nimis <strong>and</strong> 

Zappa (1988) advise against elimination, as degeneration of 

the thallus leaves the surface porous <strong>and</strong> more vulnerable 

to environmental acids, whereas the damage caused by the 

lichens is limited due to their extremely slow growth. 

In the past, normal restoration practice involved the mechanical 

removal of the lichen thalli. Today this is done 

only for foliose <strong>and</strong> fruticose lichens since they do not 

adhere strongly to the substrate. Mechanical removal of 

crust-like lichens, the thallus of which penetrates deeply 

into the stone, requires the use of hard brushes <strong>and</strong> much 

washing with water <strong>and</strong> detergents. This process disperses 

the spores <strong>and</strong> causes much stone to be lost. To avoid these 

problems, Clarke (1976) advised applying an ammonium 

hydroxide solution to kill the thalli <strong>and</strong> loosen them, before 

mechanical removal. Nimis et al. (1987) advised against 

mechanical removal of thalli with soredia in order to avoid 

dispersal of these asexual reproductive structures <strong>and</strong> spread 

of the lichens. 

<strong>Lichens</strong> are generally removed by application of chemical 

products (see Appendix A) which may be combined 

with mechanical methods. A large variety of biocides that 

damage either the phycobiont (algicides) or the mycobiont 

(fungicides) are available (Lloyd, 1971). The most 

important parameters to consider in the choice of products 

to use for restoration are e cacy, nonreactivity with 

the substrate <strong>and</strong> toxicological characteristics. E cacy 

against the lichen should be a maximum at the minimum 

dose. The biocides should be of broad spectrum <strong>and</strong> have 

a long-lasting e ect. Chemical solutions should have a 

neutral pH to avoid corroding the stone <strong>and</strong> interfering 

with any soluble salts (Richardson, 1976). They should 

not cause changes in color, loss of translucence, whitening 

or increased re ectance. Richardson (1976, 1987) advises 

against the use of copper compounds <strong>and</strong> phenols because 

they color the substrate, <strong>and</strong> of borates that favor the formation 

of soluble salts. For vast areas it is preferable to 

use low toxicity products (Class III or IV, expressed as 

LD50 or LC50) so as to avoid contamination of the environment 

<strong>and</strong> poisoning of the operators. Biocides may be 

applied by spraying, brushing or pasting. Spraying should

e avoided on windy days <strong>and</strong> brushing when the surface 

is friable. Paste is used when the agent must be kept in 

contact with the surface for a longer period. The materials 

usually used for pasting are paper pulp, rice paper, cotton 

wool <strong>and</strong> substances suchas carboxymethylcellulose (Mora 

<strong>and</strong> Mora, 1972). Richardson (1976) recommends the use 

of quaternary ammonia salts applied witha low pressure 

spray. The concentration should be chosen in relation to 

the porosity of the substrate. Samidi (1981) successfully 

used a solution of quaternary ammonia salts to eliminate 

lichens from the temple of Boradubur in Indonesia. He recommends 

performing the operation in a dry period as rain 

could cancel the inhibitory e ect of the products. Solutions 

of pentachlorophenol <strong>and</strong> PanaciderJ have been applied 

by brushto Aboriginal rock paintings in Australia (Clarke, 

1976). Small silicon channels were mounted above these 

paintings to prevent water from owing over the painted 

surface. An interesting study by Lloyd (1971) reports the 

testing of pentachloro-phenyl-laurate, Cu-8-quinolate, <strong>and</strong> 

pentachloro- <strong>and</strong> trichloro-phenoxy-isopropanol on algae of 

the genus Trebouxia <strong>and</strong> fungi of the genus Cladosporium. 

Since it is di cult to determine the moment in which the 

lichen is no longer alive because its death does not imply 

loss of its chitin structure, deciding the appropriate dose 

can be problematical. Only a long time after treatment is 

it possible to observe morphological changes indicative of 

death. In fact this problem underlies the not always positive 

results obtained by Giacobini et al. (1979) when his 

group applied uometuron, chlorobromuron, terbutryn <strong>and</strong> 

vaneide 51 by spray <strong>and</strong> paste to a large variety of stone 

substrates. More than a year was necessary to be able to 

observed lichen death directly. Bettini <strong>and</strong> Villa (1981) 

proposed the following program of treatments: 

• repeated washing with water <strong>and</strong> the non-ionic detergent 

Lito 7, brushing <strong>and</strong> nal rinsing to remove dirt <strong>and</strong> dust 

trapped in the lichens; 

• application of the biocides, Lito 3, <strong>and</strong> further brushing 

to remove dry material. 

In central India, vigorous cleaning withagents suchas 

acetic acid, hydrogen peroxide <strong>and</strong> ammonia teepol was performed 

before applying agents to kill lichens (Santobrite, 

benzal, konium chloride) on Indian temples (Sharma et al., 

1985). The use of these products is questionable as they 

have very acid or basic pH <strong>and</strong> could corrode the stone. 

3. Higher <strong>plants</strong> 

3.1. Invasion <strong>and</strong> colonization 

The growth of <strong>plants</strong> on a historical building depends on 

its state of conservation, the building material <strong>and</strong> climate 

(Fig. 1). Plants usually take more than 10 years to colonize a 

wall. In that period of time, Parietaria di usa, for instance, 

will grow to a diameter of 20–30 cm in the mortar between 

the bricks of a wall. In the colonization process, there are 

Fig. 5. The stages of one mode of colonization of a wall. Hardy pioneer 

<strong>plants</strong> grow in substrate created by break-down of the wall structure <strong>and</strong> 

are succeeded by less hardy <strong>plants</strong> on a more abundant substrate. 

always pioneer <strong>plants</strong> (herbaceous annuals <strong>and</strong> perennials) 

that cause little damage, <strong>and</strong> are replaced by more detrimental 

<strong>plants</strong> suchas small shrubs or trees (Plate 2a <strong>and</strong> 2b). 

On the basis of this sequence, two modes of colonization 

that di er in relation to position, water availability <strong>and</strong> the 

type of <strong>plants</strong> that succeed each other can be identi ed. 

Fig. 5 shows the various stages of one type of colonization 

that is independent of the positions illustrated in Fig. 7 

<strong>and</strong> occurs even when little water is available. The rst 

assault on the wall is usually the result of abiotic factors 

(Fig. 5), that create conditions suitable for the growth of bacteria, 

fungi <strong>and</strong> lichens (Hyvarinen et al., 2002). These organisms 

accelerate the deterioration of the structure <strong>and</strong> lead 

to the formation of a substrate for the germination of seeds 

of hardy pioneer <strong>plants</strong> such as Sonchus tenerrimus (herbaceous 

annual) <strong>and</strong> Parietaria di usa (herbaceous perennial). 

Atmospheric dust, bird excrement <strong>and</strong> human wastes 

contribute to the layer of substrate so that other <strong>plants</strong> can 

develop. In some cases, the detritus of the pioneer <strong>plants</strong> 

forms a suitable substrate for the <strong>plants</strong> that follow. 

The second mode of colonization occurs mostly on horizontal 

surfaces witha better water supply (Fig. 6). In this 

case the pioneer <strong>plants</strong> are mosses that trap atmospheric dust, 

leading to the formation of su cient substrate for the germination 

of other <strong>plants</strong> (Plate 2c). Plants can grow in mortar 

between stones or bricks, in building material itself <strong>and</strong> on 

substrate formed as described above. The type of stone used 

determines the species that grow. Soft, porous stone that is 

not very compact, like travertine, crumbles readily, retains 

water <strong>and</strong> hosts mosses <strong>and</strong> <strong>higher</strong> <strong>plants</strong>. In either case, 

the roots of <strong>higher</strong> <strong>plants</strong> penetrate the wall, destroying the 

materials <strong>and</strong> increasing the quantity of substrate.

Plate 2. (a) Fourteenth century tower of San Giovanni, Elba isl<strong>and</strong>. The same kinds of lichens <strong>and</strong> <strong>plants</strong> grow on the surrounding granite <strong>and</strong> the stones 

of the tower, also of local granite. (b) Archof an 18thcentury gate at Vignano, Siena. A cypress about 3 m highis growing between the bricks, to 

the detriment of the structure. (c) Crassulaceous plant (Sedum pachyphyllum) originating from a pot plant, growing on a mound of detritus consisting 

of moss <strong>and</strong> atmospheric dust at the base of the Altar of the Fatherl<strong>and</strong>, Rome (early twenties). (d) Walls of the town of San Quirico d’Orcia (Siena 

Province), hosting a plum tree. The roots of the plant have already dislodged many stones. (e) Two examples of Dittrichia viscosa growing on the 

monument to Petrarch in Arezzo. The rhizome of the lower plant has already caused the marble to crack. (f) Ruins of the town of Maktar in Tunisia. 

Plants grow mostly in cracks <strong>and</strong> are periodically removed to keep the site accessible to tourists. 

3.2. Common <strong>plants</strong> colonizing monuments 

Botanists have long been interested in <strong>plants</strong> that grow in 

the walls of European towns such as Cambridge (Rishbeth, 

1948), Durham (Woodell <strong>and</strong> Rossiter, 1959), Padua (Beguinot, 

1912, 1913, 1916a–c), Pavia (Traverso, 1898, 1899), 

Florence (Arrigoni <strong>and</strong> Rizzotto, 1994), Rome (Anzalone, 

1951), Palermo (Cannarella, 1909, 1912) <strong>and</strong> other continental 

towns (Kunick, 1982; Sukopp <strong>and</strong> Werner, 1983). In 

a recent study (Lisci <strong>and</strong> Pacini, 1993a, b), we examined 

the microsites in which <strong>plants</strong> become established, <strong>and</strong> their 

methods of colonization <strong>and</strong> reproduction.

Fig. 6. The stages of a second mode of wall colonization. A moss spore (a) falls on porous stone, such as travertine, <strong>and</strong> develops (b). Atmospheric 

dust collects on the moss (c) forming a small amount of substrate (d). A seed falls on the substrate (e), germinates (f), grows (g) <strong>and</strong> owers (h). The 

moss does not damage the substrate but the plant roots penetrate it. 

Irrespective of the age or historical interest of the building, 

<strong>plants</strong> that grow in walls have anatomical <strong>and</strong> physiological 

characteristics that enable them to survive in a dry, 

inhospitable environment with little soil. These characteristics 

include: 

1. Seeds that can reach cracks or small cavities in the 

building material. The main agents of dispersal that can 

achieve this are wind (for light seeds produced in large 

quantity: e.g. of Parietaria di usa, Centranthus ruber), 

ants (e.g. for Chelidonium majus, Mercurialis annua), 

<strong>and</strong> birds that eat eshy fruits (e.g. of Ficus carica, 

Hedera helix). Man can also act as agent in the case of 

<strong>plants</strong> that “escape” from gardens <strong>and</strong> grow wild in walls 

(e.g. Antirrhinum majus, Cheiranthus cheiri, Matthiola 

incana). 

2. Seeds that germinate with a minimum of moisture. 

3. Early sexual maturity. Arboreal species that grow in walls 

can be regarded as “natural bonsai”. 

4. Vegetative reproduction, permitting the plant to have an 

alternative method when sexual reproduction fails. 

5. Drought resistance. Centranthus ruber, for example, 

grows on the edge of the crater of Vesuvius <strong>and</strong> in 

the ruins of Pompei, environments with little water or 

substrate. 

6. Deep rooting <strong>and</strong> capable of growing in rock crevice. 

The microsites in which these <strong>plants</strong> grow are illustrated 

in Fig. 7 <strong>and</strong> described below. The positions occupied 

by <strong>plants</strong> on the ideal ruin hold for any historical 

building. 

A. Cavities at ground level. This situation hosts many 

species since substrate naturally collects there <strong>and</strong> rain water 

does not run o . The roots of the plant may even penetrate 

the earth. Plants growing in this situation su er less 

environmental stress. 

B. Cavities in inclined surfaces. This situation o ers more 

possibilities for seeds to lodge <strong>and</strong> for moisture than vertical 

surfaces. 

C: Cavities between two types of building material. The 

greater the chemical di erence between the two materials, 

the more nutrients are available. 

D. Cavities in a vertical face of homogeneous material. 

This situation is the most hostile to plant growth since water 

is limited to wind-driven rain. 

E. Cavities in horizontal surfaces. Water availability is 

good but the plant must be capable of retaining it. 

F. Cavities at the junction of vertical <strong>and</strong> horizontal surfaces. 

This situation has similar water availability to A, but 

there is less substrate. 

G. Cavities where two vertical surfaces meet. This 

situation is similar to F but o ers less moisture. 

H. Substrate formed on a horizontal porous surface.The 

situation is constituted by a corbel, a decorative element in 

Roman buildings <strong>and</strong> Baroque <strong>and</strong> Renaissance churches. 

It is usually made of marble, travertine or s<strong>and</strong>stone.

Fig. 7. The main microsites for plant growth: cavities A, at ground level; B, on inclined surface; C, at junction of two materials; D, in homogeneous 

vertical face; E, in horizontal surface; F, at junction of vertical <strong>and</strong> horizontal faces; G, at junction of two vertical faces; substrate, H, on horizontal 

porous surface; I, on ruined stonework; space, J, between wall <strong>and</strong> stone facade. Rhizomes, K, from <strong>plants</strong> growing in soil may invade the wall. Moisture 

is di erent in eachsituation. Sites B, C <strong>and</strong> D are very dry, receiving only wind-driven rain. Sites A, F <strong>and</strong> G retain some rainwater <strong>and</strong> E, H, I, J <strong>and</strong> 

K are the wetest because the horizontal substrate absorbs all the rain it receives (modi ed from Lisci <strong>and</strong> Pacini, 1993a). 

I. Substrate formed on ruined stonework. This is a special 

situation mainly found in Roman ruins. It results from the 

destruction of rubble <strong>and</strong> lime mortar- lled walls or pillars. 

J. Spaces between a wall <strong>and</strong> its stone facade. This situation 

hosts <strong>plants</strong> with rhizomes, which if vigorous, may 

detachthe facade. 

K. Wall invaded by the rhizome of a plant in adjacent 

soil. This situation occurs with retaining walls <strong>and</strong> has been 

observed withtrees <strong>and</strong> shrubs suchas Celtis australis, 

Ailanthus altissima <strong>and</strong> Syringa vulgaris, <strong>and</strong> withrhizomatous 

<strong>and</strong> stoloniferous herbs. 

Table 1 lists the most common <strong>plants</strong> <strong>and</strong> the main microsites 

that they occupy (in terms of Fig. 7). Comparison 

of Fig. 7 <strong>and</strong> Table 1 reveals that the number of <strong>plants</strong> that 

grow on buildings is greatest for horizontal surfaces (Fig. 

7: E,H,I). This is because these situation o er: (a) better 

conditions for growth(i.e. more water <strong>and</strong> substrate); (b) 

more chance of seeds l<strong>and</strong>ing; (c) a good perch for birds 

that excrete seeds; (d) a good site for ant nests where seeds 

are stored. The number of <strong>plants</strong> decreases in environments 

in which there is little precipitation <strong>and</strong> relative humidity 

is low all year round. The ruins of Roman towns in north 

Africa (Tunisia <strong>and</strong> Libya) host only drought resistant 

species. Buildings of historical interest in parts of Asia 

(Burma, China, India, Indonesia, Thail<strong>and</strong>) <strong>and</strong> America 

(Mexico, Peru), characterized by high rainfall, develop 

luxuriant vegetation which is destructive if not controlled.

3.3. Damage 

Vegetation growing on historical buildings <strong>and</strong> ruins may 

be picturesque but it is also one of the main reasons for their 

deterioration (Dia <strong>and</strong> Not, 1991; Mouga <strong>and</strong> Almeida 1994; 

Almeida et al., 1994). Both the roots <strong>and</strong> the aerial part of 

the <strong>plants</strong> damage the structure of the walls. The branches 

<strong>and</strong> leaves hide the building so that it cannot be appreciated, 

<strong>and</strong> cause static damage due to their weight, which may 

cause stones or large portions of wall to fall (Plate 2d). They 

increase the risk of re with all the destruction that it can 

cause. The roots may penetrate deep into the structure <strong>and</strong> 

grow to a large size, causing physical <strong>and</strong> chemical damage. 

The secretions of roots contain substances that attack 

building materials, <strong>and</strong> their mechanical force opens cracks, 

causes crumbling <strong>and</strong> loosens stones <strong>and</strong> large fragments 

of wall. The damage is not limited to that which the plant 

causes to the building, but also includes the consequences 

of falling stones (Plate 2D). 

Not all <strong>plants</strong> cause the same type or quantity of damage 

(Table 1). Herbaceous <strong>plants</strong> like Mercurialis annua, 

Parietaria di usa <strong>and</strong> Sonchus tenerrimus are certainly 

less destructive than trees <strong>and</strong> shrubs like Ailanthus altissima, 

Capparis spinosa, Clematis vitalba, Ficus carica, 

Hedera helix <strong>and</strong> Rubus ulmifolius (Almeida et al., 1994). 

Herbaceous perennials, suchas Cynodon dactylon, Dittrichia 

viscosa <strong>and</strong> Cheiranthus cheiri, are more harmful 

than herbaceous annuals. However, the most destructive 

<strong>plants</strong> of all are those with vegetative reproduction (Fig. 7). 

Stolons <strong>and</strong> rhizomes cause the infesting <strong>plants</strong> to increase 

in size <strong>and</strong> propagate over large areas, to the obvious detriment 

of building structure. The damage caused by this type 

of vegetation is especially destructive to statues (Plate 2e). 

A plant that is aesthetically very attractive but which 

causes many types of damage is Hedera helix. The fruit is 

eaten by birds <strong>and</strong> the seeds germinate in many microsites 

(Fig. 7). The damage varies according to whether the plant 

grows as a liana from the soil or actually germinates <strong>and</strong> 

grows in the wall. In the rst case, the damage is due to the 

weight of the aerial part; in the second, there is additionally 

the destructive action of the roots. If it grows on a stuccoed 

wall, it may cause the plaster to fall away. 

3.4. Methods of control 

Methods of control vary according to the type of plant, 

the structure of the building, its state of conservation <strong>and</strong> 

its location. Recent buildings made of classical materials 

(squared stones <strong>and</strong> bricks), should be checked to prevent 

the formation of substrate <strong>and</strong> the growth of pioneer <strong>plants</strong> 

(Fig. 1). The speed of colonization depends on building 

structure <strong>and</strong> materials (Fig. 1). Decorations suchas ledges 

<strong>and</strong> cornices favor the formation of substrate. Porous materials 

or those with cracks, like travertine <strong>and</strong> red bricks, 

provide good microsites (Fig. 6). The state of conservation 

of a building is very important in determining the appropriate 

method of control. For ruins, periodic clearing of <strong>plants</strong> 

should be performed to prevent plant growth between the 

stones <strong>and</strong> crumbling of the remains (Plate 2f). Large buildings 

of historical interest in towns should be checked for 

aesthetic reasons, in the interests of their conservation <strong>and</strong> 

for the safety of inhabitants <strong>and</strong> visitors. Two very ancient 

examples are the Pantheon <strong>and</strong> Coliseum in Rome, but even 

Baroque <strong>and</strong> Renaissance buildings can be dangerous in a 

city like Rome if they are infested with <strong>plants</strong>. Buildings 

fenced o on archaeological sites present less of a peril to 

visitors. 

The surest method of control with long term advantages 

is total removal of the <strong>plants</strong>, including their roots. After 

this the cracks are lled so that other seeds do not germinate 

in them. This method works well for herbaceous <strong>plants</strong> 

but the roots of ligneous <strong>plants</strong> are di cult to remove, 

<strong>and</strong> pieces remaining may give rise to new growth. Other 

di culties are encountered when the walls are impervious 

or very high. The surface may be damaged if tools such as 

saws, chisels <strong>and</strong> drills are used. Manual weeding is useful 

for preventing the establishment of herbaceous <strong>plants</strong>, but 

as we already mentioned, they are only the rst colonizers. 

When the wall already hosts luxuriant vegetation in which 

the herbaceous <strong>plants</strong> have been replaced by ligneous ones, 

manual weeding is no longer e cacious, <strong>and</strong> more complex 

methods must be used. These may include killing the 

plant with chemicals, or if the position of the plant in the 

building permits, removing stones, extirpating the roots <strong>and</strong> 

rebuilding. 

Chemical herbicides are much faster <strong>and</strong> more e cient 

than manual weeding, but certain environmental (climate, 

chemical <strong>and</strong> physical properties of the building materials, 

analysis of the vegetation in the area where the chemical 

is to be applied) <strong>and</strong> toxicological (toxicity, volatility, 

biodegradability) aspects need to be veri ed before they are 

used. Today herbicides with speci c properties <strong>and</strong> di erent 

mechanisms of action (contact, systemic, residual, <strong>and</strong> 

so forth) are available (Appendix A). Their careful use can 

help greatly in the maintenance <strong>and</strong> protection of buildings 

<strong>and</strong> monuments (Caneva <strong>and</strong> De Marco, 1986; Caneva 

et al., 1991). Contact <strong>and</strong> systemic herbicides belong to the 

latest generation <strong>and</strong> are especially indicated because of 

their reduced environmental impact. They may be applied 

sporadically or repeatedly, up to three times. The same product 

may be used or others may be alternated to exploit their 

di erent possibilities. 

The most commonly applied herbicidal methods are: (a) 

total elimination of all vegetation (bare ground) which has 

a long term e ect; (b) temporary (short term) weeding; (c) 

selective weeding of herbaceous <strong>plants</strong>; (d) brush control, 

consisting in removing shrubs <strong>and</strong> trees <strong>and</strong> leaving herbaceous 

vegetation. The last two methods are the most frequent 

in archaeological sites, whereas total <strong>and</strong> temporary 

weeding are used for historical buildings <strong>and</strong> monuments in 

towns. Bettini <strong>and</strong> Villa (1975) <strong>and</strong> Bettini <strong>and</strong> Cinquanta

Table 1 

List of the principal species growing on the walls of historical interest. Tree <strong>and</strong> shrub species are more destructive than herbaceous <strong>plants</strong>. Not all 

species are capable of growing on vertical surfaces. Nomenclature according to Flora Europea (Tutin et al., 1964–1980); common names from Clapham 

et al. (1962) 

Families <strong>and</strong> species 

Adiantaceae 

Common names Forms Microsites Damage 

Adiantum capillus-veneris L. 

Hypolepidaceae 

Maidenhair-fern HP A,B,C,D,J, + 

Pteridium aquilinum (L.) Kuhn 

Aspleniaceae 

Bracken HP A,B,C,D,J + 

Asplenium trichomanes L. Maidenhair Spleenwort HP A,B,C,D,J + 

Ceterach o 

Ulmaceae 

cinarum DC. Rusty-back Fern HP A,B,C,D,J + 

Celtis australis L. Nettle-tree T E,F,H,I,K ++++ 

Ulmus minor Miller 

Moraceae 

Elm S=T E,F,H,I ++++ 

Ficus carica L. 

Urticaceae 

Fig S=T A,B,C,D,E,F,H,I,K ++++ 

Parietaria di usa Mert. & KochPellitory of the Wall HP A,B,C,D,E,F,G,H,I,J + 

Urtica dubia Forska l 

Caryophyllaceae 

Nettle HP A,B,C,D,E,F,H,I + 

Arenaria serpyllifolia L. Thyme-leaved S<strong>and</strong>wort HA A,B,F,H,I + 

Minuartia hybrida (Vill.) Schischk. Fine-leaved S<strong>and</strong>wort HA A,B,F,H,I + 

Stellaria media (L.) Vill. 

Ranunculaceae 

Chickweed HA A,B,F,H,I + 

Clematis vitalba L. 

Papaveraceae 

Traveller’s Joy S F,G,H,I,J,K +++ 

Chelidonium majus L. Greater Cel<strong>and</strong>ine HP A,F,H,I ++ 

Fumaria o 

Capparidaceae 

cinalis L. Common Fumitory HA A,B,D,F,H,I + 

Capparis spinosa L. 

Cruciferae 

Caper S B,C,D,G ++++ 

Cheiranthus cheiri L. Wall ower HP A,B,C,D,E,F,H,I +++ 

Diplotaxis muralis (L.) DC. Wall Rocket HA A,F,E,I + 

Draba muralis L. Wall Whitlow Grass HA A,C,H,I + 

Matthiola incana (L.) R.Br. Stock HP A,B,C,E,F,H,I +++ 

Sinapis arvensis L. 

Crassulaceae 

Charlock HA A,E,F,G,H ++ 

Sedum album L. White Stonecrop HP B,C,D,E,F,G,H,I + 

Sedum dasyphyllum L. Thick-leaved Stonecrop HP B,C,D,E,F,G,H,I + 

Umbilicus rupestris (Salisb.) D<strong>and</strong>y 

Rosaceae 

Pennywort HP B,C,D,E,F,G,H,I + 

Rubus sp. 

Leguminosae 

Blackberry S F,G,H,I,J,K +++ 

Robinia pseudacacia L. 

Euphorbiaceae 

Acacia T A,E,I,J,K ++++ 

Mercurialis annua L. 

Simaroubaceae 

Annual Mercury HA A,E,F,H,I + 

Ailanthus altissima (Miller) Swingle 

Celastraceae 

Tree of Heaven S=T A,E,I,J,K ++++ 

Euonymus europaeus L. 

Rhamnaceae 

Spindle-tree T A,E,I,J,K +++ 

Rhamnus alaternus L. 

Guttiferae 

Buckthom S A,E,I,J,K +++ 

Hypericum perforatum L. 

Araliaceae 

Common St John’s Wort HP A,B,C,D,E,F,H,I + 

Hedera helix L. 

Oleaceae 

Ivy S A,B,C,D,G,J,K ++++ 

Syringa vulgaris L. 

Boraginaceae 

Lilac T A,E,I,J,K +++ 

Echium vulgare L. 

Labiatae 

Vipers’ Bugloss HP C,E,F,G,H,J + 

Calamintha nepeta (L.) Savi Lesser Calamint HP A,B,C,D,E,H,I,J + 

Lamium amplexicaule L. Henbit HA A,B,C,D,E,H,I,J + 

Micromeria graeca (L.) Bentham HP A,B,C,D,E,H,I,J +

Table 1. Continued 

Families <strong>and</strong> species Common names Forms Microsites Damage 

Scrophulariaceae 

Antirrhinum majus L. Snapdragon HP B,C,D,E,F,H,I,J ++ 

Cymbalaria muralis Gaer., Mey & Scher. Ivy-leaved Toad ax HP B,C,D,E,F,H,I,J + 

Caprifoliaceae 

Sambucus nigra L. Elder T A,E,I,K +++ 

Valerianaceae 

Centranthus ruber (L.) DC. Red Valerian HP B,C,D,E,F,H,I +++ 

Compositae 

Anthemis tinctoria L. Yellow Chamomile HP B,C,D,E,F,H,I,J + 

Conyza canadensis (L.) Cronq. Canadian Fleabane HA A,B,E,H,I,J + 

Crepis setosa Heller Bristly Hawk’s-beard HA D,E,F,H,I + 

Dittrichia viscosa (L.) Greuter HP A,B,C,D,E,F,G,H,I,J +++ 

Helichrysum italicum (Roth) G.Don l. HP B,C,D,E,F,G,H,I ++ 

Mycelis muralis (L.) Dum. Wall Lettuce HP D,E,G,H,I + 

Picris hieracioides L. Hawkweed Ox-Tongue HP A,E,H,I ++ 

Senecio vulgaris L. Groundsel HA A,E,F,G,H,I + 

Sonchus asper (L.) Hill Sow-Thistle HA A,B,C,D,E,H,I,J + 

Sonchus tenerrimus L. HA A,B,C,D,E,H,I,J + 

Taraxacum o cinale Weber Common D<strong>and</strong>elion HP A,E,H,I + 

Liliaceae 

Allium ampeloprasum L. Wild Leek HP A,C,E,H,I,J ++ 

Gramineae 

Bromus madritensis L. Compact Brome HA A,C,D,E,F,H,I + 

Cynodon dactylon (L.) Pers. Bermuda-grass HP A,C,D,E,F,H,I,J ++ 

Dactylis glomerata L. Cock’s-foot HP A,C,D,E,F,H,I,J ++ 

Desmazeria rigida (L.) Tutin Hard Poa HA A,C,D,E,F,H,I + 

Hordeum murinum L. Wall Barley HA A,C,D,E,F,H,I + 

HP, herbaceous perennial; HA, herbaceous annual; S, shrub; T, tree. Microsites are indicated as in Fig. 7. Score for damage to walls ranges 

from ++++ (major) to + (minor). 

Table 2 

Active principle Product <strong>and</strong> manufacturer=supplier Spectrum of action References 

3 Alkyl-trimethyl- Gloquat C–A.B.M. Chem <strong>Lichens</strong> Richardson, 1976 

ammonium chloride 

Ammonium phosamine Krenite–Du Pont <strong>Lichens</strong> <strong>and</strong> <strong>higher</strong> <strong>plants</strong> Bettini <strong>and</strong> Cinquanta, 1990 

Benzalconio chloride Hyamine 3500–Rohm & Haas <strong>Lichens</strong> Samidi, 1981 

(alkyl-dimethyl-benzylammonium 

chloride) 

Bromacile Xyvar X–Du Pont Algae, mosses, lichens <strong>and</strong> Bettini <strong>and</strong> Cinquanta, 1990 

<strong>higher</strong> <strong>plants</strong> 

Diuron Karmex–Du Pont Algae, mosses, lichens <strong>and</strong> Bettini <strong>and</strong> Cinquanta, 1990 

<strong>higher</strong> <strong>plants</strong> 

Fluometuron (3-(3- Lito 3–Ciba Geigy Algae, mosses, lichens Bettini <strong>and</strong> Villa, 1981 

tri uoromethyl-phenyl)1,1dimethylurea 

Glyphosate Roundup Rodeo–Monsanto Higher <strong>plants</strong> Bettini <strong>and</strong> Cinquanta, 1990 

Hexazinone Velpar–Du Pont Higher <strong>plants</strong>; sometimes Bettini <strong>and</strong> Cinquanta, 1990 

against mosses <strong>and</strong> lichens 

Salicylanilide Shirlan–Plus I.C.L. <strong>Lichens</strong> Clarke, 1976 

Simazine Gesatop Weedex–Ciba Geigy Higher <strong>plants</strong>; sometimes Bettini <strong>and</strong> Cinquanta, 1990 

against mosses <strong>and</strong> lichens 

Sodium dimethylthiocarbamate Vancide 51–V<strong>and</strong>erbilt <strong>Lichens</strong> Giacobini et al., 1979 

+ sodium 2-mercapto-benzothiazolo 

Sodium tetraborate Polybor–Borax Cons. Lim. <strong>Lichens</strong> Richardson, 1987 

(1990) studied the use of a variety of herbicides at di erent 

concentrations on archaeological sites <strong>and</strong> Italian monuments 

(see Appendix A). 

The methods should be chosen according to whether the 

vegetation to be destroyed is growing on archaeological remains 

or ruins in open areas, or on historical buildings <strong>and</strong>

monuments in towns. Chemicals should not be used indiscriminately 

in towns as problems of elimination could arise 

withalready polluted air. 

4. Current trends <strong>and</strong> research 

Until a few years ago, researchon this topic consisted of 

classi cation of the vegetation growing in these anthropized 

environments. As far as lichens were concerned, even this 

type of study was rare up until the sixties. Recently, however, 

there has been much interest in lichens <strong>and</strong> they have 

been studied not only from the taxonomic point of view, but 

also as environmental bioindicators or sensors of pollution, 

<strong>and</strong> as agents degrading building materials <strong>and</strong> monuments. 

As bioindicators, lichens supply useful information on atmospheric 

pollution <strong>and</strong> other parameters that a ect the 

conservation of monuments. Lichen damage to buildings 

varies in severity according to species <strong>and</strong> substrate, <strong>and</strong> is 

usually much slower than that caused by vascular <strong>plants</strong>. 

The decision to eliminate lichens with chemical substances 

should be evaluated on a case by case basis, considering in 

addition the e ects that atmospheric pollutants could cause 

to a surface devoid of lichens. As new products are tested 

for the elimination of lichens, it is necessary to st<strong>and</strong>ardize 

the methods to ascertain quickly whether the death of the 

lichen has occurred, rather than wait for the slower appearance 

of morphological changes. If environmental conditions 

do not change after treatment with chemical products, 

new forms of biological colonization will sooner or later 

occur. A new attack of lichens <strong>and</strong>=or microorganisms 

is facilitated by the porosity of the substrate left by the 

previous colony. It is therefore advisable to protect surfaces 

with substances that render them compact <strong>and</strong> resistant to 

biological agents. 

As far as <strong>higher</strong> <strong>plants</strong> are concerned, papers on the applied 

ecology of wall <strong>plants</strong> have recently appeared. The 

microsites in which these <strong>plants</strong> grow have been identi ed 

(Lisci <strong>and</strong> Pacini, 1993a, b) <strong>and</strong> germination trials are underway 

on the seeds of the most common <strong>and</strong> most harmful 

among them (Lisci <strong>and</strong> Pacini, 1994). The purpose of this is 

to nd weak points in the development of these <strong>plants</strong> with 

a view to prevention <strong>and</strong> control. In parallel, zoologists have 

been studying ways to limit bird fauna from roosting on Italian 

centers of art. The bird excrement causes four-fold damage: 

Corrosion; aesthetic damage; dispersal of seeds of wall 

vegetation (e.g. g seeds); <strong>and</strong> contributes to the formation 

of substrates for plant growth. Another line of research is 

concerned with what species to plant on archaeological sites. 

Such <strong>plants</strong> should have the following characteristics: (a) 

they should not invade the ruins; (b) they should be green 

for most of the year to grace the ruins; (c) they should be 

native to the surrounding environment; <strong>and</strong> (d) they should 

not cause allergies. 

<strong>Lichens</strong> form a patina that testi es to the aging of a monument, 

whereas <strong>plants</strong> contribute a picturesque appearance. 

These two aspects, however, need to be controlled if we are 

to enjoy the monument for as long as possible. 

Appendix A. List of some commonly used herbicides in 

Italian archaeological <strong>and</strong> monumental areas as shown in 

Table 2 

References 

Adam, J., 1988. La construction romaine. Materiaux et techniques. 

Editions A. et J. Picard, Paris. 

Aless<strong>and</strong>rini, G., 1989. The oxalate lms: origin <strong>and</strong> signi cance in the 

conservation of works of art. Centro C.N.R. “Gino Bozza”, Milano, 

pp. 381. 

Almeida, M.T., Mouga, T., Barracosa, P., 1994. The weathering ability 

of <strong>higher</strong> <strong>plants</strong>. The case of Ailanthus altissima (Miller) Swingle. 

International Biodeterioration <strong>and</strong> Niodegradation 12, 333–343. 

Anzalone, B., 1951. Flora e vegetazione dei muri di Roma. Annali di 

Botanica (Roma) 23 (3), 393–497. 

Arrigoni, P.V., Rizzotto, M., 1994. Caratteri della ora e della vegetazione 

urbana di Firenze. Allionia 32, 231–243. 

Ascaso, C., Galvan, J., Rodriguez-Pascual, C., 1976. Studies on the 

pedogenetic action of lichen acids. Pedobiologia 16, 321–332. 

Beguinot, A., 1912. La ora delle mura e delle vie di Padova. Studio 

Biogeogra co. Malpighia 24, 413–428. 

Beguinot, A., 1913. (Continuazione). Malpighia 25, 61–84. 

Beguinot, A., 1916a. (Continuazione). Malpighia 27, 244–259. 

Beguinot, A., 1916b. (Continuazione). Malpighia 27, 439–454. 

Beguinot, A., 1916c. (Continuazione e ne). Malpighia 27, 547–582. 

Bettini, C., Cinquanta, A., 1990. Vegetazione e monumenti: esigenze 

e metodologie nel controllo delle infestanti ruderali. Union Printing, 

Viterbo. 

Bettini, C., Villa, A., 1975. Il problema della vegetazione infestante 

nelle aree archeologiche. Proceedings of International Symposium “The 

Conservation of Stone”. Centro Conservazione Sculture all’Aperto, 

Bologna, pp. 191–204. 

Bettini, C., Villa, A., 1981. Description of a method for 

clearing tombstones. Proceedings of International Symposium “The 



Bracconot, H., 1825. De la presence de l’oxalate de chaux dans le regne 

mineral: existance du meme sel en quantite enorme dans les plantes 

de la famille des <strong>Lichens</strong>, et moyen avantageux d’en extraire l’acide 

oxalique. Annales de Chimimie et Physique 28, 318–322. 

Braun-Blanquet, J., 1964. P anzensoziologie. Grundzuge der 

Vegetationskunde. Springer Verlag, Wien. 

Caneva, G., De Marco, G., 1986. Il controllo della vegetazione nelle 

aree archeologiche e monumentali. Atti del Convegno Scienze e Beni 

Culturali. Lib. progetto Ed. Padova, Bressanone, pp. 553–569. 

Caneva, G., NugarI, M.P., Salvadori, O., 1991. Biology in the 

Conservation of Works of Art. International Center for the Study of 

the Preservation <strong>and</strong> the Restoration of Cultural Property, Rome. 

Cannarella, P., 1909. Flora urbica palermitana. Bullettino Societa Botanica 

Italiana 3, 73–81. 

Cannarella, P., 1912. (Continuazione). Bullettino Societa Botanica Italiana 

7, 172–183. 

Chiari, G., Sampo, S., Torraca, G., 1989. Formazione di ossalati di calcio 

su super ci marmoree da parte dei funghi. Le pellicole ad ossalato: 

origine e signi cato nella conservazione delle opere d’arte. Centro 

C.N.R. “Gino Bozza”, Milano, pp. 85–90. 

Clapham, A.R., Tutin, T.G., Warburg, E.F., 1962. Flora of the British 

Isles. Cambridge University Press, London. 

Clarke, J., 1976. A lichen control experiment at an aboriginal rock 

engraving site. Bolgart, Western Australia. ICCM Bulletin 2 (3), 

15–17.

Corball, R., Paz-Bermudez, G., Sanchez-Beisma, M.T., Prieto, B., 2001. 

Lichen colonization of coastal churces in galica: biodeterioration 

implications. International Biodeterioration <strong>and</strong> Biodegradation 47, 

157–163. 

Del Monte, M., Sabbioni, C., 1987. A study of the patina called 

“scialbatura” on Imperial Roman marbles. Studies in Conservation 32, 

114–121. 

Deruelle, S., Lallemant, R., Roux, C., 1979. La vegetation lichenique 

de la Basilique de Notre-Dame de l’Epine (Marne). Documents 

Phytosociologiques N.S. 4, 218–234. 

Dia, M.G., Not, R., 1991. Gli agenti biodeteriogeni degli edi ci 

monumentali del centro storico della citta di Palermo. Quaderni di 

Botanica Ambientale Applicata 2, 3–10. 

Enderman, J.A., Gough, L.P., White, R.W., 1977. Calcium oxalate 

as source of high ash yields in the terricolous lichen Parmelia 

chlorochroa. The Bryologist 80, 334–339. 

Fry, E.J., 1924. A suggested explanation of the mechanical action of 

lithophytic lichens on rocks (shale). Annals of Botany (London) 38, 

175–196. 

Fry, E.J., 1927. The mechanical action of crustaceous lichens on substrata 

of shale, schist, gneiss, limestone <strong>and</strong> obsidian. Annals of Botany 

(London) 41, 437–460. 

Galvan, J., Rodriguez, C., Ascaso, C., 1981. The pedogenic action of 

lichens in metamorphic rocks. Pedobiologia 21, 60–73. 

Garcia Rowe, J., Saiz-Jimenez, C., 1988. Colonization of mosaics by 

lichens: the case study of Italica (Spain). Studia Geobotanica 8, 65–72. 

Garty, J., 1992. The post re recovery of rock-inhabiting algae, microfungi 

<strong>and</strong> lichens. Canadian Journal of Botany 70, 301–312. 

Gehrmann, C., Krumbein, W.E., Petersen, K., 1988. Lichen weathering 

activities on mineral <strong>and</strong> rock surfaces. Studia Geobotanica 8, 33–46. 

Giacobini, C., Bettini, C., Villa, A., 1979. Il controllo dei licheni, alghe 

e muschi. III Congr. Intern. Deterioramento e Conservazione della 

Pietra, Venezia, pp. 305–312. 

Giamello, M., Guasparri, G., Neri, R., Sabatini, G., 1992. Building 

materials in Siena architecture: type, distribution <strong>and</strong> state of 

conservation. Science <strong>and</strong> Technology for Cultural Heritage 1, 55–65. 

Guidobaldi, F., Laurenzi Tabasso, M., Meucci, C., 1984. Monumenti in 

marmo di epoca imperiale a Roma: indagine sui residui di trattamenti 

super ciali. Bollettino d’arte 24, 121–134. 

Hale, M.E., 1974. The Biology of <strong>Lichens</strong>, 2nd Edition. Arnold, London. 

Hyvarinen, A., Meklin, T., Vespalainen, T., 2002. Fungi <strong>and</strong> actinobacteria 

in moisture-damaged building material: concentration <strong>and</strong> diversity. 

International Biodeterioration <strong>and</strong> Biodegradation 49, 21–25. 

Isk<strong>and</strong>er, I.K., Syers, J.K., 1972. Metal complex formation by lichen 

compounds. Journal of Soil Science 23, 255–265. 

Jackson, D.W., 1981. An SEM study of lichen pruina crystal morphology. 

Scanning Electron Microscopy 3, 279–284. 

Jones, D., Wilson, M.J., Tait, J.M., 1980. Weathering of a basalt by 

Pertusaria corallina. Lichenologist 12, 277–289. 

Kunick, W., 1982. Comparison of the ora of some cities of central 

European lowl<strong>and</strong>s. In: Bornkamm, R., Lee, J.A., Seaward, M.R.D. 

(Eds.), Urban Ecology. Blackwell, Oxford, pp. 13–22. 

Lisci, M., Pacini, E., 1993a. Plants growing on the walls of Italian towns 

1. Sites <strong>and</strong> distribution. Phyton (Horn, Austria) 33 (1), 15–26. 

Lisci, M., Pacini, E., 1993b. Plants growing on the walls of Italian towns 

2. Reproductive ecology. Giornale Botanico Italiano 127, 1053–1078. 

Lisci, M., Pacini, E., 1994. Germination ecology of drupelets of the g 

(Ficus carica L.). Botanical Journal of Linnean Society 114, 133–146. 

Lloyd, A.O., 1971. An approach to the testing of lichen inhibitors. 

Biodeterioration of Materials 2, 185–191. 

Modenesi, P., Lajolo, L., 1988. Microscopical investigation on a marble 

encrusting lichen. Studia Geobotanica 8, 47–64. 

Monte, M., 1991a. <strong>Lichens</strong> on monuments: environmental bioindicators. 

Science, Technology <strong>and</strong> European Cultural Heritage. Published for the 

Commission of the European Communities by Butterworth-Heinemann 

Ltd, pp. 355–359. 

Monte, M., 1991b. Multivariate analysis applied to the conservation of 

monuments: lichens on the Roman aqueduct Anio Vetus in S. Gregorio. 

International Biodeterioration 28, 133–150. 

Monte, M., 1993. The in uence of environmental conditions on the 

reproduction <strong>and</strong> distribution of epilithic lichens. Aerobiologia 9, 169– 

179. 

Mora, P., Mora, L., 1972. Metodo per la rimozione di incrostazioni su 

pietre calcaree e dipinti murali. C.N.R. Centro di studio sulle cause 

di deperimento e sui metodi di conservazione delle opere d’arte. Nota 

interna n. 12. 

Mouga, T., Almeida, M.T., 1994. Excavated monuments as environment 

for <strong>plants</strong>—Cunimbriga, portugal: a study case. III International 

Symposium on the Conservation of Monuments in the Mediterranean 

Basin, pp. 323–328. 

Nimis, P.L., Monte, M., 1988. The lichen vegetation on the cathedral of 

Orvieto (Central Italy). Studia Geobotanica 8, 77–88. 

Nimis, P.L., Zappa, L., 1988. I licheni endolitici calcicoli su monumenti. 

Studia Geobotanica 8, 125–133. 

Nimis, P.L., Monte, M., Tretiach, M., 1987. Flora e vegetazione lichenica 

di aree archeologiche del Lazio. Studia Geobotanica 7, 3–161. 

Pacini, E., 1994. Purposes <strong>and</strong> manners of representation of <strong>plants</strong> in the 

European art of 13th–17th century. Pact Journal 42, 172–180. 

Richardson, B.A., 1976. Control of moss, lichen <strong>and</strong> algae on stone. 

Proceedings of International Symposium “The Conservation of Stone”. 

Centro Conservazione Sculture all’Aperto, Bologna, pp. 225–231. 

Richardson, B.A., 1987. Control of microbial growths on stone <strong>and</strong> 

concrete. Biodeterioration, Elsevier Applied Science 7, 101–106. 

Rishbeth, J., 1948. The ora of Cambridge walls. Journal of Ecology 36, 

136–148. 

Saiz-Jimenez, C., 1981. Weathering of building materials of the Giralda 

(Seville-Spain) by lichens. Proceedings of the ICOM Sixth Triennial 

Meeting, Ottawa, pp. 92–97. 

Salvadori, O., Lazzarini, L., 1991. Lichen deterioration on stones of 

Aquileian monuments (Italy). Botanika Chronika 12, 961–968. 

Samidi, S., 1981. How to control the organic growth on Borobudur stones 

after the restoration. Proceedings of International Symposium “The 



Savoye, D., Lallemant, R., 1980. Evolution de la micro ored’un substrat 

avant er pendant sa colonisation par les lichens. I Le cas de toitures 

en amiante-ciment en zone sub-urbaine. Cryptogamie Bryologie 

Lichenolologie 1 (1), 21–31. 

Seaward, M.R.D., 1976. Performance of Lecanora muralis in an urban 

environment. In: Lichenology: Progress <strong>and</strong> Problems. Academic Press, 

London, pp. 323–357. 

Seaward, M.R.D., 1988. Lichen damage to ancient monuments: a case 

study. Lichenologist 20 (3), 291–295. 

Seaward, M.R.D., Giacobini, C., 1989. Oxalate encrustation by lichen 

Dirina massiliensis forma sorediata <strong>and</strong> its role in the deterioration of 

works of art. In: The Oxalate Films: Origin <strong>and</strong> Signi cance in the 

Conservation of Works of Art. Centro C.N.R. “Gino Bozza”, Milano, 

pp. 215–219. 

Seaward, M.R.D., Capponi, G., Giacobini, C., 1989. Biodeterioramento da 

licheni in Puglia. Proceedings of the First International Symposium “La 

Conservazione dei monumenti nel bacino del Mediterraneo”, Leterze 

Bari, pp. 243–245. 

Sharma, B.R.N., Chaturvedi, K., Samadhia, N.K., Taylor, P.N., 1985. 

Biological growthremoval <strong>and</strong> comparative e ectiveness of fungicides 

from Central India Temples for a decade in situ. Proceedings of the 

SixthInternational Congress on Deterioration <strong>and</strong> Conservation of 

Stone, Losanna, pp. 675–683. 

Shatz, A., 1963. Soil microrganisms <strong>and</strong> soil chelation. The pedogenetic 

action of lichens <strong>and</strong> lichen acids. Agricultural <strong>and</strong> Food Chemistry 

11 (2), 113–118. 

Sukopp, J., Werner, P., 1983. Urban environments <strong>and</strong> vegetation. In: 

Holzner, W., Werger, M.J.A., Ikusima, I. (Eds.), Man’s Impact on 

Vegetation. Junk, The Hague, pp. 247–260.

Syers, J.K., 1964. A study of soil formation on carboniferous limestone 

with particular reference to lichens as pedogenetica agents. Ph.D. 

Thesis, University of Durham, Engl<strong>and</strong>. 

Syers, J.K., Isk<strong>and</strong>er, I.K., 1973. Pedogenetic signi cance of lichens. 

In: Ahmadjian, V., Hale, M.E. (Eds.), The <strong>Lichens</strong>. Academic Press, 

London, pp. 225–248. 

Traverso, G.B., 1898. Flora urbica pavese. Catalogo delle piante vascolari 

che crescono spontaneamente nella citta di Pavia. (I centuria). Nuovo 

Giornale Botanico Italiano 5, 57–75. 

Traverso, G.B., 1899. Flora urbica pavese. (II centuria). Nuovo Giornale 

Botanico Italiano 6, 241–253. 

Tretiach, M., Monte, M., Nimis, P.L., 1991. A new hygrophytism index 

for epilithic lichens developed on basaltic Nuraghes in N.W. Sardinia. 

Botanika Chronika 12, 953–960. 

Tutin, T.G., Heywood, V.H., Burges, N.A., Valentine, D.H., Walters, 

S.M., Webb, D.A., Flora Europea. Cambridge University Press, 

London, 1964–1980. 

Warsheid, T., Braams, J., 2000. Biodeterioration of stone: overview. 

International Biodeterioration <strong>and</strong> Biodegradation 46, 343–368. 

Wildi, O., Orloci, L., 1988. Mulva-4, a package for multivariate analysis 

of vegetation data. Swiss Fed. Inst. For. Res. Rep., Birmensdorf. 

Wirth, V., 1980. Flechten ora. Ulmer, Stuttgart. 

Woodell, S.R.J., Rossiter, J., 1959. The ora of Durham walls. Proceedings 

of Botanical Society of BritishIsles 3, 257–273.

Lichens and higher plants on stone: a review - AseanBiodiversity.info

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?