SPECIES CONCEPTS IN THE GENUS USNEA (LICHENIZED ...

Lichenologist 30(4-5): 321–340 (1998) 

Article No. li980150 

SPECIES CONCEPTS IN THE GENUS USNEA 

(LICHENIZED ASCOMYCETES) 

Philippe CLERC* 

Abstract: Practically most of the taxonomists working on vascular plants and fungi 

are using the so-called taxonomic species concept, relying largely on morphological 

criteria or other observable patterns of discontinuity and assuming that these 

patterns are reflecting the underlying genetic integrity of species. Such a concept is 

described in detail here for the genus Usnea. The typological view versus the 

populational view of species are discussed. The diagnostic value of characters used 

by previous and recent taxonomists of the genus is analysed. The fact that Usnea has 

such a bad taxonomic reputation is discussed. The use by J. Motyka, author of the 

world monograph published in the thirties, of characters that are strongly modified 

by environmental factors together with a typological view of species are thought 

to be mainly responsible for this. More than 770 names have been published 

worldwide so far and it is suggested that around 50% are to be considered as 

synonyms. � 1998 The British Lichen Society 

Introduction 

The ‘beard-like’ lichenized ascomycete genus Usnea Hill with its fruticose 

species sharing a shrubby to pendant thallus, pale, yellowish green branches 

with radial symmetry, a cartilaginous central axis and usnic acid in the cortex 

is a beloved genus for beginners in lichenology: just stretch the branch 

and here comes the axis, this making Usnea one of the easiest lichen genera 

to identify. At the specific level, however, this is quite another matter and 

the taxonomy of Usnea is considered by most modern lichenologists to 

be exceptionally difficult, where most of the described species seem to be 

connected by a continuous array of transitional forms, impeding the recognition 

of well-defined species. Most of the herbarium material is wrongly 

identified or labelled just as Usnea sp. It has come to the point that on field 

trips some lichenologists do not even want to collect specimens of this genus 

any more. 

The purpose of this paper is to discuss the modern species concept currently 

prevailing, together with the characters that are commonly employed in the 

taxonomy of this genus. On this basis, I will try to explain why the genus Usnea 

has such a bad taxonomic reputation. Josef Motyka’s monograph ‘ Lichenum 

generis Usnea studium monographicum ’, and his species concept, will be 

briefly analysed. 

What is a Species? 

There is no general agreement on this question today and the issue is highly 

controversial. The biological species concept that defines species as being 

*Conservatoire et Jardin botaniques, Case postale 60, CH–1292 Chambésy/Genève, Switzerland. 

0024–2829/98/040321+20 r30.00/0 � 1998 The British Lichen Society

322 THE LICHENOLOGIST Vol. 30 

groups of interbreeding natural populations that are reproductively isolated from other 

such groups (Mayr 1963, 1992) and thus seen as ‘ basic ’ evolutionary units 

seems to be the most accepted definition among evolutionary biologists and 

taxonomists, especially zoologists. However the growing evidence that gene 

flow in nature is much more restricted than commonly thought (Ehrlich & 

Raven 1992) and the existence throughout the biological world of welldelimited 

asexual species (especially in lichenized ascomycetes!) has led 

biologists to reconsider their vision of the species. For instance, Mishler & 

Donoghue (1982) argue for a pluralistic approach where the unit ‘ species ’ 

would be decoupled from notions such as ‘ basic ’ evolutionary units or 

‘ integrated ’ reproductive units (see Kitcher 1992 for philosophical arguments). 

Emphasis is put on local populations and cohesive forces that 

maintain these populations into units called species (Templeton 1989). 

Species can then be seen as genetic clusters (Mallet 1995) or dense regions 

within a multidimensional environmental space (Sokal & Crovello 1992) 

where gene flow would be one among other important cohesive forces like 

stabilizing selection, historical, ecological, developmental and population 

genetic constraints (Templeton 1989). The cohesion species concept developed 

by Templeton (1989) where species are defined as the most inclusive 

population of individuals having the potential for phenotypic cohesion through 

intrinsic cohesion mechanisms (see above) is interesting because it is applicable 

to asexual species. This is especially important when considering a genus like 

Usnea where 52% of the species described so far in the world are asexual 

(Bowler & Rundel 1975). 

Recently, Purvis (1997) discussed the species concept in lichens. The 

duality of lichens, the extreme difficulty of growing them in vitro, their slow 

growth, the impossibility of making experimental crosses and lack of knowledge 

pertaining to the specific sexual cycle of lichenized ascomycetes make it 

difficult for lichenologists to discuss their species concepts in the framework of 

the general theory and its recent developments. As a matter of fact, lichen 

monographers rarely discuss extensively their criteria used to delimit species 

in the light of the existing species concepts (for an exception see however 

Kärnefelt 1979, 1997). Thus, most of the modern lichen systematists take a 

pragmatic approach (most of the vascular plant taxonomists do the same, see 

McDade 1995) relying largely on morphological criteria or other observable 

patterns of discontinuity and assuming that these patterns are reflecting the 

underlying genetic integrity of species. In this way they adopt the so-called 

taxonomic species concept (Kärnefelt 1979; Cronquist 1988). 

Modern Species Concepts in the Genus Usnea 

The work of Motyka (1936–1938) is characterized by a strong typological view 

( Fig. 1A) where species are seen as invariant units based on a ‘ perfect type ’, 

allowing thus very little opportunity for variation (one character=one species). 

Consequently Motyka described a large number of new taxa. For instance, in 

Europe, he accepted 140 taxa, 79 more than the number existing before his 

monograph!

1998 Usnea—Clerc 323 

Fig. 1. Typological and populational species concepts, as illustrated by the distribution of 

individuals of one species for a given character (Adapted from Meffe et al. 1997). A, Typological 

concept focusing on perfect ‘ types ’ represented by T1–T4. The essence of the species is the type. 

Each one of these types is considered as clearly differentiated and invariant and T1–T4 are four 

different species. Specimens not fitting into this four-species concept will be either described as 

new species or identified with such expressions as ‘ accedens ad ...’ or ‘transiens in ...’; B, 

Populational concept focusing on variation within species. Here, the essence of the species is the 

variation, that is the distribution of the character measured, and the types (T1–T4) are considered 

as an abstraction. T3 can be seen as the average representation of the species, whereas T1 

represents an extreme variant of the same species. There is here only one species where T1, T2, 

T3 and T4 are nomenclatural synonyms. 

Swinscow & Krog (1978) were the first to introduce modern populational 

thinking (Fig. 1B)inUsnea. They recognized the importance of within-species 

variation and the potential for genetic and morphological diversification 

among populations. Consequently, they regarded different morphotypes 

connected by transitional forms as constituting the same species. Moreover 

they were the first to analyse systematically the species chemistry with TLC. 

Their work on East African species (Swinscow & Krog 1974, 1975, 1976a, b, 

1978, 1979) is a precursor to modern treatments of the genus in other parts 

of the world. 

Admitting the importance of the effect of various environmental parameters 

modulating the variation among individuals inside species has been thus a very 

important step towards a modern species concept in the genus Usnea. In 

sexually reproducing species, both genetical differences and environmental 

parameters are involved in intraspecific variation, whereas in asexually reproducing 

species it is assumed that only the latter modulate the differences 

between individuals of the same clone. 

Once species are circumscribed and the amplitude of their variation 

established it is much easier to identify extreme morphotypes (as for instance 

T1 in Fig. 1B). These morphotypes are often individuals that are living under 

extreme environmental conditions or at the limits of the species distribution


area. Practically and in this context, it is essential to start a systematic study by 

looking at as many specimens as possible, in the field and in the herbarium. 

This is necessary to develop one’s own interpretation of the variation of the 

species involved and, one’s own species concept. Looking at the original 

material (types) should be the last step of this process. 

Recent papers on the systematics of Usnea (Clerc 1984, 1987a, 1997; 

Clerc & Herrera-Campos 1997; Herrera-Campos et al. 1998; Halonen et al. 

1998) develop this concept, putting special emphasis on the morphological, 

anatomical and chemical variation of each species treated. 

In recognizing intraspecific variation, we are already engaged in the process 

of circumscribing species, that is to distinguish between intra- and interspecific 

variation. Two steps are necessary here: the grouping and the ranking. 

I use the following concept (see also Herrera-Campos et al. 1998): similarities 

in morphological, anatomical and/or chemical characters are looked for and 

used to group individuals in the same taxon. Two such different groups/taxa 

are ranked as distinct species when the following conditions are fulfilled: 

(1) The existence, between these two groups, of two or more well-marked 

and correlated discontinuities in the variation of characters that are postulated 

to be independent. 

(2) The absence or very low frequency, between the two groups, of 

intermediate forms, the so called ‘ hybrids ’ in the lichenological literature 

(Brodo 1978; Clerc 1984), which is the absence of certain combinations of 

character states that usually do not occur together. In Usnea, for instance, the 

specimen cited by Clerc (1992) and characterized by the morphology of 

U. wasmuthii Räs. and the chemistry of U. subfloridana Stirt. is such an 

intermediate form. 

Thus the correlation of at least two characters that are postulated to be 

independent (e.g. soralia morphology and chemistry or pigmentation of 

cortex/medulla and chemistry) is the minimum condition for a population, or 

a group of populations, displaying these characters to be considered as good 

species. Consequently, each species of Usnea accepted by me is a unique 

combination of morphological, anatomical and chemical characters, without 

or with only exceptional intermediate forms with other species (Fig. 2). 

Moreover, it is here postulated that if new knowledge from genetics would be 

incorporated in such a framework, we would find identifiable genotypic 

clusters (Mallet 1995) that would correlate very well with the large cubes of 

Fig. 2, that are the species. In the case of sibling species (not detected by 

morphological, anatomical and chemical studies) we would find smaller, 

well-defined genotypic clusters inside a large cube of Fig. 2. 

Character Analysis in the Genus Usnea 

Analysis of character variation 

It is evident that a prerequisite of this concept is a correct analysis of the 

characters displayed in the genus. As far as it is possible, each potential 

character should be investigated and its variation analysed. The distinction 

between characters that are strongly modified by environmental factors and


Fig. 2. Three-dimensional graphic representation of the author’s species concept in Usnea. Each 

species (large cubes) consists of a ‘ cloud ’ of individuals (small cubes) characterized by the 

correlation of at least two characters (chemistry and anatomy in species a, morphology and 

chemistry in species b, all three characters are correlated in species c, A few ‘ hybrids ’ (see text) 

can be seen (small cubes outside the large cubes). 

those that are not, or little modified by these factors, is the key element for a 

successful study. In lichenized ascomycetes, such a distinction is not easy 

because it is not possible to cultivate them under different controlled 

environmental parameters. The following recommendations might be of some 

help in this search for taxonomically ‘ useful ’ characters: 

(1) Field studies are essential to understand the behaviour of certain 

characters under different environmental conditions. When possible, one 

should look especially at specimens exposed, for instance, to extreme conditions 

of light (shady or sun exposed localities) or of humidity and analyse 

the resulting modifications of the characters. In Usnea, for instance, this kind 

of observation was very useful to establish the strong variability of the colour 

of the thallus with respect to exposure (see below). Furthermore, in such 

environments, it is extremely useful to study other genera with the same 

growth form and look at the possible parallel modification of their characters. 

For instance, the study of Pseudevernia furfuracea (L.) Zopf and Bryoria 

fuscescens (Gyeln.) Brodo & D. Hawksw. growing in mixed stands with Usnea 

lapponica in windy and exposed situations at higher altitudes was important for 

interpreting the compact form of U. lapponica as a modification due to extreme 

conditions (see Fig. 13). 

(2) Interpretation of characters should be based on the analysis of numerous 

specimens collected ideally across the whole distribution range of the species 

studied. It is, for instance, unlikely that a peculiar character state will be 

present only in one or two specimens if this condition is fulfilled. In this case


% of utilization in keys 

16 

14 

12 

10 

8 

6 

4 

2 

Soralia 

Chemistry 

Shape of branches 

Isidiomorphs 

Fibrils 

Papillae-tubercles 

Pigmentation of basal part 

0 

Characters 

Fig. 3. Main characters used by taxonomists in Usnea, indicated in percentages of utilization in 

various identification keys. The following identification keys were screened: black bars (Motyka): 

keys of sections and subsections of the subgenus Euusnea in Motyka (1936–1938); white bars 

(Clerc and co-workers): keys in Clerc (1987a, 1992), Clerc & Herrera-Campos (1997), Halonen 

et al. (1998); Herrera-Campos et al. (1998), Clerc (unpublished key of Usnea in South Eastern 

North America). 

such an occurrence would suggest with high probability a special kind of 

modification due, for example, to the presence of a parasite or a teratologic 

change. Usnea leucosticta Vain., an extremely rare species (!), characterized 

by its strongly capitate form with farinose soredia, without equivalent in the 

genus, is certainly such an example. As a corollary, descriptions of new 

species of lichenized ascomycetes should not be made on the basis of scant 

collections. 

Discussion of characters used in Usnea 

Figure 3 gives the main characters used by taxonomists in the genus Usnea. 

The diagnostic value of these characters in delimiting species is discussed 

below in the order of their growing importance in modern taxonomic 

treatment of this genus. 

Colour of the thallus 

I agree with Swinscow & Krog (1978) that the colour of the thallus is not a 

good diagnostic character. Thalli exposed to strong insolation are typically 

yellowish green, due to the probable high concentration of usnic acid in the 

CMA 

Brilliancy of cortex 

Habitus 

Length of thallus 

Foveoles 

Colour 

Geography 

Thickness of cortex 

Rigidity of thallus


cortex, supposed to protect the algae against excessively strong light (Henssen 

& Jahns 1974). Thalli in shady situations are greyish green to glaucous, 

probably reflecting the low concentration of usnic acid in the cortex. All 

intermediates can be seen between these two extremes. Moreover colour 

perception is highly subjective and differs among different people. Usnea 

glaucescens Vain. is such a species described on the basis of the colour of the 

thallus. It possesses all the diagnostic characters of U. hirta (L.) Wigg. and is 

probably a shade form of this species (Clerc 1987a). 

Thickness of main branches 

This character is highly variable within species and can thus rarely be used 

to differentiate closely related species. Most of the taxa have an average branch 

diameter of between 0·7 mm and 1·5 mm. Known exceptions are U. trichodea 

Ach., U. merrillii Motyka and U. cavernosa Tuck. in Agassiz, that regularly 

have very thin branches, between 0·4 mm and 0·7 mm thick. Some taxa, 

like the North American protocetraric chemotype of U. cornuta Körb. have 

distinctly larger branches, between 1·2 and 3·1 mm. 

Geography 

Lichenized ascomycetes belong to a very old group of organisms 

(Hawksworth & Hill 1984) and many species have a very large, sometimes 

worldwide distribution, such as some species of Cladonia Hill ex Browne 

(Stenroos 1989a, b). The genus Usnea is not an exception to this and many 

species such as, for example, U. hesperina Motyka (Fig. 4), U. hirta (Clerc 

1997) and U. dasaea Stirt. (Clerc & Herrera-Campos 1997) have nearly 

worldwide distribution. To use the geographic criterion a fortiori, as done by 

Motyka (1936–1938), has led to the situation shown in Fig. 4 where the same 

taxon has been described many times in different parts of the world. 

Foveoles 

Foveoles, nearly circular depressions in the cortex (Figs. 5, 6, 8) or 

transverse furrows (Fig. 7), seem to originate from mechanical disturbances of 

the cortex. Moreover, evidence of regeneration of the cortex are often 

associated with foveoles. This hypothesis is supported by Fig. 11, showing that 

two species with a thin cortex, U. hirta (see Fig. 9 as well) and U. cavernosa, 

are regularly associated with such structures, whereas U. subfloridana Stirt. 

and U. madeirensis Motyka (see Fig. 10 as well) with a thick cortex never have 

foveoles. Usnea rigida s. lat. with a somewhat intermediate cortex thickness 

may or may not have foveoles or similar structures. As a consequence, this 

character should be used with caution, in connection with the thickness of the 

cortex. Usnea foveata Vain. corresponds to a strongly foveolated morphotype 

of U. hirta (Clerc 1992). 

Length of thallus 

This is a very variable character that has been exaggeratedly used as a 

diagnostic character in the past. If a few species, for example, U. esperantiana 

P. Clerc, U. glabrata Vain. and U. wirthii P. Clerc are characterized by very 

short thalli, between 2 and 5 cm, most of the species have a length between


Fig. 4. World distribution of U. hesperina. Described by Motyka (1936–1938) in Europe and 

North America, this species corresponds to U. subplicata (Vain.) Motyka, U. subgracilis Vain. in 

Central America and U. elongata Motyka in South America. In Africa, this species has been 

included in the U. gigas aggr. (Swinscow & Krog 1978, fig. 19) or left as an ‘ undetermined 

morphotype ’ (Swinscow & Krog 1978, fig. 25). In southeastern Asia, U. hesperina has been 

described under the name U. schadenbergiana Göpp. & Stein. The type of U. pseudomontis-fuji 

Asah. has not been seen by the author but the description in the protologue fits quite well with 

that of U. hesperina. A proposal will be made elsewhere to conserve U. hesperina against 

U. schadenbergiana and U. subgracilis. 

2 and 15 cm (shrubby to subpendulous taxa) or between 2 cm and more than 

2 m (pendulous taxa). Moreover, the length of the thallus is strongly associated 

with the age of the lichen and with environmental conditions. Thalli of 

U. filipendula Stirt. are, for instance, much shorter in areas influenced by air 

pollution. 

Habitus 

This character allows rough separation of most of the species in two groups: 

shrubby to subpendulous and pendulous taxa. However, it should be used 

with caution for two reasons: (1) Some species such as, for instance, U. 

madeirensis (see Clerc 1991, Figs 1, 2), U. subscabrosa Motyka. (Fig. 12) and 

U. rubicunda Stirt. are very variable in this respect and might display all three 

types of habitus (Clerc 1991, 1992); (2) Taxa that are normally pendulous 

might appear shrubby when young or when living under environmental 

conditions that are not optimal. Short and compact thalli are characteristic for 

harsh environments, for instance exposure to strong winds at higher altitude 

(Fig. 13). Many Usnea species show compact forms, as for example U. 

subfloridana, U. substerilis Motyka, U. diplotypus Vain., U. lapponica Vain. 

(Fig. 13d). Usnea compacta Motyka corresponds to U. glabrescens (Vain.) Vain. 

(Clerc 1987a).


Figs 5–10. Characters used in Usnea. Figs 5,6. Foveoles of U. cavernosa, Arnold Exs. 573 (G). 

Fig. 7. Transverse furrows of U. hirta, Arnold Exs. 967 (G). Fig. 8. Foveoles of U. cavernosa, Regel 

(G). Fig. 9. Longitudinal section of branch of U. hirta, Arnold Exs. 967 (G). Fig. 10. Longitudinal 

section of branch of U. madeirensis, Feuerer 28 vii 1987 (G). Scales: Figs 5–8=1 mm; Figs 

9–10=0·5 mm.


Number of specimens 

60 

50 

40 

30 

20 

10 

0 

1–1.5 

FOVEOLES 

2–2.5 

3–3.5 

4–4.5 

+ 

5–5.5 

6–6.5 

7–7.5 

8–8.5 

Thickness of the cortex 

Brilliancy of cortex 

Although it might in some cases be difficult to decide whether the cortex 

(longitudinal section) is matt or glossy (especially in old herbarium specimens), 

it seems to be a constant feature in most of the species (Clerc 1987). 

For instance, U. subscabrosa typically has a thick and glossy cortex (Fig. 14). 

It is a most important character providing phylogenetic clues in the genus 

(Clerc & Herrera-Campos 1997). For example, in the U. fragilescens aggr. (U. 

cornuta Koerb., U. esperantiana, U. fragilescens Lynge, U. glabrata, U. wirthii), 

a glossy cortex is strongly correlated with a thin cortex and lateral branches 

that are constricted at the base. 

Thickness of cortex, medulla and central axis 

Following the method described by Clerc (1984, 1987a), and for each 

species studied, width of the cortex, the medulla and the axis can be 

represented statistically (Fig. 11). Once the variation of each of these 

characters is understood, these data provide valuable information to understand 

and delimit taxa. As an example, width of the medulla is an important 

additional character helping to separate U. madeirensis from U. subfloridana 

and U. wasmuthii (Clerc 1991) or to synonymize two taxa (Clerc 1994). 

9–9.5 

10–10.5 

FOVEOLES 

– 

Fig. 11. Correlation between the thickness of the cortex and the occurrence of foveoles. Species 

with a thin cortex (2–6·5%) such as U. cavernosa (thick solid line on the left) and U. hirta (thin 

broken line on the left) are richly foveolate. In contrast, U. madeirensis (thick broken line on the 

right) and U. subfloridana (middle-sized broken line on the right) characterized by a thicker cortex 

(7–16%) do not have foveoles. Usnea rigida s. lat. (thin solid line), with a somewhat intermediate 

cortex thickness, is variable concerning the presence/absence of foveoles. 

11–11.5 

12–12.5 

13–13.5 

14–14.5 

15–15.5 

16–16.5 

17–17.5 

18–18.5

Figs. 12–13. Variability of characters in Usnea. Fig. 12. Habitus of U. subscabrosa, thalli 

shrubby-erect to long pendulous, Purvis et al. 5155 (BM). Fig. 13. Compact forms of Hypogymnia 

physodes (L.) Nyl. (a), Pseudevernia furfuracea (b), Bryoria fuscescens (c) and Usnea lapponica (d), on 

Pinus cembra, on ridge exposed to strong winds at 2200 m., Aletsch area, Valais, Switzerland, Clerc 

13 viii 1997 (G). Scales=20 mm.


Pigmentation of basal part 

As already stated by Motyka (1947), it is important to collect Usnea with the 

basal part. The pigmentation in the first few millimetres from the fixation 

point is diagnostic for many species and does not seem to be much influenced 

by environmental conditions. For instance, this part of the thallus in U. florida 

(L.) Wigg. em Clerc, U. subfloridana and U. madeirensis is always pigmented jet 

black. Usnea subscabrosa typically has a reddish pigment close to the basal part, 

whereas in U. ceratina Ach., this pigment is tinged orange (but not always 

present, however). Many species such as U. flammea Stirt., U. glabrata, and 

U. hirta have no special pigments and their base is of the same colour as the 

main branches or slightly paler. In some species, this character is not constant, 

as for instance in U. cornuta and U. lapponica, where the bases might be 

pigmented black or not. 

Papillae—tubercles 

The diagnostic value of these small and short cortex outgrowths (Fig. 15) 

(presence of medulla in tubercles) is somewhat difficult to evaluate, for the 

reason that their growth and development seem to be largely influenced by 

environmental parameters. However, the potential of each species to produce 

these structures or not seems to have an important diagnostic value. Usnea 

cavernosa, U. hirta and U. glabrata, for instance, never seem to produce 

papillae. Myllys (1994) and Halonen & Puolasmaa (1995) mention, however, 

the occurrence of ‘ few and indistinct ’ papillae in some specimens of the two 

latter species. These specimens need to be studied further because it is at 

present impossible to differentiate morphologically papillae with a very early 

developmental stage of fibrils appearing at the surface of the cortex. Ontogenetical 

studies of papillae, tubercles and fibrils are badly needed here. Most 

of the other species have the potential to produce papillae in varying amounts, 

from very few (sometimes totally absent in some specimens) to numerous. 

Short tubercle-like structures appearing after the breaking-away of fibrils were 

named fibercles by Clerc & Herrera-Campos (1997). It is important to 

differentiate them from true tubercles since they have a different origin. 

Fibrils 

These short branch-like appendages ( Fig. 15) 8 with a central axis that is not 

attached to the central axis of the branches on which they occur (Clerc & 

Herrera-Campos 1997) are probably in many species efficient short-range 

propagules (for example, in U. filipendula and U. dasaea). To my knowledge, 

U. cavernosa is the only species where fibrils are said to be absent. Here again, 

however, it is very difficult to distinguish morphologically between a fibril and 

a young branch in its early development. The density of fibrils on branches, 

their localization on the thallus (close to the basal part only or on the whole 

thallus), their length (fibrils shorter than 2–3 mm are usually called spinules) 

and their disposition in a ‘ fish-bone like ’ appearance (Swinscow & Krog 

1978) are useful diagnostic characters in some species. 

Isidiomorphs 

So far, I have seen no true isidia in this genus. What have been called isidia 

in Usnea (Fig. 16) are morphologically and anatomically identical with the


Figs. 14–15. Characters used in Usnea. Fig. 14. Brilliancy of the cortex (arrows) of U. 

subscabrosa, Clerc 89/108 (G). Fig. 15. Papillae (thick arrows) and fibrils (thin arrows) of 

U. lapponica, Bonner 2 iv 1961 (G). Scale=0·5 mm. 

‘ isidioid spinules ’ in Bryoria furcellata (Fr.) Brodo and D. Hawksw. (Brodo 

& Hawksworth 1977). Detailed anatomical and ontogenetic studies are 

needed at this level but preliminary observations suggest that these structures 

are not formed as an outgrowth of the cortex, but from medulla 

hyphae after perforation of the cortex, initiating the development of soralia 

once they break away. This would be in accordance with Hawksworth & Hill 

(1984) describing them as ‘ soredia starting to grow into filaments while 

still in soralia . . . in some fruticose Bryoria and Usnea species ’. The 

presence or absence of isidiomorphs in soralia is a very important diagnostic 

character in Usnea. Some species, such as U. esperantiana, U. lapponica or 

U. fulvoreagens (Räs.) Räs., never produce isidiomorphs. In other species 

with soralia, it is important to note whether isidiomorphs are present or 

not on young and/or mature soralia. For instance, in U. glabrescens isidiomorphs 

are rare on young soralia and totally absent on mature ones, whereas 

in U. wasmuthii,. they are present in young soralia and rare in mature


Figs. 16–17. Characters used in Usnea. Fig. 16. Isidiomorphs of U. mutabilis Stirt., Clerc 88/96 

(G). Fig. 17. Lateral branches of U. glabrata (Ach.) Vain. distinctly narrowed at point of 

attachment, Müller 30 ix 1867 (G). Scales: Fig. 16=0·5 mm; Fig. 17=1 mm. 

ones. Usnea subfloridana has mature soralia characteristically covered with 

isidiomorphs. 

Shape of branches 

The shape of main branches (in the sense of Swinscow & Krog 1978) in 

transverse and longitudinal sections is important taxonomically, especially in 

pendulous species (for details, see Herrera-Campos et al. 1998). Secondary 

branches may be distinctly narrowed or not at their point of attachment (Fig. 

17) and this character has proved to be especially important in the taxonomy 

of this genus (Clerc 1987a). Species with secondary branches clearly constricted 

at their base and other highly correlated characters constitute a 

distinct group inside the genus (see under brilliancy of cortex). 

Chemistry 

Usnea species produce a range of depsides, depsidones and fatty acids 

(Asahina 1956; Culberson 1969; Fiscus 1972; Swinscow & Krog 1988;


Clerc 1992). In Usnea, as in lichenology in general, lichen secondary 

substances have turned out to be an indispensable guide to taxonomic studies 

(see Fig. 3). The use of chemical characters in lichen taxonomy has been 

subject to numerous discussions (Hawksworth 1976; Brodo 1986). One of the 

most revolutionary papers in modern lichenology, published by Culberson 

et al. (1988) on gene flow in lichens, has brought new insights so far as sexually 

reproducing species are involved, but no general acceptance has been found 

on this matter to date. It would be outside of the scope of this article to discuss 

in detail this problem but below I briefly explain my own concept concerning 

the use of lichen substances in the taxonomy of Usnea. 

More than 8000 TLC analyses coupled with morphological and anatomical 

studies have caused me to give, in Usnea, the same weight to chemical data as 

to morphological or anatomical data. That means that a chemical character 

has a diagnostic value at the species level, only when it is strongly correlated 

with one or more morphological or anatomical characters. Consequently I do 

not accept chemical species, and thus individuals differing only by their 

chemistry are treated as chemotypes. 

Soralia 

Soralia are a most important character in the taxonomy of Usnea (see 

Fig. 3). Not only the presence versus the absence of soralia is important but 

likewise their morphology (Clerc 1987b) and the structures on which they 

start their development (Clerc & Herrera-Campos 1997). When looking at the 

morphology of soralia, it is essential, as for other characters such as spores in 

Rinodina or Buellia species, to observe and describe only mature soralia, on 

well-developed specimens that are not parasitized by lichenicolous fungi or 

other organisms. Under these conditions most of the asexual species can 

usually be very well characterized by their soralia. 

An important exception to the species concept described above is made 

when considering primary versus secondary species (Poelt 1970, 1972). For 

these pairs, the unique difference seems indeed to be the presence versus the 

absence of soralia. In connection with this, I would like to make two 

comments: 

(1) Even if there is good evidence for some of them, it is not yet proved at 

all that the species pairs mentioned in the literature are really species pairs 

sensu Poelt (1970), meaning that the asexual taxon is genetically derived from 

the sexual one. Moreover more than two taxa may be involved (Poelt 1994). 

Furthermore, Mattsson & Lumbsch (1989) discuss the possibility that an 

asexual taxon would have to generate a new species, so that the problem seems 

to be much more complicated than initially thought. Only studies with the 

help of molecular biology techniques will give, in each particular case, more 

information. If the same fungus is really involved in both sexual and asexual 

taxa of one species pair, then nomenclatural consequences will have to follow 

(see Tehler 1982). As the name of a lichen applies to the fungal component, 

both names will have to be synonymized. To distinguish between these two 

conspecific forms, we could then adopt a terminology of the type proposed by 

Heidmarsson et al. (1997) concerning a parallel case, the photosymbiodemes 

(Armaleo & Clerc 1991).


(2) At the moment, and waiting for further molecular evidence on the 

problem, I agree with Du Rietz (1924) and Mattsson & Lumbsch (1989) that 

differences in distribution and/or ecology are important features to be 

considered when ranking the member of a species pair. In Europe there is so 

far only one Usnea species pair known for sure: U. florida (fertile species) 

and U. subfloridana (sterile species). They are morphologically, anatomically 

and chemically indistinguishable and few individuals with both apothecia and 

soralia have been found, especially in Great Britain. However, both species 

have quite different types of distribution and ecology. Usnea florida has 

a suboceanic pattern of distribution (Clerc 1984) and grows mostly on 

deciduous trees in localities with high atmospheric humidity. In central 

Europe, it is a species in danger of extinction (Clerc et al. 1992). Usnea 

subfloridana has a much wider pattern of distribution and can be found in more 

continental areas (Frey 1952). Moreover, this species grows more often on 

conifers and in drier localities. In species pairs occurring mainly in tropical 

areas, the asexual taxa have usually a much wider pattern of distribution than 

the sexual ones, the latter occurring often only in a very restricted area 

(Herrera-Campos et al. 1998). 

For all these reasons, I prefer, at the moment, to keep both the sexual and 

the asexual taxa as distinct species. 

Usnea—Why This Reputation for Being Such a Difficult Genus? 

The genus Usnea is often mentioned as crux lichenogorum by lichenologists 

themselves. Intrinsic and extrinsic factors can be held to be responsible for this 

bad taxonomic reputation: 

Intrinsic factors 

From field experience, it is evident that fruticose lichens seem to be 

especially prone to morphological variation through environmental influences. 

When studying Usnea in the field, under different ecological conditions, it 

is evident that environmental parameters such as light or humidity exert 

profound effects on thallus morphology of the species. Characters such as 

colour and length of the thallus, density of ramifications and papillae seem to 

react with an enormous plasticity to the variation of these factors. It is certainly 

not a coincidence if Bryoria, another genus of fruticose, pendulous species 

showing considerable morphological variation, has nearly the same type of 

reputation. 

Extrinsic factors 

Extrinsic factors are those considered in relation to the philosophical and 

scientific environment of the taxonomist and the taxonomist himself. Some 

of these factors might explain why a taxonomist is influenced by a certain 

concept or why he or she puts strong emphasis on certain characters, and thus 

might have played a major role in making Usnea a very difficult genus to 

understand at species level.


(1) In the 1930s, populational thinking was not yet a widespread philosophical 

approach among lichenologists. Magnusson (Weber 1968) and 

Motyka were the most typical examples of taxonomists that have adopted a 

strong typological view of species (see Fig. 1 and discussion above). In a letter 

to E. Frey dated 25 i 1949, Motyka wrote the following sentences: ‘ Vous 

n’avez pas dans votre herbier que peu de formes typiques. Usnea maxima, 

U. cembricola, U. freyi, etc., bien faciles à reconnaître dans leurs types, sont 

dans votre herbier souvent difficiles à classifier. Doit-on les nommer d’une 

telle ou telle autre manière? Faire d’elles des variétés ou des formes, ou les 

classifier comme ‘ formae intermediae ’?’ This statement illustrates at best the 

typological way of thinking of J. Motyka. In a genus where modification is 

responsible for a great diversity of form, this typological approach of the 

species has resulted in a high number of ‘ one character species ’ with an 

unreasonably inflated nomenclature as an ultimate consequence. It is thus 

not totally surprising that this genus appears now totally unworkable to 

contemporary lichenologists. 

(2) There are striking differences in the choice and weighting of characters 

between Motyka (1936–1938) and recent workers (Clerc and co-workers, see 

legend Fig. 3) as shown in Fig. 3. Three groups of characters can be 

distinguished: 

(A) Characters used by Motyka but not or very occasionally by Clerc and 

co-workers: colour of the thallus, thickness of branches, foveoles and length of 

thallus. Environmental factors are supposed to exert profound effects on these 

characters (see discussion above for each of these characters); 

(B) Characters used both by Motyka and Clerc and co-workers: habitus, 

papillae-tubercles, fibrils, shape of branches, chemistry and soralia. However, 

even if these characters have been recognized as important by everybody, they 

have not been used in the same way. For instance, concerning the soralia, 

Motyka used mainly the presence versus absence of these structures, whereas 

Clerc and co-workers used mainly the morphology, the size and the presence 

versus the absence of soralia. Motyka used mainly the density and morphology 

of papillae whereas Clerc and co-workers restricted their use of papillae mainly 

to their presence versus absence. 

(C) Characters used only by Clerc and co-workers and not or very 

occasionally by Motyka: brilliancy of cortex, thickness of internal structures 

(CMA), pigmentation of basal part and isidiomorphs. All these characters give 

very important additional information to understand and delimit Usnea 

species. 

The emphasis given by Motyka on characters (group A) that are strongly 

modifiable by external parameters is, to my opinion, one of the main reasons 

for the confusion existing in the taxonomy of Usnea. This, added to Motyka’s 

narrow typological view of the species, makes it very difficult for lichenologists 

to identify material with the help of Motyka’s monograph. 

(3) The physical shape and health of a taxonomist might play an important 

role and have decisive consequences on the circumscription of species. When 

he was young, J. Motyka lost an eye during an unsuccessful surgery operation. 

Every systematist knows how continuous observation under the microscope 

is stressful for the eyes. Having only one eye was probably a considerable


handicap for Motyka and could have, for instance, influenced, consciously or 

not, his choice of taxonomically important characters in the genus. With only 

one eye, it is indeed less tiresome to look at the colour of the thallus, the 

habitus, the length of the thallus, the thickness of branches, the rigidity of the 

thallus (Fig. 3) than at some small morphological differentiation of soralia or 

at isidiomorphs, or at small variations in the brilliancy of the cortex. 

(4) The technical advancement of science is another important factor. 

When he was writing his monograph, Motyka had only the K test solution at 

his disposal, which is nothing in comparison with TLC or HPLC today. The 

K reagent is often misleading, the intensity of the reaction depending strongly 

on variations in the concentrations of the lichen substances tested. This shows 

in the monograph (Motyka 1936–1938) where many of the indications based 

on the K solution are unreliable, adding to the difficulty of identifying species. 

(5) Motyka did not have the opportunity to travel widely outside Poland in 

areas where Usnea is most diverse and abundant. Of most of the new species 

based on material collected outside Europe, he saw only herbarium specimens. 

Without the possibility of making extended field studies (see discussion 

above under character analysis in the genus), it was very difficult for him to 

understand the variation of his characters, thus reinforcing him in his 

typological view of the species. 

Conclusion 

I hope that this article will contribute to a rehabilitation and a better 

understanding of this genus. I would like to emphasize that the relatively high 

number of species recently reduced to synonymy (Clerc 1987a, 1994, 1997; 

Clerc & Herrera-Campos 1997) is in no way due to an exaggerated lumping 

as a consequence of a too-wide species concept. A careful reinterpretation of 

the characters used in the past, in the light of a modern understanding of the 

infraspecific variation, extended field and herbarium studies, detailed observations 

of types and routine utilization of TLC have given the present author 

a wide understanding of the species and their variation. More than 770 names 

have been published worldwide (Clerc unpublished) and probably some 50% 

of them will have to be reduced to synonymy. On the other side, however, the 

genus is still poorly known, especially in the tropics, and it is expected that 

many new species will be described in the future. 

My sincerest thanks for my colleagues and friends Maria Herrera-Campos (Arizona State 

University), Pekka Halonen (University of Oulu), Daniel Jeanmonod (CJB) and Mathias Vust 

(CJB) for stimulating collaboration and discussions. I would like to thank Mathias Vust for 

drawing Figs 1 and 2. Last but not least I would like to express my gratitude to Mats Wedin and 

William Purvis who organized this meeting in London, perfectly. 

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Accepted for publication 18 April 1998

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