Ecology is the science that deals with the habitats of plants and their response to the environment of climate or of substratum. Ecology in the lichen kingdom is habitat “writ large,” and though it will not be possible in so wide a field to enter into much detail, even a short examination of lichens in this aspect should yield interesting results, especially as lichens have never, at any time, been described without reference to their habitat. In very early days, medicinal Usneas were supposed to possess peculiar virtues according to the trees on which they grew and which are therefore carefully recorded, and all down the pages of lichen literature, no diagnosis has been drawn up without definite reference to the nature of the substratum. Not only rocks and trees are recorded, but the kind of rock and the kind of tree are often specified. The important part played by rock lichens in preparing soil for other plants has also received much attention Several comprehensive works on Ecology have been published in recent times and though they deal mainly with the higher vegetation, the general plan of study of land plants is well adapted to lichens. A series of definitions and explanations of the terms used will be of service: Thus in a work by Moss 1. “A plant formation is the whole of the vegetation which occurs on a definite and essentially uniform habitat.”—All kinds of plants are included in the formation, so that strictly speaking a lichen formation is one in which lichens are the dominant plants. Cf. p. 394. The term however is very loosely used in the literature. A uniform habitat, as regards lichens, would be that of the different kinds of soil, of rock, of tree, etc. 2. “A plant association is of lower rank than a formation, and is characterized by minor differences within the generally uniform habitat.”—It represents a more limited community within the formation. 3. “A plant society is of lower rank than an association, and is marked by still less fundamental differences of the habitat.”—The last-named term represents chiefly aggregations of single species. Moss adds that: “plant community is a convenient and general term used for a vegetation unit of any rank.” Climatic conditions and geographical position are included in any consideration of habitat, as lichens like other plants are susceptible to external influences. Ecological plant-geography has been well defined by Macmillan B. External InfluencesThe climatic factors most favourable to lichen development are direct light (already discussed) a. Temperature. Lichens, as we have seen, can endure the heat of direct sunlight owing to the protection afforded by thickened cortices, colour pigments, etc. Where such heat is so intense as to be injurious the gonidia succumb first Lichens endure low temperatures better than other plants, their xerophytic structure rendering them proof against extreme conditions: the hyphae have thick walls with reduced cell lumen and extremely meagre contents. Freezing for prolonged periods does them little injury; they revive again when conditions become more favourable. Efficient protection is also afforded by the thickened cortex of such lichens as exist in Polar areas, or at high altitudes. Thus various species of Cetrariae with a stout “decomposed” amorphous cortex can withstand very low temperatures and grow freely on the tundra, while Cladonia rangiferina, also a northern lichen, but without a continuous cortex, cannot exist in such cold conditions, unless in localities where it is protected by a covering of snow during the most inclement seasons. b. Humidity. A high degree of humidity is distinctly of advantage to the growth of the lichen thallus, though when the moist conditions are excessive the plants become turgid and soredial states are developed. The great abundance of lichens in the western districts of the British Isles, where the rainfall is heaviest, is proof enough of the advantage of moisture, and on trees it is the side exposed to wind and rain that is most plentifully covered. A series of observations on lichens and rainfall were made by West Though a fair number of lichens are adapted to life wholly or partly under water, land forms are mostly xerophytic in structure, and die off if submerged for any length of time. The Peltigerae are perhaps the most hydrophilous of purely land species. Many Alpine or Polar forms are covered with snow for long periods. In the extreme north it affords more or less protection; and Kihlman c. Wind. Horizontal crustaceous or foliose lichens are not liable to direct injury by wind as their close adherence to the substratum sufficiently shelters them. It is only when the wind carries with it any considerable quantity of sand that the tree or rock surfaces are swept bare and prevented from ever harbouring any vegetation, and also, as has been already noted, the terrible winds round the poles are fatal to lichens exposed to the blasts unless they are provided with a special protective cortex. After crustaceous forms, species of Cetraria, Stereocaulon and Cladonia are best fitted for weathering wind storms: the tufted Upright fruticose lichens are necessarily more liable to damage by wind, but maritime Ramalinae and Roccellae do not seem to suffer in temperate climates, though in regions of extreme cold fruticose forms are dwarfed and stunted. The highest development of filamentous lichens is to be found in more or less sheltered woods, but the effect of wind on these lichens is not wholly unfavourable. Observations have been made by Peirce Schrenk d. Human Agency. Though lichens are generally associated with undisturbed areas and undisturbed conditions, yet accidents or convulsions of nature, as well as changes effected by man, may at times prove favourable to their development. The opening up of forests by thinning or clearing will be followed in time by a growth of tree and ground forms; newly planted trees may furnish a new lichen flora, and the building of houses and walls with their intermixture of calcareous mortar will attract a particular series of siliceous or of lime-loving lichens. A few lichens are partial to the trees of cultivated areas, such as park-lands, avenues or road-sides. Among these are several species of Physcia: Ph. pulverulenta, Ph. ciliaris and Ph. stellaris, some species of Placodium, and those lichens such as Lecanora varia that frequently grow on old palings. On the other hand lichens are driven away from areas of dense population, or from regions affected by the contaminated air of industrial centres. In our older British Floras there are records of lichens collected in London during the eighteenth century—in Hyde Park and on Hampstead Heath—but these have long disappeared. A variety of Lecanora galactina seems to be the only lichen left within the London district: it has been found at Camden Town, Notting Hill and South Kensington. So recently as 1866, Nylander The question of atmospheric pollution in manufacturing districts and its effect on vegetation, more especially on lichen vegetation, has received special attention from Wheldon and Wilson Saxicolous lichens have also suffered severely in South Lancashire; not only the number of species, but the number of individuals is enormously reduced and the specimens that have persisted are usually poorly developed. The smoke-producing towns are situated in the valley-bottoms, and the smoke rises and drifts on to the surrounding hills and moorlands. The authors noted that crustaceous rock-lichens were in better condition on horizontal surfaces such as the copings of walls, or half-buried stones, etc. than on the perpendicular or sloping faces of rocks or walls. This was probably due to what they observed as to the effect of water trickling down the inclined substrata and becoming charged with acid from the rock surfaces. They also observed further that a calcareous substratum seemed to counteract the effect of the smoke, the sulphuric acid combining with the lime to form calcium sulphate, and the surface-washings thus being neutralized, the lichens there are more favourably situated. They found in good fruiting condition, on mortar, cement or concrete, the species Lecanora urbana, L. campestris, L. crenulata, Verrucaria muralis, V. rupestris, Thelidium microcarpum and Staurothele hymenogonia. Some of these occurred on the mortar of sandstone walls close to the town, “whilst on the surface of the sandstone itself no lichens were present.” Soil-lichens were also strongly affected, the Cladoniae of the moorlands being in a very depauperate condition, and there was no trace of Stereocaulon or of Sphaerophorus species, which, according to older records, previously occurred on the high uplands. The influence of human agency is well exemplified in one of the London districts. In 1883 Crombie published a list of the lichens recorded from R. Paulson and P. Thompson C. Lichen CommunitiesLichen communities fall into a few definite groups, though, as we shall see, not a few species may be found to occur in several groups—species that have been designated by some workers as “wanderers.” The leading communities are: 1. Arboreal, including those that grow on leaves, bark or wood. 2. Terricolous, ground-lichens. 3. Saxicolous, rock-lichens. 4. Omnicolous, lichens that can exist on the most varied substrata, such as bones, leather, iron, etc. 5. Localized Communities in which owing to special conditions the lichens may become permanent and dominant. In all the groups lichens are more or less abundant. In arboreal and terricolous formations they may be associated with other plants; in saxicolous and omnicolous formations they are the dominant vegetation. It will be desirable to select only a few of the typical communities that have been observed and recorded by workers in various lands. 1. ARBOREALArboreal communities may be held to comprise those lichens that grow on wood, bark or leaves. They are usually the dominant and often the sole vegetation, but in some localities there may be a considerable development of mosses, etc., or a mantle of protococcaceous algae may cover the bark. Certain lichens that are normally corticolous may also be found on dead wood or may be erratic on neighbouring rocks: Usnea florida for instance is a true corticolous species, but it grows occasionally on rocks or boulders generally in crowded association with other foliose or fruticose lichens. Most of the larger lichens are arboreal, though there are many exceptions: Parmelia perlata develops to a large size on boulders as well as on trees; some species of Ramalinae are constantly saxicolous while there are only rare instances of Roccellae that grow on trees. The purely tropical or subtropical genera are corticolous rather than saxicolous, but species that have appeared in colder regions may have acquired the saxicolous habit: thus Coenogonium in the tropics grows on trees, but the European species, C. ebeneum, grows on stone. a. Epiphyllous. These grow on Ferns or on the coriaceous leaves of evergreens in the tropics. Many of them are associated with Phycopeltis, Phyllactidium or Mycoidea, and follow in the wake of these algae. Observations are lacking as to the associations or societies of these lichens whether they grow singly or in companies. The best known are the Strigulaceae: there are six genera in that family, and some of the species have a wide distribution. The most frequent genus is Strigula associated with Phycopeltis which forms round grey spots on leaves, and is almost entirely confined to tropical regions. Chodat Other genera, such as those of Ectolechiaceae, which inhabit fern scales and evergreen leaves, are associated with Protococcaceae. Pilocarpon leucoblepharum with similar gonidia grows round the base of pine-needles. It is found in the Caucasus. In our own woods, along the outer edges, the lower spreading branches of the fir-trees are often decked with numerous plants of Parmelia physodes, a true “plant society,” but that lichen is a confirmed “wanderer.” Biatorina Bouteillei, on box leaves, is a British and Continental lichen. b. Corticolous. In this series are to be found many varying groups, the type of lichen depending more on the physical nature of the bark than on the kind of trees. Those with a smooth bark such as hazel, beech, lime, etc., and younger trees in general, bear only crustaceous species, many of them with a very thin thallus, often partly immersed below the surface. Among the first to pay attention to the ecological grouping of corticolous lichens was A.L. FÉe
Fries More extensive studies of habitat were made by Krempelhuber Arnold Bruce Fink On trees with smooth bark he records in the Minnesota district:
On rough bark he records:
Finally, as generally representative of the commonest lichens in our woods of deciduous trees, including both smooth- and rough-barked, the community of oak-hazel woods as observed by Watson
And on the soil of these woods:
Paulson c. Lignicolous. Lichens frequenting the branches of trees do not long continue when these have fallen to the ground. This may be due to the lack of light and air, but Bouly de Lesdain On worked wood, such as old doors or old palings, light and air are well provided and there is often an abundant growth of lichens, many of which seem to prefer that substratum: the fibres of the wood loosened by weathering retain moisture and yield some nutriment to the lichen hyphae which burrow among them. Though a number of lichens grow willingly on dead wood, there are probably none that are wholly restricted to such a habitat. A few, such as the species of Coniocybe, are generally to be found on dead roots of trees or creeping loosely over dead twigs. They are shade lichens and fond of moisture. The species on palings—or “dead wood communities”—most familiar to us in our country are:
These may be found in very varying association. It has indeed been remarked that the dominant plant may be simply the one that has first On wood:
On rotten stumps and prostrate logs: Peltigera canina, Cladonia fimbriata var. tubaeformis, Cl. gracilis, Cl. verticillata, Cl. symphicarpia, Cl. macilenta, Cl. cristatella. Except for one or two species such as Buellia turgescens, Cladonia symphicarpia, etc., the associations could be easy paralleled in our own country, though with us Peltigera canina, Cladonia gracilis and Cl. verticillata are ground forms. 2. TERRICOLOUSIn this community other vegetation is dominant, lichens are subsidiary. In certain conditions, as on heaths, they gain a permanent footing, in others they are temporary denizens and are easily crowded out. As they are generally in close contact with the ground they are peculiarly dependent on the nature of the soil and the water content. There are several distinct substrata to be considered each with its characteristic flora. Cultivated soil and grass lands need scarcely be included, as in the former the processes of cultivation are too harassing for lichen growth, and only on the more permament somewhat damp mossy meadows do we get such a species as Peltigera canina in abundance. Some of the earth-lichens are among the quickest growers: the apothecia of Baeomyces roseus appear and disappear within a year. Thrombium epigaeum develops in half a year; Thelidium minutulum in cultures grew from spore to spore, according to Stahl There are three principal types of soil composition: (1) that in which there is more or less of lime; (2) soils in which silica in some form or other predominates, and (3) soils which contain an appreciable amount of humus. Communities restricted to certain soils such as sand-dunes, etc., are treated separately. a. On calcareous soil. Any admixture of lime in the soil, either as chalk, limy clay or shell sand is at once reflected in the character of the lichen flora. On calcareous soil we may look for any of the squamulose Lecanorae or Lecideae that are terricolous species, such as Lecanora crassa, L. lentigera, Placodium fulgens, Lecidea lurida and L. decipiens. There are also the many lichens that grow on mortar or on the accumulated debris mixed with lime in the crevices of walls, such as Biatorina coeruleonigricans, species of Placodium, several species of Collema and of Verrucariaceae. Bruce Fink
This particular association occupied the slope of a hill that was washed by lime-impregnated water. It was normally a dry habitat and the lichens were distinguished by small closely adnate thalli. There are more lichens confined to limy than to sandy soil. Arnold
It is interesting to note how many of these lichens specialized as to habitat are forms of species that grow in other situations. b. On siliceous soil. Lichens are not generally denizens of cultivated soil; a few settle on clay or on sandbanks. Cladonia fimbriata and Cl. pyxidata grow frequently in such situations; others more or less confined to sandy or gravelly soil are, in the British Isles:
These very generally grow in extended societies of one species only. In his enumeration of soil-lichens Arnold
Mayfield On bare heaths of gravelly soil in Epping Forest Paulson and Thompson
And on flints in the soil: Lecidea crustulata and Rhizocarpon confervoides. They found that Peltigera spuria colonized very quickly the burnt patches of earth which are of frequent occurrence in Epping Forest, while on wet sandy heaths amongst heather they found associated Cladonia sylvatica f. tenuis and Cl. fimbriata subsp. fibula. c. On bricks, etc. Closely allied with siliceous soil-lichens are those that form communities on bricks. As these when built into walls are more or less smeared with mortar, a mixture of lime-loving species also arrives. Roof tiles are more free from calcareous matter. Lesdain There are many references in literature to lichens that live on tiles. A fairly representative list is given by Lettau
Several of these are more or less calcicolous and others are wanderers, indifferent to the substratum. Though certain species form communities on bricks, tiles, etc., none of them is restricted to such artificial substrata. d. On humus. Lichens are never found on loose humus, but rocks or stumps of trees covered with a thin layer of earth and humus are a favourite habitat, especially of Cladoniae. One such “formation” is given by Bruce Fink
e. On peaty soil. Peat is generally found in most abundance in northern and upland regions, and is characteristic of mountain and moorland, though there are great moss-lands, barely above sea-level, even in our own country. Such soil is of an acid nature and attracts a special type of plant life. The lichens form no inconsiderable part of the flora, the most frequent species being members of the Cladoniaceae. The principal crustaceous species on bare peaty soil in the British Isles are Lecidea uliginosa and L. granulosa. The former is not easily distinguishable from the soil as both thallus and apothecia are brownish black. The latter, which is often associated with it, has a lighter coloured thallus and apothecia that change from brick-red to dark brown or black; Wheldon and Wilson To quote again from Arnold
The last is a very rare lichen in Central Europe and is generally found on sandy soil. Arnold considered that near Munich, for various reasons, there was a very poor representation of turf-lichens. f. On mosses. Very many lichens grow along with or over mosses, either on the ground, on rocks or on the bark of trees, doubtless owing to the moisture accumulated and retained by these plants. Besides Cladoniae the commonest “moss” species in the British Isles are Bilimbia sabulosa, Bacidia muscorum, Rinodina Conradi, Lecidea sanguineoatra, Pannaria brunnea, Psoroma hypnorum and Lecanora tartarea, with species of Collema and Leptogium and Diploschistes bryophilus. Wheldon and Wilson
As already described Lecanora tartarea g. On fungi. Some of the fungi, such as Polyporei, are long lived, and of hard texture. On species of Lenzites in Lorraine, Kieffer 3. SAXICOLOUSLichens are the dominant plants of this and the following formations, they alone being able to live on bare rock; only when there has been formed a nidus of soil can other plants become established. a. Characters of Mineral Substrata. It has been often observed that lichens are influenced not only by the chemical composition of the rocks on which they grow but also by the physical structure. Rocks that weather quickly are almost entirely bare of lichens: the breaking up of the surface giving no time for the formation either of thallus or fruit. Close-grained rocks such as quartzite have also a poor lichen flora, the rooting hyphae being unable to penetrate and catch hold. Other factors, such as incidence of light, and proximity of water, are of importance in determining the nature of the flora, even where the rocks are of similar formation. b. Colonization on Rocks. When a rock surface is laid bare it becomes covered in time with lichens, and quite fresh surfaces are taken possession of preferably to weathered surfaces The question of colonization Some of these lichens had doubtless come from the boulders, others from the trees; the Cladonias were all known to occur within a few miles, but most of the species had been wind-borne from some distance. The Stereocaulon present did not exist elsewhere in Iowa; it had evidently been brought by the railroad cars, possibly on telegraph poles. A similar wall on the south side of the railway, subject to even more xerophytic conditions but with less disintegration of the surface, had a larger number of individuals though fewer species. Only one Cladonia and one Parmelia had gained a footing, the rest were crustaceous, Buellia myriocarpa being one of the most frequent. There are two types of rock of extreme importance in lichen ecology: those mainly composed of lime (calcareous), and those in which silica or silicates preponderate (siliceous). They give foothold to two corresponding groups of lichen communities, calcicolous and silicicolous. c. Calcicolous. The pioneer in this section of lichen ecology is H. F. Link, who was a Professor of Natural Science and Botany at Rostock, then at Breslau, and finally in Berlin. He Most calcicolous lichens are almost completely dependent on the lime substratum which evidently supplies some constituent that has become necessary to their healthy growth. Calcareous rocks are usually of softer texture than those mainly composed of silica, and not only the rhizoidal hyphae but the whole thallus—both hyphae and gonidia—may be deeply embedded. Only the fruits are visible and they are, in some species, lodged in tiny depressions (foveolae) scooped out of the surface by the lichen-acids acting on the easily dissolved lime. Those obligate lime species may be found in associations on almost any calcareous rock. Watson The flora recorded by Flagey Bruce Fink
Forssell d. Silicicolous. There is greater variety in the mineral composition and in the nature of the surface in siliceous than in calcareous rocks; they are also more durable and give support to a large number of slow-growing forms. Silicon enters into the composition of many different types, from the oldest volcanic to the most recent of sedimentary rocks. Some of these are of hard unyielding surface on which only a few lichens are able to attach themselves. Such a rock is instanced by Servit A difference has been pointed out by Bachmann Other rocks, such as granite, in which the different crystals, quartz, mica and felspar are of varying hardness, are favourite habitats as affording not only durability but a certain openness to the rhizoidal hyphae, though in Shetland, West It will only be possible to give a few typical associations from the many that have been published. Crustaceous forms are the most abundant. On granite and on quartzite not disintegrated Malinowski
On granite and quartzite disintegrated:
On these disintegrated rocks there is a constant struggle for existence between the various species; the victorious association finally consists of Lecanora badia, L. cenisia and Lecidea confluens with occasional growths of the following species:
A number of rock associations have been tabulated by Wheldon and Wilson
On siliceous rocks in West Lancashire the same authors On the Silurian series of rocks in the same district they describe a somewhat brighter coloured flora: “First Stereocaulons invite attention, and greenish or yellowish shades are introduced by an abundance of Lecanora sulphurea, L. polytropa, Rhizocarpon geographicum and Parmelia conspersa, often beautifully commingled with grey species such as Lecidea contigua and L. stellulata, and reddish angular patches of Lecanora Dicksonii. Also an abundance of orbicular patches of Haematomma ventosum with its reddish-brown apothecia.” A brightly coloured association on the cretaceous sand-rocks of Saxon Switzerland has been described as “Sulphur lichens.” These have recently 4. OMNICOLOUS LICHENSSome account must be taken in any ecological survey of those lichens that are indifferent to substrata. Certain species have become so adapted to some special habitat that they never or rarely wander; others, on the contrary, are true vagabonds in the lichen kingdom and settle on any substance that affords a foothold: on leather, bones, iron, pottery, etc. There can be no sustenance drawn from these supports, or at most extremely little, and it is interesting to note in this connection that while some rock-lichens are changed to a rusty-red colour by the infiltration of iron—often from a water medium containing iron-salts—those that live directly on iron are unaffected. The “wanderers” are more or less the same in every locality and they pass easily from one support to another. Bouly de Lesdain The following were noted by Lesdain and other workers: On iron—Xanthoria parietina, Physcia obscura and var. virella, Ph. ascendens, Placodium (flavescens) sympageum, Pl. pyraceum, Pl. citrinum, Candelariella vitellinum, Rinodina exigua, Lecanora campestris, L. umbrina, L. galactina, Lecania erysibe, Bacidia inundata. Xanthoria parietina is one of the commonest wandering species; it was found by Richard On tar—Lecanora umbrina. On charcoal—Rinodina exigua, Lecanora umbrina. On bones—Xanthoria parietina, Physcia ascendens, Ph. tenella, Placodium citrinum, Pl. lacteum, Rinodina exigua, Lecanora galactina, L. dispersa, L. umbrina, Lecania erysibe, L. cyrtella, Acarospora pruinosa, A. Heppii, Bacidia inundata, B. muscorum, Verrucaria anceps, V. papillosa. In Arctic regions in Ellesmere Land and King Oscar Land, Darbishire On charcoal—Rinodina exigua, Lecanora umbrina. On dross or clinkers—Parmelia dubia, Physcia obscura, Ph. ascendens f. tenella, Ph. pulverulenta, Xanthoria parietina, Placodium pyraceum, Pl. citrinum, Rinodina exigua, Lecanora dispersa, L. umbrina, Lecania erysibe. On glass On earthenware, china, etc.—Physcia ascendens f. tenella, Lecanora umbrina, L. dispersa, Lecania (? Biatorina) cyrtella, Verrucaria papillosa, Bacidia inundata. On leather—Nearly fifty species or varieties were found by Lesdain on old leather on the dunes. Cladonias, Parmelias and Physcias were well represented with one Evernia and a large series of crustaceous forms. He adds a note that leather is an excellent substratum: lichens covered most of the pieces astray on the dunes. Similar records have been made in Epping Forest by Paulson and Thompson On pasteboard—Even on such a transient substance as this Lesdain found a number of forms, most of them, however, but poorly developed: Cladonia furcata (thallus), Parmelia subaurifera (beginning), Xanthoria parietina (beginning), Physcia obscura, Placodium citrinum (thallus), Pl. On linoleum—Xanthoria parietina, Physcia ascendens f. tenella, Rinodina exigua, Lecanora umbrina. On indiarubber—Physcia ascendens f. tenella. On tarred cloth—Xanthoria parietina, Placodium citrinum, Pl. pyraceum, Rinodina exigua, Lecanora umbrina, Lecania erysibe, Bacidia inundata. On felt—Bacidia inundata, B. muscorum. On cloth (cotton, etc.)—Bacidia inundata. On silk—Physcia ascendens, Ph. obscura, Placodium citrinum (thallus), Lecanora umbrina, Bacidia inundata. On cord—Physcia ascendens f. tenella, Placodium citrinum (thallus). On excreta—One would scarcely expect to find lichens on animal droppings, but as some of these harden and lie exposed for a considerable time, some quick-growing species attain to more or less development on what is, in any case, an extremely favourable habitat for fungi and for many minute organisms. Paulson and Thompson found tiny fruiting individuals of Cladonia macilenta and Cl. fimbriata var. tubaeformis growing on the dry dung of rabbits in Epping Forest. On the same type of pellets Lesdain records Physcia ascendens f. leptalea, Cladonia pyxidata, Bacidia inundata and B. muscorum; and on sheep pellets: Physcia ascendens f. leptalea and Placodium citrinum; while on droppings of musk-ox in Ellesmere Land Darbishire found Biatorina globulosa, Placodium pyraceum, Gyalolechia subsimilis, Lecanora epibryon, L. verrucosa, Rinodina turfacea and even, firmly attached, Thamnolia vermicularis. It would be difficult to estimate the age of these lichens, but it seems evident that the “wanderers” are all more or less quick growers, and the lists also prove conclusively their complete indifference to the substratum, as the same species occur again and again on the very varied substances. 5. LOCALIZED COMMUNITIESLichens may be grouped ecologically under other conditions than those of substratum. They respond very readily to special environments, and associations arise either of species also met with elsewhere, or of species restricted to one type of surroundings. Such associations or communities might be multiplied indefinitely, but only a few of the outstanding ones will be touched on. a. Maritime Lichens. This community is the most specialized of any, many of the lichens having become exclusively adapted to salt-water surroundings. They are mainly saxicolous, but the presence of sea-water is the factor of greatest influence on their growth and distribution, and they occur The greater or less proximity of the salt water induces in lichens, as in other maritime plants, a distribution into belts or zones which recede gradually or abruptly according to the slope of the shore and the reach of the tide. Weddell In Ireland, a thorough examination has been made of a rocky coast at Howth near Dublin by M. C. Knowles
(1) The Ramalina belt. In this belt there are two zones of lichen vegetation: those in the upper zone consist mainly of barren plants of Ramalina siliquosa Ramalina siliquosum in both zones takes several distinct forms, according to exposure to light, wind or spray, the effects of which are most marked in the upper zone. The plants growing above the ordinary spray zone generally form sward-like growths (Fig. 124); at the higher levels the sward growth is replaced by isolated tufts with a smaller more amorphous thallus which passes into a very small stunted condition. The latter form alone has gained and retained a footing on the steep faces of the hard and close-grained quartzite rocks. “On the western faces, indeed, it is the only visible vegetation.” The dwarfed tufts with lacerated fronds measuring from 1/4 to 1/2 an inch in height are dotted all over the quartzites. On the sea faces the plants are larger, but everywhere they are closely appressed to the rock surface. At lower levels the fronds lengthen to more normal dimensions. “On these steep rock-faces there is a complete absence of any of the crustaceous species. The problem, therefore, as to how the Ramalina has obtained a foothold on these very hard precipitous rocks, which are too inhospitable even for crustaceous species is an interesting and puzzling one.” In the Ramalina zone along with the dominant species there occur occasional tufts of R. Curnowii and R. subfarinacea, the latter more especially in shady and rather moist situations. There are also numerous foliaceous and crustaceous lichens mingling with the Ramalina vegetation (Fig. 125), several Parmelias, Physcia aquila, Xanthoria parietina, Buellia canescens, B. ryssolea, Lecanora atra, L. sordida, Rhizocarpon geographicum and others. In the main these are arranged in the following order descending towards the sea:
Parmelia prolixa is the most abundant of the Parmelias: it covers large spaces of the rocks and frequently competes for room with the Ramalinas, or in other areas with Physcia aquila and Lecanora parella. A number of crustaceous species which form the sub-vegetation of the Ramalina belt, and also on the same level, clothe the steeper rock faces where shelter and moisture are insufficient to support the foliose forms. “In general the sub-vegetation of the eastern and northern coasts is largely composed of species that are common in Alpine and upland regions. This is due to the steepness of the rocks and also to the colder and drier conditions prevailing on these coasts.” An association of Rhizocarpon geographicum, Lecanora (sordida) glaucoma and Pertusaria concreta f. Westringii forms an almost continuous covering in some places, descending nearly to sea-level. On sunnier and moister rocks with a south and south-west aspect the association is of more lowland forms such as Buellia colludens, B. stellulata, Lecanora smaragdula and L. simplex f. strepsodina. (2) The Orange belt. “Below the Ramalinas, and between them and the sea, several deep yellow or orange-coloured lichens form a belt of varying
“On the stones and low shore rocks that lie just above the ordinary high-tide level Placodium lobulatum grows abundantly, covering the rocks with a continuous sheet of brilliant colour.” With these brightly coloured lichens are associated several with greyish thalli such as:
(3) The Lichina vegetation, and (4) The Verrucaria maura belt. These two communities are intermingled, and it will therefore be better to consider them together. There are only two species of Lichina on this or any other shore, L. pygmaea and L. confinis; the latter grows above the tide-level, and sometimes high up on the cliffs, where it is subject to only occasional showers of spray: it forms on the Howth coast a band of vegetation four to five inches wide above the Verrucaria belt. Lichina pygmaea occurs nearer the water, and therefore mixed with and below Verrucaria maura. Those three zones were first pointed out by Nylander Verrucaria maura is one of the most abundant lichens of our rocky coasts, and is reported from Spitzbergen in the North to Graham Land in the Antarctic. It grows well within the range of sea-spray, covering great stretches of boulders and rocks with its dull-black crustaceous thallus. At Howth it is submerged only by the highest spring tides. Though it is the dominant lichen on that beach, other species such as V. memnonia, V. prominula, and V. aquatilis form part of the association, and more rarely V. scotina along with Arthopyrenia halodytes, A. leptotera and A. halizoa. (5) The belt of marine Verrucarias. This association includes the species that are submerged by the tide for a longer or shorter period each day. The dominant species are Verrucaria microspora, V. striatula and V. mucosa. Arthopyrenia halodytes is also abundant; A. halizoa and A. marina are more rarely represented. Among the plants of Fucus spiralis, Verrucaria mucosa, the most wide-spreading of these marine forms, is “very conspicuous as a dark-green, almost black, band of greasy appearance stretching along the shore.” When growing in the shade, the thallus is of a brighter green colour. An examination
A somewhat similar series of “formations” was determined by Sandstede
While in a higher position on similar boulders:
And more rarely a few species of Lecidea. b. Lichens of Sand-dunes. These lichens might be included with those of the terricolous communities, but they really represent a maritime community of xerophytic type, subject to the influence of salt spray but not within reach of the tide. They are sun-lichens and react to the strong light in the deeper colour of the thallus. In such a sun-baked area at Findhorn a luxuriant association of lichens was observed growing among short grass and plant debris. It consisted chiefly of:
On very arid situations the species of Cladonia are those that have a well-developed rather thick primary thallus, probably because such a thallus is able to retain moisture for a prolonged period When the loose sand on the dunes of South Lancashire becomes cemented by algae and mosses several rare Lecideae are to be found on the decaying vegetation, and with further accumulation of humus Cladoniae appear and spread rapidly along with several species of Peltigera and the ubiquitous Parmelia physodes. The latter starts on dead twigs of Salix repens and spreads on to the surrounding soil where it forms patches some inches in diameter. The association also includes Lecidea uliginosa and Bilimbia sphaeroides. On the more inland portions of the dunes numerous rather poorly developed Cladoniae and Cetraria aculeata were associated, while on the sides of “slacks” or “dune-pans” Collema pulposum, Cladonia sylvatica and several crustaceous lichens covered the soil. The wetter parts of the dunes were not found to be favourable to lichen growth. Sandstede A very careful tabulation of lichens at Blakeney Point in Norfolk was made by McLean (1) Bare sand, which is the first association listed, is an area practically without phanerogams; the few lichen plants, Cladonia furcata and Cetraria aculeata f. acanthella, are attached by slight embedding in the soil. (2) Grey dune. The sand-loving lichens of the association grow in company with Hypnum cupressiforme and attain their greatest development. Other species which also occur there are Parmelia physodes and Evernia prunastri var. stictocera. (3) Derelict dune. This part of the dune formation occurs here and there on the seaward margin where the grey dune has been worn down by the wind. It is more shingly, hence the presence of stone lichens; dune phanerogams are interspersed and with them a few fruticose lichens, such as Cladonia furcata. (4) High shingle. The term indicates shingle aggregated into banks lying well above all except the highest tides. A large percentage of sand may be mixed with the stones and if no humus is present and the stones of small size, lichens may be absent altogether. Those occurring in the “loose shingle” are saxicolous. In the “bound shingle” where there is no grass the stones, fixed in a mixture of sand and humus, are well covered with lichens. With the presence of grass, a thin layer of humus covers the stones and a dense lichen vegetation is developed both of shingle and of dune species. (5) Low shingle. This last association lies in the hollows among plants of Suacda fruticosa. Stability is high and tidal immersions regular and frequent. The dominant factor of the association is the quantity of humus and mud deposited around and over the stones. The lichens cover almost every available spot on the firmly embedded pebbles. The characteristic species of such areas are Lecanora badia and L. (Placodium) citrina which effect the primary colonization. To these succeed Lecanora atra and Xanthoria parietina. In time the mud overwhelms and partly destroys the lichens, so that the phase of luxuriant growth is only temporary. Lecanora badia is conspicuously abundant at the sand end of this formation. Lecanora (Placodium) citrina disappears as the mud is left behind. Collema spp. also occur frequently on the mixture of mud and sand round the stones. The species on “low shingle” are those most tolerant of submersion: Verrucaria maura is confined to this area, where it is covered by the tide several hours each day.
McLean adds that Xanthoria parietina in its virescent form on Suaeda fruticosa also endures constant immersion; Lecanora badia does not occur c. Mountain Lichens. On the mountain summits of our own and other lands are to be found lichens very similar to those of the far North the climatic conditions being the chief factors of importance in determining the formations. These regions are occupied by what Wheldon and Wilson
Again on the summit of Ben-y-Gloe the same authors On the Continent an interesting study of the lichens of high altitudes was made by Maheu
He found that as he climbed higher and higher foliaceous species became rarer and crustaceous more abundant. The colour of the lichens on the high summits was slightly weakened and the thallus often reduced, but all were fertile and the apothecia normal and sporiferous. Lichens at less high altitudes where they emerge from the snow covering for longer periods and enjoy light and sunshine are, as already observed, often very brightly coloured and of luxuriant growth. d. Tundra Lichens. In phyto-geography the term “tundra” is given to great stretches of country practically treeless and unsheltered within the Polar climate; the tundra extends from the zone of dwarfed trees on to the permanent ice or snow fields. The vegetation includes a few dwarfed trees, shrubs, etc., but is mainly composed of mosses and lichens; the latter being the most abundant. These are true climatic lichen formations. Leighton The accounts of tundra lichens that have been given by various travellers deal chiefly with the more prominent terricolous forms. They have been classified as “Cladina tundra,” including Cladonia rangiferina and Sphaerophorus coralloides, “Cetraria tundra,” and “Alectoria heath,” the latter the hardiest of all. Great swards of these lichens often alternate with naked stony soil. Kihlman The same kinds of lichens occur all over these northern regions. Birger Nilson Darbishire Crustaceous lichens are not wanting: Lecanora tartarea f. frigida, L. epibryon and others are to be found in great sheets covering the mosses or the soil, or spreading over the stones and boulders. Cold has no deterrent effect, and their advance is only checked by the presence of perpetual snow. e. Desert Lichens. The reduced rainfall of desert countries is unfavourable to general lichen growth and only the more xerophytic species—those with a stout cortex—can flourish in the adverse conditions of excessive light and dryness. Lichens, however, there are, in great numbers as far as individuals are concerned, though the variety is not great. The abundance of the crustaceous Lecanora esculenta in the deserts of Asia has already been noted. Flagey Bruce Fink At 5000 feet and upwards lichens are, in general, exceedingly abundant on all rock substrata and represent 57 species or subspecies, only three of these being arboreal: Buellia triphragmia occurs rarely, Xanthoria polycarpa is frequent on sage brush, while Candelariella cerinella though a rock-lichen grows occasionally on the same substratum. Caloplaca (Placodium) elegans is one of the most successful and abundant species and along with Lecanora (nine forms), Acarospora (seven forms) and Lecidia (five forms) comprises three-fourths of the rock surface occupied by lichens. The addition of Rinodina with two species and Gyrophora with four brings the computation of individuals in these desert rock formations up to nine-tenths of the whole. As the desert rocks pass to the Alpine, Gyrophora becomes easily the dominant genus followed by Acarospora, Caloplaca and Lecidea. “The colouring characteristic of the rock ledges of the desert and caÑon walls is often entirely due to lichens, and in a general way they form the only brilliant plant formations in a landscape notable for its subdued pale monotonous tones. Most conspicuous are Acarospora chlorophana and Caloplaca elegans, which form striking landmarks when covering great crags and rock walls. The next most conspicuous lichens are Rinodina oreina and Lecanora rubina and its allies, which often entirely cover immense boulders and northerly sloping rock walls.” Herre concludes that though desert conditions are unfavourable to most species of lichens, yet some are perfectly at home there and the rocks are just as thickly covered as in regions of greater humidity and less sunshine. f. Aquatic Lichens. There is only one of the larger lichens that has acquired a purely aquatic habit, Hydrothyria venosa, a North American plant. It grows on rocks The nearest approach to aquatic habit among the foliose forms in our In a discussion of lake shore plants Conway Macmillan M. C. Knowles Bruce Fink Species with closed fruits such as Pyrenolichens, or with apothecia deeply sunk in the thallus and thus also well protected, seem to be best adapted to the aquatic life. Such in our own country are Lecanora lacustris, Bacidia inundata and others, with a number of Verrucariae: V. aethiobola, V. hydrela, V. margacea, etc. Lettau
In their ecological study of Perthshire lichens Wheldon and Wilson
The second group of species usually inhabits damp, shaded rocks of ravines or large boulders by streams or near waterfalls. It includes species of Collema, Sticta, Peltigera, Solorina, Pannaria, etc., with Opegrapha zonata, Porina lectissima and Verrucaria nigrescens. The last-mentioned lichen grows by preference on limestone, but in excessive moisture D. Lichens As Pioneersa. Soil-formers. The part played by lichens in the “Economy of Nature” is of very real importance: to them is allotted the pioneer work of breaking down the hard rock surfaces and preparing a soil on which more highly developed plants can grow. This was pointed out by Linnaeus Similar observations have been made since Linnaeus’s day, among others by Guembel The decaying parts of the lichen thallus add to the soil material as observed by Linnaeus, and in time mosses follow, and, later, phanerogams. Goeppert Goeppert Salter Bachmann Lucy E. Braun In tropical countries the first vegetation to settle on bare rocks would seem to be blue-green gelatinous algae. Three years after the eruption of Krakatoa, dark-green layers of these plants were found by Treub b. Outposts of vegetation. Lichens are the only plants that can survive extreme conditions of cold or of heat. They grow in Polar regions where no other vegetation could obtain sustenance; they are to be found at great heights on mountains all over the globe; and, on arid desert rocks they persist through long dry seasons, depending almost entirely on night dews for the supply of moisture. Here we have true lichen formations in the sense of modern ecology. |