ECOLOGY
A. GENERAL INTRODUCTION
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[1116].
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[1117] we read “The flora is composed of the
individual species: the vegetation comprises the groupings of these
species into ensembles termed vegetation units or plant communities.” And
again:
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[1118]
as “the science which treats of the reciprocal relation between
physiographic conditions and life requirements of organisms in so far as
such relations manifest themselves in choice of habitats and method of
establishment upon them ... resulting in the origin and development of
plant formations.”
B. EXTERNAL INFLUENCES
The climatic factors most favourable to lichen development are direct
light (already discussed)[1119], a moderate or cold temperature, constant
moisture and a clear pure atmosphere. Wind also affects their growth.
_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[1120].
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[1121] and have been published since his death.
He has remarked in more than one of his papers that a most favourable
situation for lichen growth is one that is subject to a drive of wind
with much rain. In localities with an average of 216 days of rain in
the year, he found abundant and luxuriant growths of the larger foliose
species. In West Ireland there were specimens of _Ricasolia laetevirens_
measuring 165 by 60 cm. In West Scotland with an “average of total days
of rain, 225,” he found plants of _Ricasolia amplissima_ 150 × 90 cm. in
size, of _R. laetevirens_ 120 × 90 cm., while _Pertusaria globulifera_
formed a continuous crust on the trees as much as 120 × 90 cm. _Lecanora
tartarea_ seemed to thrive exceptionally well when subject to driving
mists and rains from mountain or moorland, and was in these circumstances
frequently the dominant epiphyte. Bruce Fink[1122] also observed in his
ecological excursions that the number of species and individuals was
greater near lakes or rivers.
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[1123] and others have remarked on
the scarcity of lichens in localities denuded of the snow mantle and
exposed to severe winter cold. On the other hand lichens on the high
Alpine summits that are covered with snow the greater part of the year
suffer, according to Nilson[1124], from the excessive moisture and the
deprivation of light. Foliose and fruticose forms were, he found, dwarfed
in size; the crustaceous species had a very thin thallus and in all of
them the colour was impure. _Gyrophorae_ seemed to be most affected:
folds and outgrowths of the thallus were formed and the internal tissues
were partly disintegrated. Lichens on the blocks of the glacier moraines
which are subject to inundations of ice-cold water after the snow has
melted, were unhealthy looking, poorly developed and often sterile,
though able to persist in a barren state. Lindsay[1125] noted as a result
of such conditions on _Cladoniae_ not only sterility but also deformity
both of vegetative and reproductive organs; discolouration and mottling
of the thallus and an increased development of squamules of the primary
thallus and on the podetia.
_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[1126] cushion-like growth adopted by these lichens gives them
mutual protection, not only against wind, but against superincumbent
masses of snow. Kihlman[1123] has given us a vivid account of wind action
in the Tundra region. He noted numerous hollows completely scooped out
down to the sand: in these sheltered nooks he observed the gradual
colonization of the depressions, first by a growth of hepatics and mosses
and by such ground lichens as _Peltigera canina_, _P. aphthosa_ and
_Nephromium arcticum_; they cover the soil and in time the hollow becomes
filled with a mass of vegetation consisting of Cladonias, mosses, etc.
On reaching a certain more exposed level these begin to wither and die
off at the tips, killed by the high cold winds. Then arrives _Lecanora
tartarea_, one of the commonest Arctic lichens, and one which is readily
a saprophyte on decayed vegetation. It covers completely the mound of
weakened plants which are thus smothered and finally killed. The collapse
of the substratum entails in turn the breaking of the _Lecanora_ crust,
and the next high wind sweeps away the whole crumbling mass. How long
recolonization takes, it was impossible to find out.
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[1127] on two American pendulous lichens which are
dependent on wind for dissemination. On the Californian coasts a very
large and very frequent species, _Ramalina reticulata_ (Fig. 64), is
seldom found undamaged by wind. In Northern California the deciduous oaks
_Quercus alba_ and _Q. Douglasii_ are festooned with the lichen, while
the evergreen “live oak,” _Q. chrysolepis_, with persistent foliage,
only bears scraps that have been blown on to it. Nearer the coast and
southward the lichen grows on all kinds of trees and shrubs. The fronds
of this _Ramalina_ form a delicate reticulation and when moist are easily
torn. In the winter season, when the leaves are off the trees, wind- and
rain-storms are frequent; the lichen is then exposed to the full force of
the elements and fragments and shreds are blown to other trees, becoming
coiled and entangled round the naked branches and barky excrescences, on
which they continue to grow and fruit perfectly well. A succeeding storm
may loosen them and carry them still further. Peirce noted that only
plants developed from the spore formed hold-fasts and they were always
small, the largest formed measuring seven inches in length. Both the
hold-fast and the primary stalk were too slight to resist the tearing
action of the wind.
Schrenk[1128] made a series of observations and experiments with the
lichens _Usnea plicata_ and _U. dasypoga_, long hanging forms common on
short-leaved conifers such as spruce and juniper. The branches of these
trees are often covered with tangled masses of the lichens not due to
local growth, but to wind-borne strands and to coiling and intertwining
of the filaments owing to successive wetting and drying. Tests were
made as to the force of wind required to tear the lichens and it was
found that velocities of 77 miles per hour were not sufficient to cause
any pieces of the lichen to fly off when it was dry; but after soaking
in water, the first pieces were torn off at 50 miles an hour. These
figures are, however, considered by Schrenk to be too high as it was
found impossible in artificially created wind to keep up the condition of
saturation. It is the combination of wind and rain that is so effective
in ensuring the dispersal of both these lichens.
_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[1129] made a list of the lichens growing
in the Luxembourg gardens in Paris; the chestnuts in the alley of the
Observatory were the most thickly covered, and the list includes about
35 different species or varieties, some of them poorly developed and
occurring but rarely, others always sterile, but quite a number in
healthy fruiting condition. All of them were crustaceous or squamulose
forms except _Parmelia acetabulum_, which was very rare and sterile;
_Physcia obscura_ var. and _Ph. pulverulenta_ var., also sterile;
_Physcia stellaris_ with occasional abortive apothecia and _Xanthoria
parietina_, abundant and fertile. In 1898, Hue[1130] tells us, there were
no lichens to be found on the trees and only traces of lichen growth on
the stone balustrades.
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[1131] in their account of the
lichens of South Lancashire, a district peculiarly suitable for such an
inquiry, as nowhere, according to the observations, are the evil effects
of impure air so evident or so wide-spread. The unfavourable conditions
have prevailed for a long time and the lichens have consequently become
very rare, those that still survive leading but a meagre existence. The
chief impurity is coal smoke which is produced not only from factories
but from private dwellings, and its harmful effect goes far beyond
the limits of the towns or suburbs, lichens being seen to deteriorate
as soon as there is the slightest deposition of coal combustion
products—especially sulphur compounds—either on the plants or on the
surfaces on which they grow. The larger foliose and fruticose forms have
evidently been the most severely affected. “While genera of bark-loving
lichens such as _Calicium_, _Usnea_, _Ramalina_, _Graphis_, _Opegrapha_,
_Arthonia_ etc. are either wholly absent or are poorly represented in
the district,” corticolous species now represent about 15 per cent.
of those that are left; those that seem best to resist the pernicious
influences of the smoky atmosphere are, principally, _Lecanora varia,
Parmelia saxatilis, P. physodes_ and to a less degree _P. sulcata_, _P.
fuliginosa_ var. _laetevirens_ and _Pertusaria amara_.
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
Epping Forest during the nineteenth century. They numbered 171 species,
varieties or forms, but, at the date of publication, many had died out
owing to the destruction of the older trees; the undue crowding of the
trees that were left and the ever increasing population on the outskirts
of the Forest. Crombie himself made a systematic search for those that
remained, and could only find some 85 different kinds, many of them in a
fragmentary or sterile condition.
R. Paulson and P. Thompson[1132] commenced a lichen exploration of the
Forest 27 years after Crombie’s report was published, and they have found
that though the houses and the population have continued to increase
round the area, the lichens have not suffered. “Species considered by
Crombie as rare or sterile are now fairly abundant, and produce numerous
apothecia. Such are _Baeomyces rufus_, _B. roseus_, _Cladonia pyxidata_,
_Cl. macilenta_ var. _coronata_, _Cl. Floerkeana_ f. _trachypoda_,
_Lecanora varia_, _Lecidea decolorans_ and _Lecidea tricolor_.” They
conclude that “some at least of the Forest lichens are in a far more
healthy and fertile condition than they were 27 years ago.” They
attribute the improvement mainly to the thinning of trees and the opening
up of glades through the Forest, letting in light and air not only to the
tree trunks but to the soil. In 1912[1133] the authors in a second paper
reported that 109 different kinds had been determined, and these, though
still falling far short of the older lichen flora, considerably exceed
the list of 85 recorded in 1883.
C. LICHEN COMMUNITIES
Lichen 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. ARBOREAL
Arboreal 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[1134] records a sterile species, _S. Buxi_, on box leaves from the
neighbourhood of Geneva.
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. As the trees become older and the bark takes on a more ragged
character, other types of lichens gain a foothold, such as the thicker
crustaceous forms like _Pertusaria_, or the larger foliose and fruticose
species. The moisture that is collected and retained by the rough bark is
probably the important factor in the establishment of the thicker crusts,
and, as regards the larger lichens, both rhizinae and hold-fasts are able
to gain a secure grip of the broken-up unequal surface, such as would be
quite impossible on trees with smooth bark.
Among the first to pay attention to the ecological grouping of
corticolous lichens was A.L. Fée[1135], a Professor of Natural Science
and an Army doctor, who wrote on many literary and botanical subjects.
In his account of the Cryptogams that grow on “officinal bark,” he
states that the most lichenized of all the _Cinchonae_ was the one known
as “Loxa,” the bark of which was covered with species of _Parmelia_,
_Sticta_ and _Usnea_ along with crustaceous forms of _Lecanora_,
_Lecidea_, _Graphis_ and _Verrucaria_. Another species, _Cinchona
cordifolia_, was completely covered, but with crustaceous forms only:
species of Graphidaceae, _Lecanora_ and _Lecidea_ were abundant, but
_Trypethelium_, _Chiodecton_, _Pyrenula_ and _Verrucaria_ were also
represented. On each species of tree some particular lichen was generally
dominant:
A species of _Thelotrema_ on _Cinchona oblongifolia_.
A species of _Chiodecton_ on _C. cordifolia_.
A species of _Sarcographa_ on _C. condaminea_.
Fries[1136], in his geography of lichens, distinguished as arboreal and
“hypophloeodal” species of Verrucariaceae, while the Graphideae, which
also grew on bark, were erumpent. _Usnea barbata_, _Evernia prunastri_,
etc., though growing normally on trees might, he says, be associated with
rock species.
More extensive studies of habitat were made by Krempelhuber[1137] in his
_Bavarian Lichens_. In summing up the various “formations” of lichens, he
gives lists of those that grow, in that district, exclusively on either
coniferous or deciduous trees, with added lists of those that grow on
either type of tree indifferently. Among those found always on conifers
or on coniferous wood are: _Letharia vulpina_, _Cetraria Laureri_,
_Parmelia aleurites_ and a number of crustaceous species. Those that are
restricted to the trunks and branches of leafy trees are crustaceous
with the exception of some foliose Collemaceae such as _Leptogium
Hildenbrandii_, _Collema nigrescens_, etc.
Arnold[1138] carried to its furthest limit the method of arranging
lichens ecologically, in his account of those plants from the
neighbourhood of Munich. He gives “formation” lists, not only for
particular substrata and in special situations, but he recapitulates the
species that he found on the several different trees. It is not possible
to reproduce such a detailed survey, which indeed only emphasizes the
fact that the physical characters of the bark are the most important
factors in lichen ecology: that on smooth bark, whether of young trees,
or on bark that never becomes really rugged, there is a preponderance
of species with a semi-immersed thallus, and very generally of those
that are associated with _Trentepohlia_ gonidia, such as Graphidaceae or
Pyrenulaceae, though certain species of _Lecidea_, _Lecanora_ and others
also prefer the smooth substratum.
Bruce Fink[1139] has published a series of important papers on lichen
communities in America, some of them similar to what we should find in
the British Isles.
On trees with smooth bark he records in the Minnesota district:
_Xanthoria polycarpa._
_Candelaria concolor._
_Parmelia olivacea_, _P. adglutinata_.
_Placodium cerinum._
_Lecanora subfusca._
_Bacidia fusca-rubella._
_Lecidea enteroleuca._
_Graphis scripta._
_Arthonia lecideella_, _A. dispersa_.
_Arthopyrenia punctiformis_, _A. fallax_.
_Pyrenula nitida_, _P. thelena_, _P. cinerella_, _P. leucoplaca_.
On rough bark he records:
_Ramalina calicaris_, _R. fraxinea_, _R. fastigiata_.
_Teloschistes chrysophthalmus._
_Xanthoria polycarpa_, _X. lychnea_.
_Candelaria concolor._
_Parmelia perforata_, _P. crinita_, _P. Borreri_, _P. tiliacea_,
_P. saxatilis_, _P. caperata_.
_Physcia granulifera_, _Ph. pulverulenta_, _Ph. stellaris_, _Ph.
tribacia_, _Ph. obscura_.
_Collema pycnocarpum_, _C. flaccidum_.
_Leptogium mycochroum._
_Placodium aurantiacum_, _Pl. cerinum_.
_Lecanora subfusca._
_Pertusaria leioplaca_, _P. velata_.
_Bacidia rubella_, _B. fuscorubella_.
_Lecidea enteroleuca._
_Rhizocarpon alboatrum_, _Buellia parasema_.
_Opegrapha varia._
_Graphis scripta._
_Arthonia lecideella_, _A. radiata_.
_Arthopyrenia quinqueseptata_, _A. macrospora_.
_Pyrenula nitida_, _P. leucoplaca_.
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[1140] in Somerset may
be quoted:
_Collema flaccidum._
_Calicium hyperellum._
_Ramalina calicaris_, _R. fraxinea_ with var. _ampliata_, _R.
fastigiata_, _R. farinacea_ and _R. pollinaria_.
_Parmelia saxatilis_ and f. _furfuracea_, _P. caperata_, _P. physodes_.
_Physcia pulverulenta_, _Ph. tenella_ (_hispida_).
_Lecanora subfusca_, _L. rugosa_.
_Pertusaria amara_, _P. globulifera_, _P. communis_, _P. Wulfenii_.
_Lecidea_ (_Buellia_) _canescens_.
_Graphis scripta._
And on the soil of these woods:
_Cladonia pyxidata_, _Cl. pungens_, _Cl. macilenta_, _Cl. pityrea_,
_Cl. squamosa_ and _Cl. sylvatica_.
Paulson[1141], from his observations of lichens in Hertfordshire, has
concluded that the presence or absence of lichens on trees is influenced
to a considerable degree by the nature of the soil. They were more
abundant in woods on light well-drained soils than on similar communities
of trees on heavier soils, though the shade in the former was slightly
more dense and therefore less favourable to their development; the cause
of this connection is not known.
_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[1142] has suggested that
the chemical reactions produced by the decomposition of the bast fibres
are fatal to them, _Lecidea parasema_ alone continuing to grow and even
existing for some time on the detached shreds of bark.
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:
_Usnea hirta._
_Cetraria diffusa._
_Evernia furfuracea._
_Parmelia scortia_, _P. physodes_.
_Xanthoria parietina._
_Placodium cerinum._
_Rinodina exigua._
_Lecanora Hageni_, _L. varia_ and its allies.
_Lecidea ostreata_, _L. parasema_.
_Buellia myriocarpa._
Cladoniaceae and Caliciaceae (several species).
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
gained a footing, though the larger and more vigorous lichens tend to
crowd out the others. Bruce Fink[1143] has recorded associations in
Minnesota:
On wood:
_Teloschistes chrysophthalmus._
_Placodium cerinum._
_Lecanora Hageni_, _L. varia_.
_Rinodina sophodes_, _R. exigua_.
_Buellia parasema_ (_disciformis_), _B. turgescens_.
_Calicium parietinum._
_Thelocarpon prasinellum._
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. TERRICOLOUS
In 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[1144], in three months.
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[1145] found in N.W. Minnesota an association on exposed
calcareous earth as follows:
_Heppia Despreauxii._
_Urceolaria scruposa._
_Biatora_ (_Lecidea_) _decipiens_.
_Biatora_ (_Bacidia_) _muscorum_.
_Dermatocarpon hepaticum._
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[1146]
gives a list of those he observed near Munich on the former habitat:
_Cladonia sylvatica_ f. _alpestris_.
_Cladonia squamosa_ f. _subsquamosa_.
_Cladonia rangiformis_ f. _foliosa_.
_Cladonia cariosa_ and f. _symphicarpa_.
_Peltigera canina_ f. _soreumatica_.
_Solorina spongiosa._
_Heppia virescens._
_Lecanora crassa._
_Urceolaria scruposa_ f. _argillacea_.
_Verrucaria_ (_Thrombium_) _epigaea_.
_Lecidea decipiens._
_Dermatocarpon cinereum._
_Collema granulatum._
_Collema tenax._
_Leptogium byssinum._
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:
_Baeomyces roseus._
_Baeomyces rufus._
_Baeomyces placophyllus._
_Endocarpon_ spp.
_Gongylia viridis._
_Dermatocarpon lachneum_
_Dermatocarpon hepaticum._
_Dermatocarpon cinereum._
These very generally grow in extended societies of one species only.
In his enumeration of soil-lichens Arnold[1146] gives 40 species that
grow on siliceous soil, as against 57 on calcareous. Many of them
occurred on both. Those around Munich on siliceous soil only were:
_Cladonia coccifera._
_Cladonia agariciformis._
_Secoliga_ (_Gyalecta_) _bryophaga_.
_Baeomyces rufus._
_Lecidea gelatinosa._
_Psorotichia lutophila._
Mayfield[1147] in his account of the Boulder Clay lichen flora of Suffolk
found only four species that attained to full development on banks and
hedgerows. These were: _Collema pulposum_, _Cladonia pyxidata_, _Cl.
furcata_ var. _corymbosa_ and _Peltigera polydactyla_.
On bare heaths of gravelly soil in Epping Forest Paulson and
Thompson[1148] describe an association of such lichens as:
_Baeomyces roseus._
_Baeomyces rufus._
_Pycnothelia papillaria._
_Cladonia coccifera._
_Lecidea granulosa._
_Cladonia macilenta._
_Cladonia furcata._
_Cetraria aculeata._
_Peltigera spuria._
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[1149]
noted that on the dunes, though stray bricks were covered by algae,
lichens rarely or never seemed to gain a footing.
There are many references in literature to lichens that live on tiles.
A fairly representative list is given by Lettau[1150] of “tegulicolous”
species.
_Verrucaria nigrescens._
_Lecidea coarctata._
_Candelariella vitellina._
_Lecanora dispersa._
_Lecanora galactina._
_Lecanora Hageni._
_Lecanora saxicola._
_Parmelia conspersa._
_Placodium teicholytum._
_Placodium pyraceum._
_Placodium decipiens._
_Placodium elegans._
_Placodium murorum._
_Xanthoria parietina._
_Rhizocarpon alboatrum_ var.
_Buellia myriocarpa._
_Lecidea demissa._
_Physcia ascendens._
_Physcia caesia._
_Physcia obscura._
_Physcia sciastrella._
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[1151] from N. Minnesota; with the exception of
_Cladonia cristatella_, the species are British as well as American:
_Cladonia furcata._
_Cladonia cristatella._
_Cladonia gracilis._
_Cladonia verticillata._
_Cladonia rangiferina._
_Cladonia uncialis._
_Cladonia alpestris._
_Cladonia turgida._
_Cladonia coccifera._
_Cladonia pyxidata._
_Cladonia fimbriata._
_Peltigera malacea._
_Peltigera canina._
_Peltigera aphthosa._
_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[1152] remarked that after the burning of the
heath it was the first vegetation to appear and covered large spaces with
its grey thallus. Another peat species is _Icmadophila ericetorum_, but
it prefers damper localities than the two Lecideae.
To quote again from Arnold[1153]: 24 species were found on turf around
Munich, 13 of which were _Cladoniae_, but only four species could be
considered as exclusively peat-lichens. These were:
_Cladonia Floerkeana._
_Biatora terricola._
_Thelocarpon turficolum._
_Geisleria sychnogonioides._
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[1154] have listed the lichens that they found in
Perthshire on subalpine heath lands, on the ground, or on banks amongst
mosses:
_Leptogrum_ spp.
_Peltigera_ spp.
_Cetraria_ spp.
_Parmelia physodes._
_Psoroma hypnorum._
_Lecanora epibryon._
_Lecanora tartarea._
_Lecidea coarctata._
_Lecidea granulosa._
_Lecidea uliginosa._
_Lecidea neglecta._
_Bilimbia sabulosa._
_Bilimbia liguiaria._
_Bilimbia melaena._
_Baeomyces_ spp.
_Cladonia_ spp.
As already described _Lecanora tartarea_[1155] spreads freely over the
mosses of the tundra. Aigret[1156] in a study of _Cladoniae_ notes that
_Cl. pyxidata_, var. neglecta chooses little cushions of acrocarpous
mosses, which are particularly well adapted to retain water. _Cl.
digitata_, _Cl. flabelliformis_ and some others grow on the mosses which
cover old logs or the bases of trees.
_g._ ON FUNGI. Some of the fungi, such as Polyporei, are long lived, and
of hard texture. On species of _Lenzites_ in Lorraine, Kieffer[1157] has
recorded 15 different forms, but they are such as naturally grow on wood
and can scarcely rank as a separate association.
3. SAXICOLOUS
Lichens 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[1158]. The number of
species is largest at first and the kind of lichen depends on the flora
existing in the near neighbourhood. Link[1159], for instance, has stated
that _Lichen candelarius_ was the first lichen to appear on the rocks he
observed, and, if trees were growing near, then _Lichen parietinus_ and
_Lichen tenellus_ followed soon after. After a time the lichens change,
the more slow-growing being crowded out by the more vigorous. Crustaceous
species, according to Malinowski[1160], are most subject to this struggle
for existence, and certain types from the nature of their thallus are
more easily displaced than others. Those with a deeply cracked areolated
thallus become disintegrated in the older central areas by repeated
swelling and contracting of the areolae as they change from wet to dry
conditions. Particles of the thallus are thus easily dislodged, and bare
places are left, which in time are colonized again by the same lichen
or by some invading species. There may result a bewildering mosaic
of different thalli and fruits mingling together. Some forms such as
_Rhizocarpum geographicum_ which have a very close firm thallus do not
break away. In the course of time lichen communities come and go, and
the plants of one locality may be different from those of another for no
apparent reason.
The question of colonization[1161] was studied by Bruce Fink[1162] on
a “riprap” wall of quartz, 30 years old, built to protect and brace
a railway in Iowa. Near by was a grass swamp which supplied moisture
especially to the lower end of the wall. A few boulders were present in
the vicinity, but the nearest lichen “society” was on trees about 150
metres away and these bore corticolous Parmelias, Physcias, Ramalinas,
Placodiums, Lecanoras and Rinodines which were only very sparingly
represented on the riprap. Moisture-loving species never gained a
footing; the extreme xerophytic conditions were evidenced by the
character of the lichens, _Biatora myriocarpoides_ (_Lecidea sylvicola_)
occupying the driest parts of the wall. Lower down where more moisture
prevailed _Bacidia inundata_ and _Stereocaulon paschale_ were the
dominant species. Some 30 species or forms were listed of which 11 were
Cladonias that grew mainly on debris from the disintegration of the wall.
With the exception of two or three species the number of individuals was
very small.
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[1163] published in 1789, while
still at Rostock, an account of limestone plants in his neighbourhood,
most of them being lichens. In a later work he continues his Botanical
Geography or “Geology” and gives more precise details as to the plants,
some of which are essentially calcicolous though many of them he records
also on siliceous rocks.
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[1164] has given us a list of species that
inhabit carboniferous limestone in Britain. Wheldon and Wilson[1165]
have described in West Lancashire the “grey calcareous rocks blotched
with black patches of Pannarias (_Placynthium nigrum_) and Verrucarias,
or dark gelatinous rosettes of Collemas. White and grey _Lecanorae_
and _Verrucariae_ spread extensively, some of them deeply pitting the
surface. These more sombre or colourless species are enlivened by an
intermixture of orange-yellow _Physciae_ (_Xanthoriae_) and _Placodii_
by the ochrey films of _Lecanora ochracea_ and lemon-yellow of _Lecanora
xantholyta_. Amongst the greenish scaly crusts of _Lecanora crassa_ may
be seen the bluish cushions of _Lecidea coeruleonigricans_, the whole
forming an exquisite blend of tints.”
The flora recorded by Flagey[1166] on the cretaceous rocks of Algeria in
the Province of Constantine does not greatly differ, some of the species
being identical with those of our own country. Placodiums and Rinodinas
were abundant, as also _Lecanora calcarea_, _Acarospora percaenoides_
and _Urceolaria actinostoma_ var. _calcarea_. Also a few _Lecideae_
along with _Verrucaria lecideoides_, _V. fuscella_, _V. calciseda_ and
_Endocarpon monstrosum_. The rocks of that region are sometimes so
covered with lichens that the stone is no longer visible.
Bruce Fink[1167] gives a typical community on limestone bluffs in
Minnesota:
_Pannaria_ (_Placynthium_) _nigra_.
_Crocynia lanuginosa._
_Omphalaria pulvinata._
_Collema plicatile._
_Collema pustulatum._
_Leptogium lacerum._
_Placodium citrinum._
_Bacidia inundata._
_Rhizocarpon alboatrum_ var.
_Dermatocarpon miniatum._
_Staurothele umbrinum._
Forssell[1168] pointed out an interesting selective quality in
the Gloeolichens which are associated with the gelatinous algae,
_Chroococcus_, _Gloeocapsa_ and _Xanthocapsa_. The genera containing the
two former grow on siliceous rocks with the exception of _Synalissa_. The
genera _Omphalaria_, _Peccania_, _Anema_, _Psorotichia_ and _Enchylium_,
in which _Xanthocapsa_ is the gonidium, grow on calcareous rocks.
_Collemopsidium_ is the only _Xanthocapsa_ associate that is silicicolous.
_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[1169] as
occurring in Bohemia, and is known as Lydite or Lydian stone, a black
flinty jasper. The association of lichens on this smooth rock was
almost entirely _Acarospora chlorophana_ and _Rinodina oreina_, which
as we shall see occur again as a “desert” association in Nevada; these
two lichens grow equally well in sun or shade, and either sheltered or
exposed as regards wind and rain. _Acarospora chlorophana_, according
to Malinowski[1170], arrives among the first on rocks newly laid bare,
and forms large societies, though in time it gives place to _Lecanora
glaucoma_ (_L. sordida_), a common silicicolous lichen.
A difference has been pointed out by Bachmann[1171] between the lichens
of acid and of basic rocks. The acid series, such as quartz- and
granite-porphyry, contain 70 per cent. and more of oxide of silica;
the basic—diabase and basalt—not nearly 50 per cent. He observed that
_Rhizocarpon geographicum_ was the most frequent lichen of the acid
porphyry, while on basalt there were only small scattered patches.
_Pertusaria corallina_ was abundant only on granitic rocks. On the other
hand _Pertusaria lactea_ f. _cinerascens_, _Diploschistes scruposus_,
_D. bryophilus_ and _Buellia leptocline_ preferred the basic substratum
of diabase and basalt. In this case it is the chemical rather than
the physical character of the rocks that affects the lichen flora, as
porphyry and basalt are both close-grained, and are outwardly alike
except in colouration.
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[1172] found the granitic rocks bare
owing to their too rapid weathering. In these rocks the softer basic
constituents such as the mica are colonized first; the quartz remains
a long time naked, though in time it also is covered. Wheldon and
Wilson[1173] point out that the sandstone near to intrusive igneous
rocks has become close-grained and indurated and bears _Lecanora
squamulosa_, _L. picea_, _Lecidea rivulosa_ and _Rhizocarpon petraeum_,
which were not seen on the unaltered sandstone. It was also observed by
Stahlecker[1174], that, in layered rocks, the lichen chose the surface at
right angles to the layering as the hyphae thus gain an easier entrance.
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[1175] listed:
_Acarospora chlorophana._
_Lecanora glaucoma._
_Rhizocarpon viridiatrum._
_Lecidea tumida._
_Biatorella sporostatia._
_Biatorella testudinea._
On granite and quartzite disintegrated:
_Aspicilia cinerea._
_Aspicilia gibbosa._
_Aspicilia tenebrosa._
_Buellia coracina._
_Catillaria_ (_Biatorina_) _Hochstetteri_.
_Rhizocarpon petraeum._
_Rhizocarpon geographicum_ vars.
_Biatorella cinerea._
_Lecanora badia._
_Lecanora cenisia._
_Lecidea confluens._
_Lecidea fuscoatra._
_Lecidea platycarpa._
_Lecidea lapicida._
_Haematomma ventosum._
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:
_Aspicilia cinerea._
_Haematomma ventosum._
_Rhizocarpon geographicum_ vars.
_Biatorella cinerea._
_Lecidea platycarpa._
A number of rock associations have been tabulated by Wheldon and
Wilson[1176] for Perthshire. Among others they give some of the most
typical lichens on granitic and eruptive rocks:
_Sphaerophorus coralloides._
_Sphaerophorus fragilis._
_Platysma Fahlunense._
_Platysma commixtum._
_Platysma glaucum._
_Platysma lacunosum._
_Parmelia saxatilis._
_Parmelia omphalodes._
_Parmelia Mougeotii._
_Parmelia stygia._
_Parmelia tristis._
_Parmelia lanata._
_Gyrophora proboscidea._
_Gyrophora cylindrica._
_Gyrophora torrefacta._
_Gyrophora polyphylla._
_Gyrophora flocculosa._
_Lecanora gelida._
_Lecanora atra._
_Lecanora badia._
_Lecanora tartarea._
_Lecanora parella._
_Lecanora ventosa._
_Lecanora Dicksonii._
_Lecanora cinerea._
_Lecanora peliocypha._
_Pertusaria dealbata._
_Stereocaulon Delisei._
_Stereocaulon evolutum._
_Stereocaulon coralloides._
_Stereocaulon denudatum._
_Psorotichia lugubris._
_Lecidea inserena._
_Lecidea panaeola._
_Lecidea contigua._
_Lecidea confluens._
_Lecidea lapicida._
_Lecidea plana._
_Lecidea mesotropa._
_Lecidea auriculata._
_Lecidea diducens._
_Lecidea aglaea._
_Lecidea rivulosa._
_Lecidea Kochiana._
_Lecidea pycnocarpa._
_Buellia atrata._
_Rhizocarpon Oederi._
On siliceous rocks in West Lancashire the same authors[1177] depict
the lichen flora as follows: “There are many grey _Parmeliae_ and
_Cladoniae_ with coral-like _Sphaerophorei_ on the rocks, and on the
walls smoky-looking patches of _Parmelia fuliginosa_ and ragged fringes
of _Platysma glaucum_ and _Evernia furfuracea_. On the higher scars, flat
topped tabular blocks exhibit black scaly _Gyrophoreae_, dingy green
_Lecidea_ (_Rhizocarpon_) _viridiatra_ and mouse-coloured _L. rivulosa_.
Suborbicular (whitish) patches of _Pertusaria lactea_ and _P. dealbata_
enliven the general sadness of tone, and everywhere loose rocks and
stones are covered with the greyish-black spotted thallus of _Lecidea
contigua_.”
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[1178] been
determined as chiefly _Lepraria chlorina_, in less abundance _Lecidea
lucida_ and _Calicium arenarium_, with occasional growths of _Coniocybe
furfuracea_ and _Calicium corynellum_.
4. OMNICOLOUS LICHENS
Some 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[1179] made a
tabulation of such as he found growing on varied substances on the dunes
round Dunkirk and they well represent these omnicolous communities. It is
in such a no man’s land that one would expect to find an accumulation of
derelict materials, not only favourably exposed to light and moisture,
but undisturbed for long periods and bordering on normal lichen
associations of soil, tree and stones. Arnold[1180] also noted many of
these peculiar habitats.
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[1181] on an old cannon lying near water,
that was exfoliated by rust.
=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[1182]
found on bones: _Lecanora varia_, _L. Hageni_, _Rinodina turfacea_ and
_Buellia parasema_ (_disciformis_). He could not trace any effect of the
lichens on the substratum.
=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=[1183]—_Physcia ascendens_ f. _tenella_, _Buellia canescens_.
Richard has recorded the same lichens on the broken glass of walls and
in addition: _Xanthoria parietina_, _Lecanora crenulata_, _L. dispersa_,
_Lecania erysibe_, _Rinodina exigua_, and _Buellia canescens_.
=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[1184] who found _Cladonia
fimbriata_ var. _tubaeformis_ and _Lecidea granulosa_ growing on an old
boot. These authors connect the sodden condition of the leather with its
attraction for lichens.
=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. pyraceum_, _Lecanora umbrina_, _Bacidia inundata_ and
_Polyblastia Vouauxi_ var. _charticola_.
=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 COMMUNITIES
Lichens 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 indifferently on any kind of shore rock either siliceous
or calcareous. Wheldon and Wilson[1185] noted this indifference to
substratum on the Arran shores, where a few calcicolous species such as
_Verrucaria nigrescens_, _V. maculiformis_, _Placodium tegularis_ and
_Pl. lobulatum_, grow by the sea on siliceous rocks. They suggest that
the spray-washed habitat affords the conditions, which, in other places,
are furnished by limestone.
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[1186] on the Isle d’Yeu delimited three such zones:
(1) marine, those nearest the sea and immersed for a longer or shorter
period at each tide; (2) semi-marine, not immersed but subject to the
direct action of the waves, and (3) maritime or littoral, the area beyond
the reach of the waves but within the influence of sea-spray. In the
course of his work he indicates the lichens of each zone.
[Illustration: Fig. 122. _Ramalina siliquosa_ A. L. Sm. Upper zone of
barren plants (after M. C. Knowles, R. Welch. _Photo._).]
In Ireland, a thorough examination has been made of a rocky coast at
Howth near Dublin by M. C. Knowles[1187]. She recognizes five distinct
belts beginning with those furthest from the shore though within the
influence of the salt water:
1. =The Ramalina belt.=
2. =The Orange belt.=
3. =Lichina Vegetation.=
4. =Verrucaria maura belt.=
5. =The belt of Marine Verrucarias.=
(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_[1188], rather dark or glaucous in colour with
much branched fronds which are incurved at the tips (Fig. 122). They
are beyond the direct action of the waves. The lower zone consists
also mainly of the same _Ramalina_, the plants bearing straight,
stiff, simple, or slightly branched fertile fronds of a pale-green or
straw colour (Fig. 123). The pale colour may be partly due to frequent
splashings by sea-spray.
_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:
1. _Parmeliae._
2. _Physcia aquila._
3. _Xanthoria parietina._
4. Crustaceous species.
[Illustration: Fig. 123. _Ramalina siliquosa_ A. L. Sm. Lower zone of
fertile plants (after M. C. Knowles, R. Welch, _Photo._).]
[Illustration: Fig. 124. Sward of young _Ramalinae_ (after M. C. Knowles,
R. Welch, _Photo._).]
_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.
[Illustration: Fig. 125. Crustaceous communities in the _Ramalina_ belt.
_Lecanora atra_ Ach. (grey patches) and _Buellia ryssolea_ A. L. Sm.
(dark patches). (After M. C. Knowles, R. Welch, _Photo._)]
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 width all round the coast. In summer, the colour of these
lichens is so brilliant that the belt is easily recognized from a
considerable distance.” The most abundant species occur mainly in the
following order descending towards the sea:
1. _Xanthoria parietina._
2. _Placodium murorum._
3. _Placodium tegularis._
4. _Placodium decipiens._
5. _Placodium lobulatum._
“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:
_Lecanora prosechoides._
_Lecanora umbrina._
_Lecanora Hageni._
_Rhizocarpon alboatrum._
_Biatorina lenticularis._
_Rinodina exigua var. demissa._
_Opegrapha calcarea f. heteromorpha._
(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[1189] at Pornic, where however they were all submerged at
high tide.
_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[1190] of the west coast of Ireland yielded much the same
results, but with a still higher “white belt” formed mainly of _Lecanora
parella_ and _L. atra_ which covered the rocks lying above high-water
mark, “giving them the appearance of having been whitewashed.” A more
general association for the same position as regards the tide is given by
Wheldon and Wilson[1191] on the coasts of Arran as:
_Physcia aquila._
_Xanthoria parietina._
_Lecanora parella._
_Lecanora atra._
_Lecanora campestris._
_Placodium ferrugineum_ var. _festivum._
_Placodium tegularis._
_Ramalina cuspidata._
_Physcia stellaris._
_Physcia tenella._
_Verrucaria maura._
A somewhat similar series of “formations” was determined by
Sandstede[1192] on the coast of Rügen. On erratic granite boulders washed
by the tide he found:
_Verrucaria maura._
_Lichina confinis._
_Lecanora prosechoides._
_Placodium lobulatum._
While in a higher position on similar boulders:
_Lecanora exigua._
_Lecanora dispersa._
_Lecanora galactina._
_Lecanora sulphurea._
_Lecanora saxicola._
_Lecanora caesiocinerea._
_Lecanora gibbosa._
_Lecanora atra._
_Lecanora parella._
_Lecidea colludens._
_Lecidea lavata._
_Lecidea nigroclavata_ f. _lenticularis._
_Xanthoria parietina_ and f. _aureola._
_Physcia subobscura._
_Physcia caesia._
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:
_Parmelia physodes._
_Evernia prunastri._
_Cetraria aculeata._
_Cladonia cervicornis._
_Cladonia endiviaefolia._
_Peltigera_ spp.
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[1193].
On shifting sand, as in the desert, there are no lichens; it is only
on surfaces more or less fixed by marram grass that lichens begin
to develop, though in the cool damp weather of autumn and winter, as
observed by Wheldon and Wilson[1194], certain species associated with
Myxophyceae, such as Collemaceae, may make their appearance, among others
_Leptogium scotinum_, _Collemodium turgidum_ and _Collema ceranoides_.
Watson[1195] makes the same observation in his study of sand-dunes.
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[1196] found on the sandy shores of Rügen, from the shore
upwards: first a stretch of bare sand, then a few dune grasses with
scattered scraps of _Cladoniae_, _Peltigerae_ and _Cetraria aculeata_.
Next in order sandbanks with _Parmelia physodes_, _Cladonia sylvatica_,
_Cl. alcicornis_ and _Stereocaulon paschale_. All these are species
that occur on similar shores in the British Islands. Sandstede adds an
extensive list of maritime species observed by him in Rügen.
A very careful tabulation of lichens at Blakeney Point in Norfolk was
made by McLean[1197] and the table on p. 386 is reproduced from his
paper. Sand, he writes, is present in all the associations and the
presence or absence of stones marks the great difference between the two
formations determined by dune and shingle.
(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.
FORMATION ASSOCIATION PRINCIPAL SPECIES
{1. Bare Sand _Cetraria aculeata_ f. _acanthella_
{ _Cladonia furcata_
Dune {2. Grey Dune _Cladonia rangiferina_,
{ _Peltigera rufescens_
{ _Cladonia furcata_, _Cl. alcicornis_
{3. Derelict Dune _Cladonia furcata_, _Parmelia fuliginosa_
{ _Rhizocarpon confervoides_
{4. High Shingle
{ {_Lecanora atra_, _L. galactina_
{ {With sand {_Rhizocarpon confervoides_
{ { {_Lecanora citrina_
{ Loose
{ { {_Physcia tenella_, _Lecanora citrina_,
{ {Without sand {_Xanthoria parietina_
{ { {_Squamaria saxicola_
{ {_Parmelia saxatilis_, _P. fuliginosa_
Shingle{
{ {_Cladonia rangiferina_, _Cl. furcata_,
{ {With grasses {_Cl. pungens_
{ { {_Cetraria aculeata_
{ Bound
{ { {_Xanthoria parietina_,
{ { {_Biatorina chalybeia_,
{ {Without {_Lecanora atra_
{ { grasses {_Aspicilia gibbosa_, _Buellia colludens_,
{ { {_Verrucaria microspora_
{ {_Physcia tenella_, _Lecanora atroflava_
{
{ _Rhizocarpon confervoides_,
{ _Lecanora citrina_ var. _incrustans_
{5. Low Shingle _L. badia_, _L. atra_, _Xanthoria
parietina_
_Verrucaria maura_
McLean adds that _Xanthoria parietina_ in its virescent form on _Suaeda
fruticosa_ also endures constant immersion; _Lecanora badia_ does not
occur above the tidal line and _Lecanora galactina_ does not descend
below tidal limits; the latter is an arenicolous species and colonizes
some of the loosest and sandiest areas of shingle. _Rhizocarpon
confervoides_ is ubiquitous.
_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[1198] describe as “a zone of Arctic-Alpine vegetation,” and
they have recorded a series of lichen associations belonging to that zone
on the schistose summits of the Perthshire mountains. The following is
one of the most typical:
_Euopsis granatina._
_Sphaerophorus coralloides._
_Sphaerophorus fragilis._
_Gyrophora polyphylla._
_Cetraria tristis._
_Cetraria nivalis._
_Lecanora tartarea_ var. _frigida_.
_Lecanora upsaliensis._
_Aspicilia oculata._
_Pertusaria dactylina._
_Pertusaria glomerata._
_Stereocaulon denudatum._
_Parmelia saxatilis._
_Parmelia omphalodes._
_Parmelia lanata._
_Parmelia stygia._
_Stereocaulon tomentosum._
_Stereocaulon alpinum._
_Cladonia coccinca._
_Cladonia gracilis._
_Cladonia uncialis._
_Cladonia destricta._
_Cladonia racemosa._
_Lecidea arctica._
_Parmelia alpicola._
_Cetraria aculeata._
_Cetraria crispa._
_Cetraria islandica._
_Lecidea limosa._
_Lecidea alpestris._
_Lecidea demissa._
_Lecidea uliginosa._
_Lecidea cuprea._
_Lecidea Berengeriana._
_Lecidea cupreiformis._
_Lecidea atrofusca._
Again on the summit of Ben-y-Gloe the same authors[1199] have recorded
_Gyrophora erosa_, _G. torrefacta_ and _G. cylindrica_, _Parmelia
alpicola_, _Lecanora tartarea_ var. _frigida_, _Lecidea limosa_ and _L.
arctica_, the last two lichens thriving in the most bleak and exposed
situations. _Cladonia cervicornis_ grew in reduced squamulose cushions;
_Stereocaulon_ and _Sphaerophorus_ in very compact forms, the outer
stalks prostrate, the next inclined, the central ones erect so that
points only are exposed and no lateral stress is caused by wind storms.
Erect fruticose lichens are absent in this region, being represented
only by _Parmelia lanata_, a semi-decumbent plant, and by _Thamnolia
vermicularis_ which is prostrate on the ground except where the points
of the stalks turn up to catch the dew. Many of the _Lecideae_ were
observed to have large fruits and very little thallus: “the hyphae ramify
in the minute interstices of the stone and the gonidia cluster under the
lea of the apothecia: this is especially the case on loose stones where
conditions are extremely dry.”
On the Continent an interesting study of the lichens of high altitudes
was made by Maheu[1200] in the Savoyard Oberland. On the Great Casse at
a height of 3861 m. he collected four mosses and sixteen lichens. These
were:
_Stereocaulon condensatum._
_Gyrophora cylindrica._
_Gyrophora spodochroa._
_Solorina crocea._
_Solorina saccata._
_Parmelia encausta._
_Candelaria concolor._
_Caloplaca pyracea_ var. _nivalis_.
_Haematomma ventosum._
_Acarospora smaragdula._
_Psora decipiens._
_Buellia discolor._
_Buellia stellulata._
_Lecidea contigua_ var. _steriza_.
_Lecidea confluens._
_Dermatocarpon hepaticum._
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[1201], in describing lichens from Arctic America brought home
by the traveller, Sir John Richardson, quotes from the latter that: “the
terrestrial lichens were gathered on Great Bear, and Great Slave Lakes
before starting on our summer voyages after the snow had melted....
The barren grounds are densely covered for many hundreds of miles with
_Corniculariae_ and _Cetrariae_, and where the ground is moist with
_Cladoniae_, while the boulders thickly scattered over the surface are
clothed with _Gyrophorae_.... The smaller stones on the gravelly ridges
of the Barren Grounds are covered with lichens.”
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[1202] has noted, as characteristic of tundra formations, the
compact cushion-like growth of the mosses which are thus enabled to store
up water and to conduct it by capillarity throughout the mass to the
highest stalks. Certain tundra lichens take on the same growth character
as adaptations to the strenuous life conditions. _Cetraria glauca_ f.
_spadicea_ with f. _congesta_ and _C. crispa_ are examples of this
compact growth: they form a soft thick carpet of a yellowish-grey colour.
_Cladoniae_ also grow in crowded tufts, but are generally to be found in
the more sheltered positions, in valleys between the tundra hills and in
the clefts of the rocks, or between great boulders and stones where there
is also more moisture.
The same kinds of lichens occur all over these northern regions. Birger
Nilson[1203] gives as the principal earth-lichens in Swedish Lappland,
_Alectoria ochroleuca_, _A. nigricans_, _Cetraria nivalis_, _C.
cucullata_, _Cladonia uncialis_, _Thamnolia_ (_Cerania_) _vermicularis_
and _Sphaerophorus coralloides_.
Darbishire[1204] speaks of the extensive beds of various species of
_Cetraria_ in Ellesmere Land and King Oscar Land. _Alectoria nigricans_
and _A. ochrolenca_ were often found in pure communities, but even more
frequently in close company with mosses. Though these fruticose lichens
are not represented by many species in Arctic regions, they cover a very
extensive area and form a very important feature in the vegetation.
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[1205] found it one of the
dominant species at Biskra in the Sahara where it grows on the rocks.
Patouillard[1206] in describing the flora of Tunis speaks of the great
patches (societies) of _Lecanora crassa_ f. _deserti_ which at a distance
look like milk spilled on the ground, or if growing on unequal surfaces
take the aspect of plaster that has been passed over by some wheeled
vehicle. At Biskra species of _Heppia_ grow on the sand. Steiner[1207]
also records the frequency of _Heppia_ and of _Endocarpon_ in the Sahara
as well as of Gloeolichens which, as they are associated with gelatinous
blue-green algae, can endure extreme and long-continued desiccation.
These lichens, however, only form communities in clefts among the rocks
where these abut on the desert. In the great plains the sand is too
mobile and too often shifted by the sirocco to enable them to settle.
Bruce Fink[1208] discusses desert lichens and their adaptive characters:
crustaceous species with a stout cortex are best able to withstand the
long dry periods; conspicuously lobed thalli are lacking, as are lichens
with fruticose structure though he thinks the latter are prevented
from developing by the exposure to high winds and driving sand storms.
Herre’s[1209] study of the desert lichen flora at Reno, Nevada, is
full of interest. The district is situated at an altitude of 4500 feet
east of the Sierra Nevada Mountains. The annual rainfall averages 8·21
inches, and a large part falls as snow during the winter months or as
early spring rain. The summer is hot and dry and the diurnal changes of
temperature are very great. Strong drying winds from the west or north
are frequent.
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[1210] in the beds of streams, covering them
often with a thick felt; it is attached at the base and the rather narrow
fronds float freely in the current. The gonidium is _Nostoc_ sp., and
the thallus is of a bluish-grey colour; the fruits are small discoid
reddish apothecia with an evanescent margin. It is closely allied to
_Peltigerae_, some of which are moisture-loving though not truly aquatic.
The nearest approach to aquatic habit among the foliose forms in our
country is _Dermatocarpon aquaticum_, with thick coriaceous rather
contorted lobes; it inhabits rocks and stones in streams and lakes.
Somewhat less continuously aquatic is _D. miniatum_ var. _complicatum_
which grows on damp rocks exposed to spray or occasionally to inundation.
Lindsay[1211] has described it “on boulders by the side of the Tay,
frequently covered by the river when flooded, and of a deep olive colour
when under water”: both these lichens have a wide distribution in Europe,
Africa, America and New Zealand.
In a discussion of lake shore plants Conway Macmillan[1212] describes on
the flat shores a _Dermatocarpon_ zone on the wet area nearest the lake,
behind that a _Biatora_ zone and further landward a _Cladonia_ zone. On
rounded rocky shores the same zones followed each other but were less
broad: they were so close together that the _Cladoniae_, which with
_Stereocaulon paschale_ grow in profusion on all such shores, occurred
within a couple of feet of the high-water mark.
M. C. Knowles[1213] reports concerning the lichen flora of some mountain
lakes in Waterford, that a band of _Dermatocarpon miniatum_ var.
_complicatum_ six feet wide grew all the way round the lakes between the
winter and summer level of the water. Below that zone _D. aquaticum_
formed another belt mingled with the moss _Fontinalis_ and several
species of crustaceous lichens _Staurotheleae_, _Polyblastiae_, etc.
Bruce Fink[1214] gives as a typical “amphibious angiocarpous lichen
formation” of wet rocks in Minnesota: _Dermatocarpon aquaticum_, _D.
miniatum_ var. _complicatum_, _Staurothele clopima_ and _Verrucaria
viridula_. These “formations,” he says, “may be seen complete in places
along the shores of Vermillion Lake and less well represented at other
portions of the lake shore.” Macmillan found that on the rocky shores of
Lake Superior the _Dermatocarpon_ zone also occurred nearest the water.
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[1215] gives as “formations” on rocks or boulders in the beds of
streams in Thuringia:
_Verrucaria aethiobola._
_Verrucaria hydrela._
_Dermatocarpon aquaticum._
_Bacidia inundata._
_Lecanora aquatica._
In their ecological study of Perthshire lichens Wheldon and Wilson[1216]
give two “formations.” The first is on rocks submerged for long periods,
though in dry weather the lichens may be exposed, and can withstand
desiccation for a considerable time:
_Pterygium Kenmorensis._
_Collema fluviatile._
_Lecanora lacustris._
_Lecanora epulotica._
_Bacidia inundata._
_Rhizocarpum obscuration._
_Rhizocarpum petraeum._
_Lecidea contigua._
_Lecidea albocoerulescens._
_Dermatocarpon miniatum var. complicatum._
_Dermatocarpon aquaticum._
_Verrucaria laevata._
_Verrucaria aethiobola._
_Verrucaria margacea._
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[1217], as by the sea-side, the substratum seems to be
of minor importance.
D. LICHENS AS PIONEERS
_a._ 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[1218]
who thus describes the succession of plants: “Crustaceous lichens,” he
writes, “are the first foundation of vegetation. Though hitherto we have
considered theirs a trifling place among plants, nevertheless they are
of great importance at that first stage in the economy of nature. When
the rocks emerge from the seas, they are so polished by the force of
the waves, that scarcely any kind of plant could settle on them, seen
more especially near the sea. But very soon, in truth, the smallest
crustaceous lichens begin to cover those arid rocks, and are sustained by
minute quantities of soil and by imperceptible particles brought to them
by rain and by the atmosphere. These lichens in time become converted by
decay into a thin layer of humus, so that at length imbricate lichens
are able to thrust their rhizoids into it. As these in turn change to
humus by natural decay, various mosses such as _Hypnum_, _Bryum_ and
_Polytrichum_ follow, and find suitable place and nourishment. In time
there is produced by the dying down of the mosses such a quantity of soil
that herbs and shrubs are able to establish themselves and maintain their
existence.”
Similar observations have been made since Linnaeus’s day, among others
by Guembel[1219] in his account of _Lecanora ventosa_. Either by the
excretion of carbon dioxide which acidifies the surrounding moisture, or
by the mechanical action of hyphae and rhizinae, the component particles
of rocks such as granite are gradually dissolved and broken up. Rocks
exposed to weather alone are unchanged, while those covered with lichens
have their surface disintegrated and destroyed.
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[1220] has pointed out the succession observed on roofs of
houses as: “first some lichen such as _Lecanora saxicola_, then the moss
_Grimmia pulvinata_, which forms compact cushions on which later grow
_Poa compressa_, small crucifers, etc.”
Goeppert[1220] has noted as special rock-destroyers some foliaceous
species, _Parmelia saxatilis_, _P. stygia_ and _P. encausta_, the
underlying rock being roughened and broken up by their rhizoids. Species
of _Gyrophora_ and _Sphaerophorus_ have the same disintegrating effect,
so that the surface of the rock may in time lose its coherence to a depth
of 2 to 4 inches. Crustaceous species such as _Lecanora polytropa_,
_Candelariella vitellina_, etc., exercise an equally powerful solvent
action, while underneath closely appressed growers like _Lecanora atra_
and _Acarospora smaragdula_ the stone is converted to a friable substance
that can be sliced away with a knife.
Salter[1221] concluded that oxalic acid was the principal agent in
disintegration. He found that it acted more or less rapidly on minerals
and almost any class of saline compounds; it even attacked glass finely
powdered, though silica remained unchanged.
Bachmann[1222] found that granite was reduced by lichens to a clay-like
granular yellow mass in a comparatively short time, the lichen seizing on
the particles of mica first; but the spread of the lichen over the rock,
he observes, is largely directed by the amount of humidity and by the
chance of gaining a foothold. In the case of calcareous rocks he[1223]
tested the relative dampness of those containing lichens and those that
were lichen-free. In the former case water was absorbed more freely and
retained much longer than in the barren rock, thus encouraging further
vegetation.
Lucy E. Braun[1224] has described the successive colonization of
limestone conglomerate in Cincinnati. The rock is somewhat resistant
to erosion and stands out in irregular outcrops on the hillsides of
the region. The first plants to gain a footing are certain crustaceous
lichens, _Lecidea_ sp., _Pertusaria communis_, _Staurothele umbrina_,
_Verrucaria muralis_ and _Placodium citrinum_ which occur as patches on
the smoother and more exposed rock faces. With these were associated
small quantities of a moss, _Grimmia apocarpa_. In the second stage of
growth _Dermatocarpon miniatum_, and, to a lesser degree, a gelatinous
_Omphalaria_ sp. were the most prominent plants, but mosses were more in
evidence, and the next stage consisted almost exclusively of mosses and
hepatics with _Peltigera canina_. A thick layer of humus was gradually
built up by these plants on which Phanerogamous plants were able to
flourish.
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[1225] on
the surface of the pumice and ash, and on the loose stones in the ravines
of the mountain. It was only at a later stage that lichens appeared.
_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.