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.