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.