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:

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.