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.