362 ECOLOGY 



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 1 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" 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 1833. 



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 saxi- 

 colous 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 Paulson and Thompson 191 1. 3 Paulson and Thompson 1912. 



