66 A Study of the Vegetation of 



of succession. Starting from the vantage ground of a crack or 

 crevice, the small tufts collect wind-blown soil and lichen remains 

 about them and building upon this substratum increase their 

 area. Well-worn rocks are frequently covered with the black 

 mats of the Grimmia consocies. 



Less xerophytic but far more abundant is Tortula rurolis. 

 Growing in places less dry than those occupied by Grimmia, it 

 surrounds the base of rocky outcrops and occurs on thin stony 

 soil where its soft cushiony mats may cover areas of many square 

 meters. While Tortula is very abundant on basaltic ledges it is 

 sometimes partly or wholly replaced, especially about quartzite 

 rocks, by species of Polytrichum, especially P. piliferum and P. 

 juniperinum. 



In other situations, and especially on canyon walls with north 

 and northeast exposure other mosses are abundant, often covering 

 large areas of sheer rock surface or growing luxuriantly over the 

 thin stony soil. The following lists include the most important 

 of these together with a number of lichens which usually occur 

 with the mosses. Some of these lichens are less xerophytic than 

 the foliaceous forms already mentioned. Indeed, they are the 

 most exacting members of the foliose lichen community. 



Brachythecium albicans Ceratodon purpureus 



Brachythecium erythrorrhizon Encalypta ciliata 



Camptothecium aeneum Grimmia apocarpa 



Camptothecium lutescens Mnium affine 



Cladonia pyxidata Ochrolechia upsaliensis 



Diploschistes scruposus Peltigera canina 



Poa-Polygonum Associes 



In the thin soil between the fragments of broken rock, an area 

 over which Tortula ruralis and other mosses for a time have 

 control, bunches of June grass and plants of Polygonum majus 

 and Plantago purshii begin to appear (Fig. 18). Changes of 

 light and root competition react unfavorably upon the mosses, 

 while shallow-rooted plants are well adapted to dominate just 



66 



