SUCCESSIONS ON ROCK 227 



An area of very hard rock affords no available water for plants 

 except from dew and rain. Soft and porous rocks, however, con- 

 tain varying amounts of water and this may be a factor of con- 

 siderable importance. The presence or absence of cracks and 

 crevices also are important in their influence on the rate of weather- 

 ing and on the establishment of certain kinds of plants. The slope 

 of rock surfaces may vary all the way from horizontal to vertical 

 and this leads to great variations in exposure to climatic factors. 

 A south-facing cliff is extremely xeric while a north-facing slope, 

 because of its protection from heat and evaporation, may be almost 

 mesic. 



On a hard rock surface the first pioneer plants are crustose lichens 

 of various species. They vary in color, some being pale green but 

 others gray, black, yellow or red. Some are so thin that they appear 

 like spots of paint on the rock. They grow very slowly and seldom 

 cover the entire surface but they gradually disintegrate the rock 

 superficially and produce a slight roughness which makes possible 

 the growth of some xeric foUose lichens. These also grow very 

 slowly but they hasten the disintegration of the rock somewhat and 

 they catch some wind-blown dust and so start the building of a soil. 



A very small amount of soil is enough to prepare the way for xeric 

 mosses which will grow along with the lichens. As more soil accumu- 

 lates xeric grasses and some other herbaceous seed plants appear. 

 The species of seed plants that come in as pioneers depend upon the 

 nature of the feeder vegetation, that is, the surrounding communities 

 from which migrations may take place. They are likely to be such 

 plants as golden-rod (Solidago), cinquefoil {Pontentilla), chickweed 

 {Cerastium) , harebell (Campanula americana) , etc. The accumulation 

 of soil will now go on more rapidly and such xeric shrubs as black- 

 berries (Rubiis), smooth sumach (Rhm glabra), and sassafras 

 (Sassafras mriifoliiim) begin to appear. By this time the rock 

 will be completely covered with soil containing a supply of humus, 

 and trees gradually enter. The first trees are likely to be xeric oaks 

 and hickories accompanied by various other species and these are 

 followed in order by the climax mesic trees, hard maple and beech, 

 accompanied by red oak (Quercus rubra), basswood (Tilia ameri- 

 cana), etc. (Fig. 103). 



The rock succession discussed above is outlined as it would nor- 

 mally occur on an area that was essentially horizontal. In the_^case of 

 a vertical cliff the conditions are even more severe and succession is 



