220 THE STUDY OF PLANT COMMUNITIES ■ Chapter IX 



continuous series of zones girdling a lake that is gradually filling 

 in. Borings of the soil under any zone will show the partially de- 

 composed remains, in vertical sequence, of each of the previous 

 stages of succession that contributed to the development of that 

 zone. 



Xerarch succession on rock follows a definite pattern, whose 

 progress is controlled by the rate at which soil forms and accumu- 

 lates. Pioneers on rock surfaces are either lichens or mosses ca- 

 pable of growing during the brief periods when water is available 

 to them and lying more or less dormant through the usually longer 

 periods of drought. The pioneer lichens are crustose and foliose 

 types, which usually contribute little to succession since they are 

 not mat-forming. 186 However, they do probably cause corrosion 

 of the rock surface and thus provide some anchorage for other 

 species. Pioneer mosses, on the other hand, are in tufts or clumps, 

 which catch dust and mineral matter from wind and water. This 

 material, combined with the remains of mosses, forms a gradually 

 thickening mat with a periphery of young plants that spreads over 

 bare rock (and the pioneer lichens) and with a central area that 

 may become thick enough to support foliose lichens (Cladonia 

 especially), larger mosses such as Polytrichum, or often species 

 of Selaginella. Such bushy plants catch and hold still more mineral 

 material, and their death adds much organic soil to the mat. 



When soil has built up sufficiently to provide the necessary an- 

 chorage and water-retaining ability, seed plants appear on the 

 mats. A number of hardy, annual herbs, often weeds of field and 

 garden, appear first and are followed by biennials and perennials, 

 of which grasses are most abundant. Later a shrub stage becomes 

 dominant, which usually includes some species of sumac (Rhus) 

 and several ericaceous shrubs. By this time, the mats may be sev- 

 eral inches or a foot thick and then trees make their appearance. 



Just as a series of girdles of vegetation usually surrounds a lake 

 and indicates the sequence of succession from open water to solid 

 ground, so the progress of succession on rock may be seen as a 

 series of girdles of vegetation from the periphery to the center of 

 an old mat. Pioneers are at the outer margin of the mat, and each 

 successive stage of dominance is nearer the center where, on the 

 thickest soil, trees may be present. 



