lyi] FLORA OF COLUMBIA AND VICINITY 49 



found even on the high ridges. On hills about the Pinnacles the 

 river birch (Betula nigra), elsewhere not found except at the 

 margins of streams, is not infrequent. The forest floor is usually 

 very weedy and grassy and rarely well-marked. 



V. Sylvales quercoides sassafrasinae. The sassafras 

 subassociation of the oak sylva occupies certain comparatively 

 light soils in the vicinity of the Pinnacles, and perhaps elsewhere. 

 Along with the sassafras grow the black and red oaks ( Quercus 

 tinctoria, Q. imbricaria, Q. rubra, and Q. Schneckii). The white 

 oaks occur in relatively less abundance. The herbs are such as 

 prefer light dry soils. Thus Phlox pilosa replaces P. divaricata, 

 so abundant in the sylva about Columbia. The forest floor is 

 well marked, showing that the black oak forest, unlike the pre- 

 ceding, is not derived, but original. 

 C. Arbustales. Plants of the mesophytic thicket zone. 



The thicket in the vicinity of Columbia is the oak forest 

 reduced to brush conditions. It is seen in all stages from the 

 second growth forest to half-cleared fields, and is found along 

 fences and other odds and ends of the farm. The peculiarity 

 of the thicket formation is the contrariety of its elements. Two 

 opposite forces are at work. The brush seeks to grow into 

 forest, and if left alone, would do so. But disintegrating forces 

 are steadily at work. These are partly due to human agency, to 

 fires, pasturings, underbrushing and the like, and partly to the 

 eroding of hill-slopes, and to the destruction of the leaf-mould 

 so essential to the forest floor. 



The first effect of the underbrushing of a wooded tract 

 is to expose the sylvan species to unwonted light. Violent 

 changes of temperature and weather are mitigated to a certain 

 extent by the forest trees and shrubs. Shade and shelter, then, 

 are taken away. On slopes, the soil, no longer protected by the 



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