30 TH E STUDY OF PLANT COMMUNITIES * Chapter II 



limit of a growth form to another may likewise have certain char- 

 acteristics, probably physiological, which limit the extent of their 

 area of favorable growth. Recently it has been shown that certain 

 grasses that seem to range throughout the latitudinal extent of the 

 prairie cannot be satisfactorily used to reseed northern areas when 

 the seed has been obtained in the south. Foresters, too, recognize 

 that it is advisable to replant with seedlings grown from locally 

 produced seed. 



The more extreme (less favorable) the climatic conditions, the 

 less diversity can there be in the species and the fewer the species 

 will be because not many will have the adaptations necessary for 

 their survival. The numbers of species in a general vegetation type 

 are by no means constant throughout, especially nearing the limits 

 of the type. Here it might be expected that conditions would be 

 something less than optimum and that some species would be less 

 well adapted to the extremes than others. The same can be said for 

 numbers of individuals of a species. As conditions favoring a spe- 

 cies vary from their maximum, the number of individuals may be 

 expected likewise to fluctuate, and, near the limits of the range of 

 a growth form, the numbers of individuals of that growth form 

 would also decline. In the same sense, but in the opposite direction, 

 this marginal area would support a few species and individuals of 

 the contiguous growth form; thus transition zones between com- 

 munities are characteristic. Sometimes these transitions are wide, 

 sometimes relatively narrow, but rarely does one community, 

 large or small, have a sharp line of demarcation between itself and 

 its neighbor. 



Local Habitats and Species Differences.— Climatic areas are of 

 considerable extent and usually include local diverse conditions of 

 soil or topography. Often these variations are so great as to result 

 in localized environments (habitats) quite unfavorable to the spe- 

 cies and even to the growth form of the region as a whole. Often 

 the conditions may be so much more favorable than those of the 

 general climate that a growth form from a neighboring region can 

 compete successfully. This is well illustrated by the trees and 

 shrubs extending far into the prairie along the streams, where the 

 favorable soil moisture is sufficient for them to compete success- 

 fully in a grassland climate. 



