PURE AND MIXED FOREST. 



41 



leaf trees and conifers. (See fig. 36 and Pis. XX, XXI.) 

 The line between gregarious and scattered species is not 

 always well marked, because it often happens that a 

 tree may be gregarious in one place, and live with many 

 others elsewhere. The Western Yellow Pine, which 

 forms, on the plateau of central Arizona, perhaps the 

 largest pure Pine for- 

 est of the earth, is 

 frequently found 

 growing with other 

 species in the moun- 

 tains, especially in 

 the Sierra Nevada of 

 central California. 

 (See figs. 34, 35.) 



Trees which oc- 

 cupy the ground to 

 the exclusion of all 

 others do so because 

 they succeed better, 

 under the condi- 

 tions, than their com- 

 petitors. (See fig. 

 37.) It may be that 

 they are able to get 

 on with less water, or 

 to grow on poorer soil, their rate of growth or power of re- 

 production may be greater, or there may be some other 

 reason why they are better fitted for their surroundings. 

 But the gregarious trees are not all alike in their ability 

 to sustain themselves in different situations, while the 

 differences between some of the mixed-forest species 

 are very marked indeed. Thus Black Walnut, as a 



FIG. 39. Sprouts of Pitch Pine from the neigh- 

 borhood of Toms River, New Jersey. 



