FOREST FORMATIONS AND FOREST TREES 



43 



shapes. 1 At the lower limit of forest growth, on the foothills and mesas 

 adjoining the plains, the trees are unable to extend out to the plains 

 largely because of the fine soil which supports a thick covering of grasses 

 and does not permit trees to become started. Differences in climate 

 are also important. The plains have colder winters and warmer sum- 

 mers, than the adjacent foothills. 2 In the mountain parks, which are 



Fig. 39. — Limber Pine near Pawnee Buttes, Colo. 

 This is the most eastward extension of its range. 



certain broad valleys of the foothill and montane regions, while forests 

 are present on the hillsides there are only a few scattered trees on the 

 level ground below. This absence of trees is probably to be accounted 

 for by the fineness of soil and greater severity of climate. 



1 This subject is discussed with some completeness by Mr. W. S. Cooper in an article referred to in the 

 bibliography. From 11,000 to 1^,000 feet may be considered the usual limit of trees. 



3 A comparison of the climates of plains and, foothills is made by the present writer in the article referred 

 to in the bibliography: "Studies in Mesa and Foothill Vegetation." 



