FOREST FORMATIONS AND FOREST TREES 



4i 



feet above the sea, is the upper limit of tree growth, although some 

 shrubby willows occur in the alpine zone above. As noted in the preced- 

 ing pages, the trees of the plains are to be found only along water courses 

 or. on the pine ridges of the high divides between streams. In the 

 following table the general distribution of each tree is stated in relation 

 to the continental divide, which extends north and south through the 



Fig. 37. — Rock Pines 

 Lower limit of tree growth on mesas and foothills. Young sumacs in the fore- 

 ground. Late winter. 



state, dividing it into nearly equal parts. It will be noticed that nearly 

 all of the trees occur both east and west of the divide, somewhere in their 

 distribution. As a matter of fact the same species does not necessarily 

 occur at opposite points east and west of the main range. Those oaks, 

 for example, which are present in the northwestern part of the state do 

 not cross to the foothills of the eastern slope in northern Colorado but 



