252 UNIVERSITY OF COLORADO STUDIES 



Distribution of species. — The tree flora of the state embraces fifty- 

 five species belonging to twenty genera. Most of the individuals, as 

 well as the species in general, are found in the foothill, montane and 

 sub-alpine zones of plant life. Timberline, at about 11,000 or 12,000 

 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 

 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 

 are found at Colorado Springs and southward. The distribution of the 

 pinyon pine is similar. Many of the high-altitude trees extend far north 

 to British Columbia and even Alaska. A considerable number range 

 to the Pacific coast, but only a few show a distribution far eastward. 



Limits of tree distribution. — The upper limit of tree growth, or timber- 

 line, is variable as to altitude. On north exposures the trees do not 

 extend so high up the mountain side as they do on the warmer and less 

 windy south slope. Cold, wind and dryness all combine to limit the 

 upward extension of forests on mountains. At timberline are found 

 scattered clumps of wind-blown trees often contorted into fantastic 

 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 



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

 bibliography. From n,ooo to u,ooo feet may be considered the usual limit of trees. 



' 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." 



