IQOS] 



COOPER— ALPINE VEGETATION IN COLORADO 



335 



retains its erect habit, but the branches point to leeward of the prev- 

 alent wind. Occasionally the trunk is prostrate, but in these cases 

 erect branches are produced, the branchlets of which are turned to 

 leeward. There is frequently a tendency in this species, when 

 growing, as it occasionally does, in much exposed places, to form long 

 narrow groups of individuals, parallel to the direction of the prevailmg 

 wind, the height gradually rising from two or three decimeters at the 

 windward end to a meter or more at the leeward end of the group. The 

 explanation of this phenomenon evidently is that some small shelter 



y^ ^*^»^ ■-■■ 



trail. 



Fig. S.—Piuus flexills grounng in a very exposed situation near the Long's 



Peak 



r 



allows a seedling to live and develop into a small tree; and this gives 

 protection in its lee to others, each of which in turn is able to nsc 

 slightly higher than the one in front. The same mode of growth i. 

 seen also in such small shrubs as Betula glandiilosa and the low alpine 

 form of Populus tremidoides, which are found in the same habitat 

 'fi'ith Picea Engelmanni. . mi 



Having now a general view of the forest at its upper limit, we ^M 

 consider the question as to whether these two trees are at presen 

 invading the grassland. The evidence here, though it is not as com- 

 plete as might be wished, points toward the general conclusion 



