i9o8] COOPER— ALPINE VEGETATION IN COLORADO 333 



the factor of soil water content. The white pine society consists of an 

 almost pure growth of Pinus flexilis. It covers the dry summits and 

 upper slopes of the ridges between the canons up to the limit of tree 

 growth. The spruce-fir society is confined to situations where there 

 is an abundant soil water content. It follows the canon bottoms 

 up to where they emerge above timberline, and extends up the sides 

 as far as there is a sufficient amount of ground water available. 

 The two trees that dominate the society arc the Engelmann spruce 

 {Picea Engelmanni) and the subalpine fir {Abies lasiocarpa). The 

 latter is more abundant close to the streams, and seldom or never m 

 this region ascends quite to timberline. 



r 



I. The Knimmlwh {figs. 7, 8) 



The altitude of the present upper limit of tree growth averages 

 about 3350°^, with a range between extremes of perhaps 150 or iSo"'. 

 Since Abies lasiocarpa does not ascend to timberline, the Krummhoh 

 is composed of two trees only, Pinns flexilis and Picea Engelmanni, 

 the former on the ridges, the latter in the canons. Both reach their 

 upper limit at approximately the same altitude, but their appearance 

 and behavior there is so strikingly different that a description of each 

 will be of interest. 



Pinus flexilis.— This tree, even when growing in its most con- 

 genial habitat, is very irregular and contorted. In its Krummhoh form 

 it assumes the most' fantastic shapes and yet in all of these ^ genera 

 uniformity of behavior may be seen. A very typical growth ot the 

 Krnmmholz form is found^ at timberline on the Long's Peak trai 

 (figs. 7, 8) . Here there is considerable exposure to the prevalent ^^ esi 

 wind which sweeps with great force over a saddle on the ridge bchinc 

 The trees form an open growth, with the individuals usually growing 

 singly. They are of great age, and often 30^"^ thick, but tcnv a 

 more than i . 5- in height. The hve branches all point to the eastj ara, 

 those on the windward sides are dry and dead, and the wmdward^po ^ 

 tions of the trunks are in nearlv ever>' case devoid of bark. rcq 

 the greater part of the trunk is worn away by the ice P.^^^icles bo 

 by the winter gales. The degre6 of exposure to the ^vmjis f °m 



west determines the height of the tree. If --^f;~be^^ 

 trunk may be nearly erect, but in the most expost^d places 



