Botany in Colorado 31 



the parent trees, near logs, rocks or in the shade of brush. In 

 many places a nurse crop of aspen furnishes the necessary shade 

 for the protection of the young trees. 



On moist north exposures and sheltered slopes in canons and 

 on benches Bull Pine occurs in mixture with Douglas Fir, Pseu- 

 dotsuga taxifolia. In such places may also be found scattered 

 individuals or groups of white Fir, Abies concolor, and Blue 

 Spruce, Picea parryana. The White Fir is a very tolerant 

 tree, small individuals may often be found beneath a rather heavy 

 cover of the associated species. 



3. The Mid-Forest Zone. 



The altitude of this zone is from 8,000 feet to about 10,000 

 feet. In most places the zone is characterized by a closer forest 

 cover since most of the bull pine has been left behind and the 

 ground has been occupied by more tolerant species. From the 

 central portion of the state northward lodgepole pine, Pinus 

 murrayana, is the most conspicuous tree. It frequently forms 

 very dense pure stands. The species is very tolerant of shade 

 when young; it may persist for many years in very dense 

 stands, but it thrives best in full light. In all close stands the 

 diameter increment is extremely low and the trees soon become 

 tall and slender with slight taper. The species produces each 

 year an abundance of seed of high rate of germination. The 

 cones may remain on the branches for many years and are only 

 opened in some cases by fire. Natural regeneration is very 

 slow, but with the aid of a light fire which is not of sufficient 

 severity to destroy the cones, but to open them and allow the 

 seeds to fall, seeding may be exceedingly thick and even. Even 

 aged stands with full top light may often be found over areas 

 which have previously been swept by fire. Full light and a 

 mineral soil free from litter are the chief requirements of good 

 reproduction. 



Bull pine also occurs in this zone, as does the limber pine, 

 Pinus flexilis, which because of its rather plastic nature is found 

 in a variety of habitats. Limber pine is found on exposed, 

 extremely dry rocky south and east exposures, on summits of 

 ridges and sometimes in moister canons and on the banks of 

 mountain streams. The species is usually found in dry, rocky, 

 very shallow soil with little or no humus. It sometimes occurs 

 on almost bare granite. The species requires much light and 

 it never forms continuous, dense stands. The foxtail pine, 

 Pinus aristata, is also found at this altitude, but it is almost 

 always confined to dry ridges and rocky ledges of south slopes. 

 In soil and light requirements this species is similar to the 



