380 BOTANICAL GAZETTE [NOVEMBER 
ASPEN ASSOCIATION 
RAMALEY (14, p. 251); YOUNG (28, p. 347). 
Botanists are familiar with the réle of Populus tremuloides in revegetation 
of burned areas, and it is prominent in the montane zone in Colorado in this 
capacity. The general area of the lower foothills, however, is too dry for 
establishment of aspens, and they occur only locally, in ravines even more 
mesophytic, perhaps, than the ordinary habitat of the canyon forest. Thus 
in the Boulder area the stations below 7200 ft. in which aspens have been 
observed in local abundance are very infrequent. Such stations are usually 
in sheltered ravines with deep humous soil, abundantly moist. The trees in 
places are large, the undergrowth very mesophytic, with Thalictrum, Heracleum, 
Castilleja spp., etc., and particularly Aquilegia coerulea. At about 7800 ft. in 
the same vicinity aspens begin to appear in small clumps on the granite- 
gravel upland, among more frequent clumps of rock pines. No connection 
with former fires could here be made out; appearances indicated that perhaps 
there the aspens might be associated with the moist patches resulting from the 
tardy disappearance of the deeper snowdrifts of winter. The conspicuous 
yellow color of the aspens in fall probably tends to exaggerate the popular 
notion of their frequency of occurrence. 
MIXED SHRUB ASSOCIATION 
DANIELS (4, p. 20); RAMALEY (12, p. 127), shrubs of the canyon forest; 
SHANTZ (22, p. 179), thicket formation, in part; notes on distribution and eco- 
logical relations of the species; RAMALEY (11); ROBBINS (16); SCHNEIDER (20). 
The shrub associations of the foothills, like the deciduous tree 
growths, are generally found in rocky or coarse soil stations with 
constant moisture supply in the substratum, which is reached by the 
deep root systems. Local distribution, as in the case of the pines, 
is probably restricted by unfavorable conditions for germination 
over a large part of the general area. It has been observed that, on 
irregular slopes where the distribution of snow in late spring is 
uneven, the shrubs occupy the moister spots determined by the 
deeper snow patches. In deep moist soil it is likely that the shrubs 
are soon succeeded by trees, as has been observed in some stations. 
The shrub species most commonly found appear in the following 
selected list. Certain of the canyon forest plants which occur with 
the shrubs in the less mesophytic stations without attaining tree 
size are included here also. It is significant that so many of the 
