NORTHERN COLORADO PLANT COMMUNITIES 23 1 



year among the trees, it makes the growing season for plants very 

 short except in the treeless portions of the park. In dense forests of 

 lodgepole pine and Engelmann spruce snow lasts until April or May at 

 Tolland, while at higher altitudes, as at Jenny Lake, there may be snow 

 fields many acres in extent as late as the middle of June. The visitor 

 who sees the forest in midsummer may well wonder at the absence of 

 undergrowth if he be not informed as to conditions in the spring months ; 

 surely dense shade alone would not account for the paucity of vegetation. 

 In more open forest areas where the sun is able to melt the snow earlier a 

 much richer flora of the forest floor is to be found. Here, in addition to 

 blueberries, which form the main undergrowth in the denser forest, 

 there are roses, junipers, various grasses and different flowering herbs. 



The principal and secondary species of the various formations are 

 quite different, as would be expected. Engelmann spruce forests offer 

 soil-moisture conditions intermediate between those of the usual dry 

 coniferous forest and the moister associations of aspen, willow and alder. 

 Hence plants characteristic of these latter formations occasionally find 

 their way as invaders among the spruces but only very few species are 

 able to pass from willow thicket to dry pine forest or vice versa. 



Most interesting in its composition is the sage-brush scrub, where one 

 may find, as it were, a piece of the foothills or plains transported to this 

 montane situation. The plants which make up this formation are 

 nearly all such as occur more abundantly at lower elevations and here 

 reach their highest altitudinal limit. 



Edaphic Considerations. — Climatic conditions are much the same 

 throughout Boulder Park. The writer's observations show an inversion 

 of temperature on still nights which makes the temperature of the lower 

 parts of the park from 2 to 5 degrees F. below that of the ridges. This 

 difference may have some influence on the distribution of trees, especially 

 in the growth of seedlings, but it certainly has very little other effect. It 

 is therefore to edaphic factors that we must look for an explanation of the 

 different plant formations. Under the term "edaphic" we include such 

 features as texture of the soil, chemical composition, moisture content and 

 temperatures below the surface; also slope, direction of exposure and 

 all other non-climatic factors. 



