FOREST TYPES IX CENTRAL ROCKY MOUNTAINS. 145 



be equally or more abundant. The disappearance of the yellow pine 

 and limber pine which at the outset found the conditions favorable 

 is a slow and almost pathetic process. They do not, except in infre- 

 quent years, suffer for lack of moisture; and as their crowns rarely 

 appear among the dominants they do not even suffer the most severe 

 winter drying, which is very rigorous for the tallest trees on this site. 

 Unlike the suppression which is so common in crowded stands on 

 less moist sites, which takes the form of complete stunting, the 

 strictly shade suppression here results in putting all of the energy 

 of the tree into height growth, the side branches being quickly 

 eliminated, and no food being left for the proper strengthening of the 

 stem. The result is a long, slender, supple pole, with only a tuft of 

 green at the top, which is invariably a little lower than the tops of 

 the surrounding firs. Limber pines and yellow pines which reach 

 this stage are usually destroyed by snow breakage. 



(4) The foot of the north slope. — The steepest portion of a slope is 

 very commonly just below the middle. Further down, deposition 

 of material from the upper slope may more than keep abreast of 

 erosion. Hence the ground tends to flatten out, and frequently a 

 bench is formed somewhat above the stream bed. For a considerable 

 part of the year the site may be shaded by the steeper ground to the 

 south. It receives by transport the best of the soil formed on the 

 entire slope; very commonly it receives also the run-off from heavy 

 rains and, with some rock formations, the seepage. There is, then. 

 a greater water supply than elsewhere, a deep, spongy, retentive soil 

 and little insolation to dissipate it. 



Probably because such a site is a catch-all for seeds as well as for 

 soil-building material, it can not long remain denuded. Rank herbs 

 appear almost immediately, and tree seedlings rarely start except 

 with the keenest competition for light. Year after year the herbs 

 grow up about the seedlings, and there is at no time opportunity for 

 any but the most shade-tolerant trees. Even these have a very 

 long struggle before they eventually find " their place in the sun." 

 The conditions, of course, do not change materially after a forest is 

 established. There will always be a dearth of light. If the stand 

 is very dense, in a protracted drought there may be a fairly complete 

 exhaustion of the moisture as deeply as the roots penetrate. The 

 most prohibitive condition, however, probably arises from the undis- 

 turbed accumulation of litter on the forest floor. After the snow has 

 melted in the spring, and this loose, jack-straw mass has slowly dried 

 out, it will in all probability not become wetted again until the next 

 snow falls. Certain greenhouse tests made by the writer have indi- 

 cated that the refusal of resinous needle litter to imbibe water creates 

 a serious problem in connection with the germination of seeds. In 

 consequence, a large part of the floor under spruce may be as barren 

 as the desert, and it is especially with such conditions that the marked 

 results of any soil disturbance^ such as occurs through the skidding 

 of logs, are to be noted in succeeding reproduction. 



The description here given refers in a degree to the lower portions 

 of all north slopes, and commonly to that portion of the stream bench 

 contiguous thereto. It is, of course, evident, that in so far as the 

 slope itself is short or moderate in gradient, it may fail to produce the 

 depicted conditions at its foot, and under certain circumstances 

 there may be no area in this position that would be prohibitive to 



73045°— 24 10 



