142 



encountered in the poorly-watered and largely unleashed soils of the 

 foothills. 



(2) Douglas fir is often in close competition with yellow pine through 

 much of the altitudinal range of the latter. Douglas fir is undoubtedly 

 more " tolerant " of shade than the pine, is more effective in photosyn- 

 thesis, and uses less water. Douglas fir seedlings root almost' as 

 deeply as those of yellow pine in the first few weeks of life ; but fir 

 seedlings are quite evidently more subject to directly injurious effects 

 from high temperatures, than are yellow pine seedlings of equal age. 

 Therefore, it is apparent that Douglas fir can tolerate almost the same 

 condition as yellow pine, and on a soil containing sufficient moisture 

 to support a moderately dense stand, Douglas fir may be the better 

 tree because of its greater photosynthetic capacity, and may even- 

 tually dominate the pine. But there is no point in the field evidence 

 clearer than the fact that Douglas fir seedlings will not tolerate full 

 sunlight on a slope where its heating effect is greatest. Therefore, 

 the fir can be a pioneer only on northerly aspects or in the shade of 

 such objects as boulders. 



It would appear to be misleading to assume, in any instance, that fir 

 replaces pine as a direct result of favorable moisture conditions. Con- 

 ditions may be conceived in which great moisture would so modify the 

 surface temperatures as to make possible fir reproduction in full and di- 

 rect sunlight, but under the conditions of the present study the moisture 

 factor is effective only through the production of canopies and shade. 



(3) Engelmann spruce is even more effective in photosynthesis than 

 Douglas fir and uses its water more economically. The seedlings of 

 spruce, however, are very slow in placing their roots at a depth, seem- 

 ing to prefer the surface layer of the soil and to branch considerably 

 in this layer. This is puzzling in view of the extreme dryness which 

 is sometimes attained by a surface layer composed almost wholly of 

 only partly decomposed litter. It is possible, however, that the roots 

 do penetrate to the top of the mineral soil, with its more certain 

 moisture, and that the seedlings are satisfied with the small supply of 

 moisture insured by this. Another puzzling fact is that spruce 

 seedlings appear to be less easily injured by excessive temperatures 

 than seedlings of Douglas fir, although when drying becomes a factor, 

 the reverse is true. 



From the present evidence the conclusion is plain that Engelmann 

 spruce occupies its cold sites and high elevations purely because of a 

 superior ability to grow with less direct light and with lower tempera- 

 ture than are required by any of the other species. Again it is mis- 

 leading to speak of the direct effects of moisture as a determinant, 

 except in the sense that spruce seedlings are evidently not adapted to 

 situations in which the surface soil dries quickly. There is, however. 

 another soil factor which the writer believes to be very important, 

 and concerning which, it is hoped, convincing evidence may be 

 presented within a short time. This is the factor of chemical com- 

 position of the types of soil commonly claimed so exclusively by spruce 

 or perhaps, to use a more familiar term, the factor of soil acidity, 

 although it is the present belief that spruce is equally tolerant of high 

 acidity and strong alkalinity. Without doubt, in some instances, it 

 is the chemical conditions of the soil, rather than moisture contents or 

 light conditions, which draw a sharp line between spruce and Douglas 

 fir forests. 



