FOREST TYPES IN CENTRAL ROCKY MOUNTAINS. 141 



one point to another that there can not be a great difference, for 

 example, between a sheltered and an exposed spot on the same slope 

 separated, say, by a distance of 50 feet. Yet it is obvious that the 

 insolation received on the one spot must have the greatest possible 

 influence on the critical conditions for the regeneration of forest trees. 

 Therefore, although air temperatures do generally outline the types 

 fairly well, one must conclude from certain important exceptions that 

 they reflect rather than define the controlling conditions. 



The study has shown not only that soil temperatures at a depth of 

 1 foot bring out more clearly the differences in aspect, which for 

 denuded sites are fully as great as the differences resulting from eleva- 

 tion, but also that the correlation between soil temperatures and the 

 reproduction of the several species is closer than that between air 

 temperatures and species. The meager data on surface-soil tempera- 

 tures that are so far available indicate that the differences between 

 sites which encourage different species are far greater than might be 

 thought possible — at least three or four times as great as the differ- 

 ences between temperatures at a depth of 1 foot. To use again the 

 example taken above, the spot on a north slope which receives full 

 insolation for two or three hours in the middle of the day may have 

 surface maxima 20° to 25° higher than the nearby sheltered spot, and 

 a similarly exposed south slope may attain surface temperatures 30° 

 to 40° higher still. The surface temperatures on south slopes, more- 

 over, occasionally attain to 150° F., a temperature which, it is 

 believed, is likely to be fatal to any young plant, independent of its 

 drying effect on either plant or soil. 



The influence of direct insolation in drying the surface of the soil is 

 hardly less potent than its temperature effect in demanding special 

 adaptations of the plant. Again, direct insolation plays an important 

 part in keeping moisture available to both young and old trees dur- 

 ing the winter season, when it is likely to be completely withdrawn 

 through the freezing in the soil. 



A frank consideration of the weak as well as the strong points in 

 the evidence which has been adduced in " Physiological Requirements 

 of Rocky Mountain Trees" (6) and in the present bulletin, compels the 

 makingof somewhat tentative conclusions as follows : 



(1) Western yellow pine is not a conservative user of water, nor is 

 it efficient in photosynthesis. It is deduced from this inefficiency that 

 it may thrive best in full sunlight and a warm atmosphere. There is 

 certainly nothing in the evidence to suggest that it prefers a dry to a 

 moist soil. However, its seedlings have the habit of rooting promptly 

 and deeply, a habit which has doubtless been developed because of 

 the need of the species for a warm situation whose surface soil dries 

 out quickly. It may, then, be said unequivocally that yellow pine 

 seeks the warmest sites at middle and low elevations, because of their 

 warmth. With yellow pine at least, the evidence is convincing that 

 warmth of the soil in winter, and availablity of moisture, is a necessity 

 throughout the life of the tree. 



At its lower limit yellow pine may come into competition with 

 pinon, or with grasses and other low plants whose life cycle is com- 

 pleted in the short period of most favorable moisture. Little is known 

 regarding pinon, but it is the writer's belief that this pine subsists on a 

 smaller water supply than yellow pine, and also that it is much less 

 sensitive than yellow pine to the high salt concentrations which are 



