FOREST TYPES IN CENTRAL ROCKY MOUNTAIN'S. 3 



At this stage it is not unimportant whether the high temperatures of the surface 

 soil react directly or indirectly, but it is of greater importance to recognize that 

 the combined effects may be the most common cause of the death of forest-tree 

 seedlings in the region under consideration. The present evidence is that the 

 action is most commonly indirect; that spruce and lodgepole pine seedlings 

 are most sensitive to excessive heat and drying, because their poorly developed 

 roots and thread-like steins do not permit them to push the necessary supplies 

 of water past the superheated section of the stem; that Douglas fir and yellow 

 pine are relatively resistant, because they are much stronger both as to length 

 of root and size of stem. 



It would seem that the species which creates carbohydrates most readily, main- 

 taining the sap of greatest density and osmotic pressure, would, bj r reason of 

 the last-named quality, extract water from the soil to the lowest point, and there 

 is no reason to believe that this is not the case. Even in carefully controlled 

 pan tests, however, and more markedly in the field, drought resistance is much 

 more than extracting the water from the soil particles with which the roots may 

 be in contact. It is the ability to reach with the roots the supply of water that 

 very largely determines which seedlings will last the longest. Thus although 

 Engelmann spruce seedlings do resist drought remarkably well when the losses 

 by transpiration are at a very low rate and when slow capillary action may assist 

 in bringing water to the roots, yet ordinarily they succumb before Douglas fir 

 or yellow pine seedlings, because the latter at the outset develop roots nearly 

 twice as long. On the contrary, spruce trees of greater age, having developed 

 a great many root branches, evidently have at their command a larger proportion 

 of the water supply of a given soil space than either yellow pine or fir, whose 

 roots are then coarse and few T in number. Both because lodgepole pine develops 

 a root as slowly as spruce, and because it apparently exerts a relatively low 

 osmotic pressure, its seedlings stand out prominently as the least drought- 

 resistant of those considered. 



As a result of the various investigations upon the relative physio- 

 logical functioning of the species named, Engelmann spruce (Picea 

 engelmanni) and Douglas fir (Pseudotsuga taxifolia) may be considered 

 to be the most highly developed plant organisms, in the sense of effect- 

 ing most readily the processes on which growth is dependent; and at 

 the other extreme are found western yellow pine (Pinus ponderosa) 

 and limber pine (P. flexilis). Although further investigation may 

 tend to alter theTelative positions of the four pines, it is believed that 

 acceptance of the order first given above will not be seriously mis- 

 leading. 



But apart from this physiological development, which is considered 

 most fundamental, there are two lines of adaptation which evidently 

 affect distribution. In the bristlecone (Pinus aristata) and limber 

 pines, both of which are "weedy" or below the stature of true forest 

 trees, and perhaps more facultative in adapting themselves to site, the 

 stomata are greatly restricted in size. This inevitably reduces gr< >wth 

 rate by making carbon dioxide less available. These trees are certainly 

 resistant to the evaporation which might result from strong wind, 

 but it is not believed that they are drought-resistant in the sense of 

 exercising a strong osmotic control over their water supplies. This 

 distinction is important, for, although the property of being resistant 

 to evaporation makes them valuable on wind-exposed sites, it does 

 not equip them for enduring the actual drought of the soil, which 

 results, for example, from competition between trees. 



On the other hand, in the early and vigorous root development of 

 yellow pine and Douglas fir, as contrasted with lodgepole pine (Pinus 

 contorta) and spruce, there is an effort on the part of the former to 

 adapt themselves to the meager moisture supplies which usually obtain 

 in the localities where these species find their proper heat conditions. 

 It is believed that the heat conditions are fundamentally controlling, 



