FOREST TYPES IX CENTRAL ROCKY MOUNTAINS. 143 



(4) Lodgepole pine is little, if any, more efficient in sunlight than 

 yellow pine, but appears to be more conseryatiye of its water supply. 

 The seedlings are as frail and shallow rooted as those of spruce, so 

 that they do not tolerate drying conditions. On the other hand, the 

 direct effect of heating seems to be less injurious with lodgepole pine 

 seedlings than with those of yellow pine, Douglas fir or spruce. An 

 additional factor in the life 01 lodgepole is the very sluggish germina- 

 tion of its seed except when stimulated by frequent and wide ranges 

 of temperature, as, ior example, a daily range of 30° or more. This 

 characteristic appears to be the outgrowth of repeated regenerations 

 on open ground, where the temperatures are not modified by coyer 

 of any sort, or perhaps are augmented by such an absorbent as char- 

 coal. In fact, every quality of lodgepole associates the tree with 

 pioneering in completely denuded ground. Even the slow rooting of 

 the seedlings may be seen to be adapted to a soil in which there is no 

 competing vegetation, and whose moisture has been made more 

 accessible by the destruction of the surface humus. But obviously 

 this slow rooting would be a very unfavorable characteristic in an 

 open situation where the surface moisture was not frequently replen- 

 ished by showers, or where it was very effectively dissipated by 

 insolation, as in typical yellow pine sites. 



Clements (10) has rendered a distinct service to students of forestry 

 by picturing vegetation as being constantly in a state of development 

 or flux as the result of changes in the site conditions and because of 

 the competition between species with different qualifications. In 

 somewhat the same vein, it may be best, since it has been shown that 

 the composition of any forest type is very much a matter of com- 

 peting elements, to consider now the typical situations which have 

 been studied at one or more stations, each in its ensemble, with the 

 hope of destroying any narrow or incomplete view which has been 

 created by considering the physical factors singly. This should be 

 done with the fact clearly in mind that the important differences 

 between sites arise almost wholly from differences in insolation. 

 Otherwise a very poor conception of causes will be obtained. 



(1) A steep south slope at middle elevation. — The gradient is such 

 that at midsummer the sun's rays fall almost normally on the surface. 

 The insolation may be rated as unity. As the resultant surface tem- 

 peratures are very high, frequently reaching 140°, and occasionally 

 150° or more, young succulent vegetation on the surface of the ground 

 might be almost instantly killed. There is an almost complete lack 

 of annual herbs. The rate of evaporation is very high. This rate 

 may be expressed as unity. The soil is almost lacking in humus. 

 The surface dries quickly after showers, but owing to the unbroken 

 mineral soil, capillary action ordinarily keeps a reasonable amount of 

 moisture near the surface. The soil has poor holding capacity because 

 of the lack of humus and also is lacking in nutrient salts. Conse- 

 quently, it can never support heavy vegetation. At the outset only 

 the sturdiest of seedlings can survive, fl they start after midsummer, 

 they must develop rapidly both as to root and lignification, in order 

 to be prepared for tne critical conditions of the next season. A 

 freely transpiring species is required, for transpiration may protect 

 the tender leaves from superheating as perspiration protects the 

 bodies of some animals. The ground will not freeze permanently if 

 at all; hence the seedling need not be inherently drought resistant, 



