148 BULLETIN 1233, U. S. DEPARTMENT OF AGRICULTURE. 



leaching. Such soils are much more fertile than the mountain soils; 

 but if they were equally moist at all times they would still present a 

 more arid condition in the physiological sense. There are still two 

 other aspects to the matter. The fine, compact soils, when rain is 

 withheld, dry out more completely and more deeply than mountain 

 or sandy soils; consequently they may be not only more arid but more 

 liable to super-heating. Again, when the plain is reached the sta- 

 bility of such soils precludes any denudation and permits the firm 

 establishment of perennial grasses; consequently competition alone 

 would preclude the establishment of tree seedlings. 



When, therefore, the question is raised whether it is lack of precipi- 

 tation or heat which excludes the coniferous forests from the plains, 

 the answer may be given unequivocally that it is both, and that it 

 is also alkalinity of soils and competition. It is nature's unanimous 

 verdict. Yet ii is to be noted that with the possible exception of 

 alkalinity, none of these conditions need absolutely inhibit tree growth. 

 For this reason, wherever there is a reasonable amount of relief and 

 nature has had extreme difficulty in establishing tree seedlings, 

 planting may still be conducted with a high degree of success. 



(8) Higher elevations. — On the several sites at higher elevations 

 there is greater light intensity, modified in its effects by lower air 

 temperatures and increased precipitation and conservation. It would 

 seem, therefore, that the conditions were exactly the opposite of 

 those at low elevations. However, there is an important exception 

 in the fact that wind movement in the higher atmosphere is much 

 freer than near the general surface of the earth, and there is no site 

 at high elevations which is entirely free from this added wind effect. 

 This tends to make the evaporation rate relatively high. 



Because the coolness of the air is largely, if not fully, compensated 

 by greater intensity of the sunlight, it is not surprising to find that 

 our light-demanding pines invade denuded areas, yellow pine to an 

 elevation of 10,000 feet and limber pine up to timberline. About 

 where yellow pine fails bristlecone pine may De expected to take its 

 place. 



Quite different, however, is the behavior of Douglas fir, which in 

 the Pikes Peak region is not seen much above 9,300 feet elevation. 

 Apparently there is no place for this species where conditions are so 

 readily made suitable for spruce by a minimum amount of shade. 

 It is not the intention to say that in an early stage of succession 

 there is no favorable place for Douglas fir but rather that in the 

 final stage there is no place, and having been crowded out of the hist 

 climax forest it has been unable to retrieve its position since the fire 

 that occurred about GO years ago. 



At the higher elevations the completely denuded areas are invaded 

 by yellow pine, limber pine, and bristlecone pines; but, with the 

 possible exception of a few rocky ridges incapable of supporting many 

 trees, every site whose elevation is more than about 10,000 feet mint 

 eventually be -conquered by spruce. On much of the area no other 

 conifer is found. A quick growth of aspen sprouts after a fire is 

 possible on almost any she. and these form abundantly heavy shade 

 lor spruce seedlings. 



(9) About lodgepole pine sites, which do not exist in the Pikes 

 Peak region, it is necessary to say a word because of the close relation 

 of lodgepole pine in many Localities to the more valuable species. 



