106 BULLETIN 1283, U. S. DEPARTMENT OF AGRICULTURE. 



The trees which tolerate or in fact thrive under such conditions 

 in this region are spruce and Douglas fir — the so-called " shade- 

 tolerant" species. It is too early to draw a clear line between the 

 two, although it is believed that the spruce is the more resistant of 

 the two. To these conditions yellow pine is plainly unadapted, but 

 limber pine fits in moderately well. 



Thus, for the first few years in the life of the trees on the sites which 

 produce the very dense forest, the conditions may give almost equal 

 encouragement to several species. Although dense shade is more 

 favorable to the tolerant spruce, Douglas fir and even the pines may 

 become established because of deeper rooting and more ready adapta- 

 bility to surface drought. On the other hand, when the trees begin 

 to be exposed above the snow blanket, and the moisture supply is 

 no longer controlled by depth of root, the struggle becomes almost 

 altogether one of resistance to long-continued drying. The evidence 

 presented in the paper on "Physiological Requirements of Rocky 

 Mountain Trees," (6) which tends to show the greater photosyn- 

 thetic activity of spruce, and which suggests that a high osmotic 

 pressure is more normal in that species and, hence, less likely to be 

 injurious, is applicable in this connection. The species which carries 

 a dense sap, especially in a limited light, not only begins to resist 

 evaporation earlier, but is least liable to injury should drying with 

 all species be carried to the same point. 



It is therefore believed that trie struggle wliich determines the 

 composition of the forest may continue long after the seedling stage 

 is passed. The evidence, which has already been used more or less, 

 indicates that on north slopes at middle elevations the original forest 

 may be largely of Douglas fir, with considerable pine; but that in 

 the second generation, from the sapling stage upward, spruce pre- 

 dominates, and must therefore form the climax. 



(2) That the spruce forest is subjected to, or that spruce may 

 tolerate, winter evaporation stresses of great magnitude is shown by 

 the high spruce burn (W-D) and timberline stations. In neither 

 place can there be much question that the soil freezing is severe and 

 continuous, with no chance for even surface melting. The quanti- 

 ties indicated — around 400 grams of evaporation — are probably more 

 nearly correct than any others that can be given, and must represent 

 the approximate limit of resistance in plant growth. It must, how- 

 ever, be conceded that, after a certain amount of drying of the leaves 

 has occurred, the process is halted until the atmospheric conditions 

 become more severe (warmer or dryer, or both), so that beyond a 

 certain time mere continuance of the stress does not comprise an 

 endurance test. 



The Fact that both limber pine and bristlecone pine follow spruce 

 nearly to the most exposed of its habitats seems to indicate either that 

 the long exposure, as has just been depicted, is not so severe as it seems, 

 or that these species possess a phenomenal ability to resist trans- 



E (ration. The sum of the evidence necessitates falling back on the 

 elief that neither of the pines occurs where the long winter drought, 

 which is possible at a high elevation, is entirely unbroken. This is 

 certainly the explanation of the conditions recorded on the high 

 ridge (F-13). Moreover, this record is open to serious question from 

 two angles. 



(3) The considerably higher winter evaporation at the Wagon 

 Wheel Gap stations, as compared with similarly situated ones at 



