THE PETRAN MONTANE FOREST. 209 



P. arizonica either occur scattered in the pine consociation or codominant 

 with it from Arizona southward into Mexico. 



Both the yellow pine and the lodgepole pine form pure stands which may 

 stretch hundreds of miles beyond the main body of the association. The 

 finest body of yellow pine on the continent is found on the Colorado plateau 

 of northern Arizona far from the central mass. Similar pure communities but 

 of less importance occur on the ranges of eastern Wyoming and the Black 

 Hills. On the mesas of western Colorado and the foothills of central Wyoming, 

 the Pinus contorta consociation breaks up into masses of varying size, sur- 

 rounded by sagebrush or grassland in the respective regions. These are out- 

 posts of the lodgepole forest and are quite different from the savannah type 

 assumed by yellow pine where conditions favor grassland. Both represent 

 the same climatic tendency, however, as is shown also by the fact that aspen, 

 Populus tremuloides, often accompanies them. 



Factor relations. — The montane forest of the Rocky Mountains has received 

 more quantitative study than any other community, with the possible excep- 

 tion of the prairie. This is due to the location in it of the Alpine Laboratory 

 and the Fremont Forest Experiment Station at Pike's Peak, where factor 

 studies have been carried on more or less continuously since 1900 and 1910 

 respectively, and of the Fort Valley Forest Experiment Station near Flagstaff, 

 Arizona, where observations have been made since 1909. In addition, the 

 Desert Laboratory has maintained stations in the montane zone of the Santa 

 Catalina Mountains since 1908. As a consequence, a large mass of factor 

 data is available, of which but a few general results can be given here. 



The rainfall limits for the montane forest are approximately 18 to 20 inches 

 for the lower margin and 22 to 23 inches for the upper. The great majority 

 of the records are for the eastern slope, but they agree closely with those for 

 western Colorado and northern Arizona. The rainfall is somewhat higher in 

 New Mexico and lower in Montana, but this is obviously compensated by the 

 evaporation. The savannah form of the pine consociation is found where 

 precipitation is as low as 15 inches, and lodgepole outposts occur at even lower 

 limits in western Colorado. The total evaporation for the growing season is 

 not known, but the relative evaporation is a third greater in the Pseudotsuga 

 consociation than in that of Picea engelmanni in the lower part of the sub- 

 alpine forest. The measurement of light values through several summers 

 has shown that there is no difference in the intensity of the light which falls 

 upon the two forest zones in the Rocky Mountains. There is a constant 

 difference in air and soil temperature, and in water relations, especially water- 

 content, the montane forest naturally showing the higher temperatures and 

 lower rainfall, humidity, and water-content. 



Serai relations. — Factor measurements show that Pinus ponderosa is the 

 most xerophytic of the three major dominants, Pseudotsuga less so, and Abies 

 somewhat less still. The most mesophytic is Picea pungens. Pinus contorta 

 is practically as xerophytic as the yellow pine, but it has a wider range of 

 adaptation. Much the same is true for P. flexilis and P. albicaulis. The 

 three southern pines resemble Pinus ponderosa in their water requirements. 

 As to light requirements, the pines are all intolerant. Picea pungens is some- 

 what more tolerant, Pseudotsuga is moderately tolerant, and Abies endures 

 still deeper shade. In Colorado the normal light intensity for the mature 



