FOREST TYPES IX CENTRAL ROCKY MOUNTAINS. 135 



Limber pine sites even at high elevations show summer tempera- 

 tures higher than those for Douglas fir, and undoubtedly reaching 

 almost as great extremes as yellow pine soils. 



It is not necessary for a proper explanation of the selective process 

 of insolation to assume, as the present writer once did (4) that the 

 temperature of the environment appreciably influences the chemical 

 effectiveness of sunlight; but it is necessary to recognize that a high 

 temperature may contribute to the superheating of the plant tissues, 

 and that a dry atmosphere and wind decrease the liability by facili- 

 tating the cooling process of evaporation. 



The idea that protection arises from the cooling caused by transpi- 

 ration is by no means new. In 1902, Barnes (2) wrote: 



34. Under the present organization of plants exposure of wet cell walls to the 

 atmosphere is indispensable for the solution of necessary gases, oxygen and 

 carbon dioxide, the plant being debarred from waterproofing the cell wall so 

 long as gas absorption is necessary. Transpiration is, therefore, considered as 

 unavoidable, though in itself a constant menace to life and activity. Advantage 

 has doubtless been taken of the xylem bundles to facilitate the movement of 

 solutes, but there is no reason to think this essential. Transpiration also has 

 become a protective factor with sun plants, whose temperature is thereby kept 

 within reasonable bounds. < (Since reading the paper the author has ascertained 

 that in certain points his view of transpiration coincides with those expressed by 

 Dr. C. E. Bessev in a paper on the function of stomata, published in Science 

 N. S. 7:13-16, 1898.) 



Each species may have, then, functional limits expressed in a 

 minimum amount of light and a certain maximum temperature. 

 It is evident also that the several species may exhibit differences not 

 only in " shade tolerance" but in light tolerance as well, which will 

 be especially marked when the seedling is very young and is subjected 

 to the reflected light from, the ground surface and to the attendant 

 temperatures which may prevail there. This limited light tolerance 

 is probably due to the direct effect of heat upon protoplasm. The 

 experiments reported have not, it is true, shown the species to be 

 sensitive to excessive heat in proportion to their shade tolerance. 

 Apparently Engelmann spruce is adapted to a much wider range of 

 light conditions than the other species, while Douglas fir is most 

 clearly sensitive to the heating effects of strong light, and with both 

 spruce and lodgepole pine, the injury to young seedlings evidently 

 arises from loss of water rather than, or commonly earlier than, 

 from superheating. 



WIND, HUMIDITY, AND EVAPORATION, 



. Wind velocities vary widely in different situations and localities 

 of the mountains, recorded velocities depending largely on the im- 

 mediate conditions of exposure. In general, wind velocities are much 

 higher in winter than in summer, and much higher at high elevations 

 than at low, exposures bein^ equal, because of the fact that the higher 

 atmosphere is more turbulent. Beyond this, comparisons of the 

 sites studied are unsafe. It is entirely in agreement with the physio- 

 logical qualities of the species to find spruce tolerating much greater 

 exposure to wind, than does yellow pine. 



Humiditj-, as measured by saturation deficits, varies more widelv 

 locally, on account of differences in air temperature and not so much 

 by reason of changes in vapor pressure. A wind in descending the 

 slope of Pikes Peak may actually gain some moisture, but its tempera- 

 ture increases so mucn more rapidly that it gains very quickly in 

 saturation deficit and evaporating power. 



