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



There are, then, no surprising differences even when widely sepa- 

 rated types are compared. It seems evident that the temperature 

 requirements of the three principal forest trees of the region — yellow 

 pine, Douglas fir, and spruce — are in accord with their physiological 

 properties, hut that lodgepole pine, which theoretically would grow 

 much better in conditions similar to those enjoyed by yellow pine, 

 is compelled to take a much cooler site because of its inability as a 

 seedling to cope with drought. Its ability to tolerate the extreme 

 heat of fresh burns and to thrive here where competition has been 

 destroyed is entirely in keeping with its physiological properties, and 

 the inability to reproduce in its own shade is further evidence that the 

 low temperatures are merely tolerated, not enjoyed. 



Somewhat similarly, limber pine, although sometimes extending 

 almost to timber line, is plainly a tree of the open. Its occupation of 

 the wind-swept northwest slope, the warmest site here studied, ma; 

 be temporary, but it is believed to result from a greater resistance 4 bo 

 wind drying than is possessed by species which function more vigor- 

 ously. Thus it seems safe to say that none of the species choo-cs 

 warmer sites than, relatively speaking, its physiological conditions 

 dictate, and that, when driven by other considerations to sites rela- 

 tively too cool, the tenancy is only temporary. 



WIND, HUMIDITY, AND EVAPORATION. 



It would appear logical to follow the discussion of air tempera- 

 tures by that of soil temperatures, for the two must be somewhat 

 closely related, and their consideration in direct sequence woulc 

 facilitate fixing in mind the temperature conditions, as a whole, for 

 various kinds of sites. There is, however, one very potent reason 

 for postponing the consideration of soil temperatures until all of the 

 atmospheric conditions have been carefully weighed. This is tin 

 fact that the soil-temperature data which we are enabled to present 

 here are not, perhaps, so important as temperatures as they are foi 

 expressing the physical status of the soil and the availability of its 

 moisture. How important it is that the soil should be frozen one 01 

 six months is, obviously, not to be determined until we have first 

 considered the atmospheric conditions and measured, in a rough wa\ . 

 the demand to give up moisture by transpiration which may he ma<' 

 upon trees during the period of soil freezing. It is therefore desira- 

 ble and in fact necessary to proceed with the consideration of atmos- 

 pheric conditions before taking up the subject of soil factors. 



WIND MOVEMENT. 



Wind movement at ih< control station.— -The topographic position o: 

 the control station has already been described in general, Th 

 station lies in a fairly wide valley formed by the junction of thre 

 stream channels. It is not in the bottom of the valley, but on a lo\ 

 ridge lying between two of the streams. As the anemometer i s 

 placed on a tower 20 feet, above the ground, the air movement is not 

 strictly that of the valley bottom, but represents a slight gradatioi 

 toward the freer movement of the higher air strata. Nevertheless, 

 the elevation of the anemometer, 50 feel or less above the Lowest 

 adjacent ground in the valley, is inconsiderable in comparison with 

 the elevation of the walls of the valley. At this point these rise 

 some 300 feet higher on the north, at an angle of is degrees, and about 



