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



Response of the species to temperatures. — There is every reason to 

 believe that insolation exercises a direct control over the distribution 

 of the species; but in the total absence of direct measurements of 

 insolation, temperature data must be relied on to carry out this idea. 

 It is, of course, difficult to distinguish between the direct effects of 

 insolation and such indirect effects as soil drying; but the total 

 evidence points to the direct and the temperature effects as influencing 

 most strongly the selection of the species for each site, except possibly 

 with spruce, which is more susceptible to drying than to heat injury. 



The range of either air or soil temperatures measured in an} 7 one 

 type of forest is, however, so small as to indicate that temperatures 

 during the growing season can not vary widely without causing the 

 elimination of the given species, or at least its domination by some 

 other species. It is true that on the lower margin of the mountain 

 forest yellow pine seems to thrive under a wide range of temperatures 

 in different localities. This apparently greater adapatability of yel- 

 low pine may be due partly to the fact that, wherever it may extend 

 downward into warmer zones it finds no important competitors, and 

 therefore may exist even if its rate of growth becomes negligible. 

 More important, however, is the consideration that air or soil tem- 

 peratures as recorded probably mean almost nothing. The tem- 

 peratures which the young plant experiences will be most intimately 

 bound up with the temperature of the surface of the soil. As may 

 be readily proven, relative to other temperatures the surface maxima, 

 which are probably most important, will vary widely according to 

 the amount of rain and the moistness of the soil during the critical 

 summer months. 



A mean growing-season soil temperature of about 58° to 62° F., at 

 a depth of 1 foot, permits the establishment of western yellow pine. 

 At oo° or 56° Douglas fir will appear with the pine. At 52° or 53° fir 

 will probably predominate, but there may still be some reproduction 

 of vellow, limber, or bristlecone pines, the species depending appar- 

 ently on winter evaporation stresses. At 50° we may expect almost 

 pure Douglas fir if moisture is favorable. At 44° to 48° and perhaps 

 nigher, Engelmann spruce reproduction will come in under fir and 

 doubtless ultimately replace it. Spruce may hold complete sway 

 where the soil temperatures are from 40° to 50°, but probably does 

 not reproduce at all where this lowest temperature prevails; the stand 

 must be thinned out before it is rejuvenated. 



Lodgepole pine, probably has temperature requirements, for satis- 

 factory reproduction, between those of yellow pine and fir. The 

 single site studied shows soil temperatures a little lower than those 

 for fir, and air temperatures no nigher than for good spruce sites. 

 However, the well-known proclivity of lodgepole reproduction for 

 denuded sites, where the radiation is intense, and some few measure- 

 ments on such sites not conducted long enough to establish averages, 

 -how the reason for the usual failure of lodgepole to reproduce under 

 its own canopy and indicate the higher heat requirement of this spe- 

 cie. Measurements by Notestein " at midsummer in 1912 showed 

 the SOU temperatures at inches to be at least 4° higher in an area 

 from which the pine had been cut and the brush burned than in the 

 virgin fotest. In each place measurements were taken at about 

 12 points. 



. i'.., forest examiner, Forest S< gross Report, Exp. R— 11," L928. 



