50 



BULLETIN 1233, U. S. DEPARTMENT OF AGRICULTURE. 



Summary of mean air temperatures. — To bring all the preceding 

 data together for closer study, the average temperature of each type, 

 as studied, and the differences within each type, may be shown as 

 follows : 





Number 



of 



stations. 



Mean temperatures. 



Type. 



(',!-,. 



"'/oar. 





Kii: ,;i-. 



Average. 1 



Range. 



A vrerage. 1 



Plains 



1 

 1 

 6 

 3 

 a 

 4 

 5 



° F. 



64.6 

 70.8 

 i 



55.2 

 56. 



5L6 



° F. 



° F. 

 47. 



Sand hills 











■>.'. 

 6.6 



3. 3 



4. 8 

 7.0 



4.0 



5.8 



4.8 



7.:i 





Liinlier pine 



33.4 



Douglas fir 





Lodgepole pine 



3i. 7 





34.7 







1 Of stations listed in four tables just preceding. 



CONCLUSIONS REGARDING AIR TEMPERATURES. 



As has already been pointed out, from the physiological standpoint 

 the air temperatures per se may mean very little. It lias been made 

 apparent by the preceding studies that the several species here con- 

 sidered show wide differences in photosynthetic capacity, a ad ii i^ 

 logical to assume that the species which makes the most effective 

 use of sunlight will succeed best in a cool environment. On this basis 

 alone the upward extension of any species would be plainly limited 

 by competition with another species of greater shade tolerance and 

 correspondingly low heat requirements for effective growth, in this 

 consideration it seems that the mean air temperatures for the growing 

 season serve as well as any other criteria to explain both the alti- 

 tudinal zonation of the mountain forests and the sharp contrasts in 



v as between opposing slopes. In fact, as regards n 

 temperatures, it has been shown that the same differei 

 produced locally as well by change of aspect as by great differences 

 in altitude. 



But even if the. zones and aspect types are accepted as temperature 

 types primarily, and as sufficiently well described and limited b\ 

 growing-season means, an explanation has not been provided 

 agreement with physiological theory, for the fact that each spe 

 has a lower altitudinal limit and an apparent maximum-temperature 

 limit. The mean air temperatures can of course give-only a 

 tion of what such limits might be, and even the aum air tem- 



peratures, it is believed, would tall so far short of d< Qg the really 



critical condition im worth while to tabulate 1 



for this purpose. When one considers all the facts of f< ! i ibu- 



tion, together with such phenomena as the marked success of yellow- 

 pine planting id the Nebraska sand bills and the vigorous growth of 

 the species there, the conclusion i^ unavoidable that the only condi- 

 tions worth studying, in the attempt to explain the natural Limita- 

 Qs of the pes, are those which have the mosl acute bearing 



on seedlings in the infant stage. Even the matter of germination may 

 confidently he put aside, for year after at numbers of ^ved- 



lings are germinated but few of them survive even to the end of 

 the first gi""\'. ing sea 3on. 



