54 



FOREST DISTRIBUTIOX 



of time, therefore, between the cool weather of spring and au- 

 tumn, which marks annually the time of awakening and of re- 

 tarding of plant life, is important as determining to a large 

 extent the limits of the distribution of species when other things 

 are equal, but hotter summers in certain areas may furnish the 

 required total of heat for the support of species which would 

 not be able to endure in another region having a growing period 

 of equal length but of lower temperature. 



The length of the frostless period in the year must deter- 

 mine, of course, the northern limit of distribution of cerlain 

 species of native as well as of crop plants. The sum total of 

 heat required for the maturing of the plant and the production 

 of seeds must in most eases be the minimum even among those 

 hardy species which resist frost, and furnish the northern limit 

 of their distribution. In this region the frostless period of the 

 year varies from 40 days to 4 months or more, depending on 

 location and altitude. Table 7 presents frost data for a number 



Fig. n. Mean temperature diagrams for the year. A. at Billing, 

 altitude, 2871 feet. B, at Havre. 200 miles north "of Billings, altitude! 

 250.5 feet. Tlie figures at the right represent the ratios of the growing 

 to the non-growing seasons. The mean temperatures for the mouths 

 are indicated ]i,t the cross-marks on the curves, midwa.v between 

 the vertical lines. 



