24 BULLETIN 1105, U. S, DEPARTMENT OF AGRICULTURE. 



and averaged around 25 per cent. For all sites in the vicinity of 

 the Fort Valley Experiment Station in the winter of 1919-20 the 

 average losses from heaving were 17 per cent for first-year and 8 

 per cent for older seedlings. These figures represent 55 and 86 per 

 cent, respectively, of the total losses from all causes. Although frost 

 heaving is responsible for much damage, particularly to first-year 

 seedlings, it is not of sufficient importance to be classed as a limiting 

 factor. 



GERMINATION AND SURVIVAL IN RELATION TO RAINFALL. 



Records at the Fort Valley Experiment Station show that germi- 

 nation and survival of pine seedlings are closely associated with 

 moisture. In the absence of adequate records of soil moisture, 

 evaporation or precipitation, or preferably both, may be used as 

 indices. The best single index is precipitation. It is believed to 

 be entirely adequate in this region for studies involving long periods 

 of time and not requiring minute distinctions. Moreover, in the 

 Southwest, ' rainfall furnishes a fairly good index to evaporation. 

 In periods of abundant rainfall evaporation is always low, because 

 of high atmospheric humidity, more or less cloudy skies, and low 

 wind movement; in periods of light or no rainfall evaporation is 

 high, because of low atmospheric humidity, prevailingly clear skies, 

 and high wind movement. These relationships do not always hold 

 for short intervals, but they do hold when periods of 10 days or 

 more are considered. 



Contrary to the usual impression, germination is not a serious 

 problem in the reproduction of western yellow pine in this region. 

 Germination takes place in July and August, because usually this is 

 the only period when both moisture and temperature conditions are 

 favorable. In April and the first half of May the soil is often moist, 

 but the temperature is too low. From the middle of May to the end 

 of June temperature requirements are satisfied, but the surface soil 

 is too dry. Usually this period is practically i^ainless. During the 

 past 11 years May or June germination has occurred in the San 

 Francisco Mountains region only once, in 1919, when for two weeks 

 in the latter part of May rain fell almost daily. Normally the sum- 

 mer rains begin between July 1 and July 15, and continue six or eight 

 weeks. During this period showers may occur almost daily during 

 continuous intervals of three weeks or more. Such conditions are 

 ideal for germination. The critical tests come in dry periods during 

 ithe first and second seasons after germination. 



Figure 2 gives a record of rainfall plotted by 10-day periods from 

 1909 to 1920. Tables and figures in the chapters " Cutting " and 

 " Brush disposal " show the fluctuations of soil moisture during the 

 growing season in 1909, 1912, 1919, and 1920. Both rainfall and 



