22 BULLETIN 1105, U. S, DEPARTMEI^^T OF AGRICULTURE. 



zation due to the clrying-up of the pollen, stating that in the cones 

 which reach maturity not over 1 per cent of the ovules develop into 

 perfect seeds. This estimate is undoubtedly much too low as an 

 average figure, although it is known that fertilization is poor in some 

 years. It is possible that late frosts have more to do with these con- 

 ditions than dry winds. Whatever may be the cause, it is probable 

 that many of the light seed crops would be good crops but for the 

 arrested development of cones in the early stages. Much damage 

 is done by weevils as the cones approach maturity, but serious in- 

 festations are usually of a local character. 



CLIMATE. 



The term " climate " embodies the primary physical factors con- 

 cerned in plant growth, namely, heat, light, and moisture. It also 

 determines in a large measure the physical and chemical composition 

 of the soil. Several publications have described in considerable de- 

 tail the forest climates of the Southwest (^^), and particularly the 

 region in' and about the San Francisco Mountains (i5, i^, l'^-, ^9). 

 It is therefore necessary here merely to point out the most prominent 

 features. 



The western yellow pine type in the Southwest occurs at altitudes 

 ranging from 6,500 to 9,000 feet. The mean temperature ranges from 

 about 42° to 47° F. for the year, and from 69° to 65° F. for the grow- 

 ing season (June to September). Extremes fall as low as —25° F. 

 and rise to 95° F., though temperatures below — 20° F. and above 90° 

 F. are unusual. The outstanding characteristics of the temperature 

 in this region are a great daily range and the prevalence of frost 

 during the growing season. The annual precipitation is about 23 

 inches, but it varies greatly in different places and in different years. 

 At the Fort Valley Experiment Station the lowest and highest an- 

 nual totals during the past 12 years are 15 and 32 inches. From the 

 standpoint of reproduction, the dominating feature is the occurrence 

 of protracted rainless periods, accompanied by high winds and high 

 evaporation, during the months of May and June. In northern New 

 Mexico this dry period is less pronounced than elsewhere in the two 

 States. Undoubtedly this condition in a large measure accounts for 

 the excellent reproduction which prevails generally on the Santa Fe 

 and Carson National Forests. 



MOISTURE THE DOMINANT FACTOR. 



All of the studies which have been made point to moisture as the 

 dominant physical factor in the^reproduction of western yellow pine. 

 Low temperature limits the upper altitudinal range of the species 

 {19) , but within its typical range this is not directly a limiting factor, 

 excepting under special conditions. In shaded situations reduced 



