686 JOURNAL OF FOREJSTRY 



sumption, and if water is deficient the engine cannot operate to its 

 maximum capacity, regardless of the heat supply. 



LIGHT 



Continuous records of light intensity are not available. Since, how- 

 ever, the saplings concerned in this study are exposed to practically 

 full sunlight, which is generally abundant in this region, and since they 

 do not stand close enough to shade one another appreciably, it is not 

 believed that variations in light intensity would affect annual height 

 growth in any measurable degree. 



BIOTIC FACTORS 



Under biotic factors may be included fungous diseases, insect attacks, 

 and damage by grazing animals. The trees considered in this study 

 have been under close observation since 1909, and are known not to 

 have suffered appreciably from any disease during this period. They 

 have, however, been subjected to sporadic injuries by the tip moth, 

 an insect which kills the terminal shoots, and thus destroys all or a 

 portion of the year's growth. Such injuries may be readily detected 

 by the appearance of the stem several years after they were inflicted. 

 Trees showing evidence of tip-moth work or other abnormalities have 

 been eliminated from the study. There has been no defoliation by in- 

 sects during the period considered. Prior to 1909, the saplings, which 

 were then mostly below 4 feet in height, were subject to severe grazing 

 damage. The terminals of nearly all plants were injured and a large 

 part of the foliage was removed. These injuries ceased in 1909, when 

 stock was excluded, but the effect appears to be reflected in the growth 

 for two or three years following. 



APPLICATION UNDER OTHER CONDITIONS 



After considering the possible influence of all factors, the conclusion 

 that height growth is determined by moisture supply during the April- 

 June period, as determined largely by the April and May precipitation, 

 appears to be fully substantiated. Having established this conclusion, 

 it is of interest to know whether it applies generally throughout the 

 range of western yellow pine in this region. The same sequence of 

 long and short internodes as shown in figure i, particularly for the 

 years 1915 to 1917, has been repeatedly observed in different localities 

 on the Colorado Plateau. It has not been possible to correlate growth 



