688 , JOURNAL OF FORESTRY 



lings have deeper roots, enabling them to draw upon the large moisture 

 supply stored in the deeper soil strata during the winter, and thus ren- 

 dering them less dependent upon spring precipitation. 



SUMMARY 



Western yellow pine in northern Arizona makes its height growth 

 during the period of lowest precipitation in the year. During this 

 period of high activity the trees are dependent almost entirely upon 

 moisture stored in the soil during the preceding winter and spring. 

 Normally the great bulk, and in some years all of this moisture, is 

 stored during the winter months, December-March. When winter 

 precipitation constitutes the sole supply, height growth in young sap- 

 lings is apt to be small. If winter precipitation is supplemented by 2 

 inches or more in April and May (the rainfall in June is rarely suffi- 

 cient to be of any consequence), a pronounced stimulus to height 

 growth results. It may be stated, as a general rule, for the sites cov- 

 ered by this study, that 2 inches or more of precipitation between April 

 I and May 31 is several times as effective as the same amount in excess 

 of the normal precipitation between December i and March 31. 



Factors reflecting atmospheric moisture conditions, including evapo- 

 ration, wind movement, relative humidity, cloudiness, and length of 

 rainless period, from April i to June 30, show a close, though not en- 

 tirely consistent, relation to height growth. 



Temperature on the sites studied appears to be important only in so 

 far as it affects moisture conditions. Since the increase in temperature 

 results in increased water consumption, height growth, if, as is usually 

 the case, there is a shortage of moisture, varies inversely with tem- 

 perature. Observations indicate that where moisture is abundant, 

 height growth increases directly with temperature. 



Complete records of soil moisture, if available, would probably show 

 even a closer relation to height growth than does precipitation. The 

 important fact pointed out by this study, however, is that it is the April 

 and May precipitation which is most important in determining the 

 amount of height growth, and presumably the moisture content of the 

 soil during the period when height growth takes place. Since height 

 growth is an excellent criterion of vigor, April and May precipitation 

 appears to play a very important role in the reproduction of western 

 yellow pine. Observations have in fact shown that the conditions 

 which favor height growth of young saplings are also those which favor 

 the survival of seedlings during the long, arid fore-summer, when mor- 



