SPRING PRECIPITATION AND HEIGHT GROWTH 685 



in trees and other perennials. Nevertheless, no constant relation be- 

 tween height growth and July-October temperatures is in evidence. 

 The same is true, though in a lesser degree, of mean annual tempera- 

 ture, whether it be correlated with the height growth of the current or 

 the following season. A more constant relation is shown by the April- 

 June temperature, which, with some exceptions, varies inversely with 

 height growth. In other words, maximum growth is recorded in the 

 years of lowest temperature. From this it might be inferred that the 

 temperature in this region is normally too high for the best develop- 

 ment of yellow pine. Other facts at hand contradict such a conclusion. 

 Recent investigations in the San Francisco Mountains indicate that the 

 upward extension of western yellow pine is limited by low tempera- 

 ture, and that the mean temperature just below the upper limit of the 

 species is only 2 or 3° F. below that at Fort Valley. Western yellow 

 pine thrives in the southern parts of Arizona and New Mexico, where 

 the mean temperature is 10 to 15° F. higher than at Fort Valley, pro- 

 viding that the water supply is sufficient. In fact, where the tree 

 occurs under such conditions, under cultivation or along watercourses, 

 it grows much more rapidly than it does at Fort Valley. The pre- 

 ponderance of evidence, therefore, supports the opinion that the tem- 

 perature conditions at Fort Valley are nearer the minimum than the 

 maximum for western yellow pine. 



The apparently contradictory sets of data presented in the preceding 

 paragraph may be harmonized in the following manner. Whatever 

 deleterious effect is produced by high temperature is the result of the 

 influence of temperature upon water relations. As long as the tree 

 can secure enough water from the soil to replace the loss from transpi- 

 ration, growth will increase with temperature up to a point far beyond 

 any temperatures recorded in this locality. If, however, the moisture 

 supply is deficient, then only a portion of the available heat energy can 

 be utilized, and the surplus becomes injurious, in that it tends to in- 

 crease evaporation from the soil and the transpiration rate beyond the 

 capacity of the roots to restore the water loss. 



As a rule, the years of highest temperature for the April- June period 

 are also the years of lowest rainfall and highest evaporation. The 

 only exception to this rule was in 1914. Thus, when the heat available 

 for plant growth is greatest, it cannot be used for lack of sufficient 

 water. In this rcsi)cct the plant may be likened to a steam-engine. 

 Given sufficient water, the power of the engine increases with the heat 

 supply; but an increase of heat is associated with increased water con- 



