30 JOURNAL OF FORESTRY 



portion of their foliage. Apparently the influence of the soil is the 

 primary cause of the difference in response to adverse conditions. 



The freeze of 1913 acted on the plants at the beginning of the grow- 

 ing season, and the damage was caused either by the destruction of or 

 changes in the cell sap, or by the breaking down of the tissues by (.he 

 low temperatures. The heat wave of June, 1917, occurred at the 

 close of the spring growing period and acted on the transpirational 

 system, all plants being subjected more or less to the same general 

 conditions. Wherever ground water was available, either on account 

 of irrigation or the closeness of the water table, the plants were able 

 to obtain sufficient water to carry on their normal functions, notwith- 

 standing the greatly increased demands caused by the high tempera- 

 tures and excessive evaporation. It was necessary, therefore, for the 

 soil to furnish the water to compensate for the loss caused by evapora- 

 tion. Only those trees escaped injury that grew in soils containing 

 sufficient available water, and the amount of injury was largely a 

 measure of the available water. 



The eft'ect of the drought depended not so much upon the species 

 as upon the soils in which they grew, for the species that were injured 

 in one soil type were entirely free from injury when growing in an- 

 other. A number of the species that are ordinarily listed as drought- 

 resistant were badly injured because of the character of soil in which 

 they were growing, while other species known to require water were 

 not appreciably hurt. Thus, the casual observer was given a wrong 

 impression of the value of the species. Table 4 indicates, for a number 

 of places where plantations were visited, the species, the nature of 

 the soil, and the character of the injury. 



The damage to trees growing in loam soil is practically negligible, as 

 Table 5 shows, but the extent of damage to those growing in sand and 

 to those growing in clay soil is nearly the same. The sandy-loam and 

 clay-loam soils in the matter of injury occupy an intermediate stage 

 between these two extremes, trees in the clay loams sufifering some- 

 what more damage than those in the sandy loams. It would be in- 

 teresting to arrange the following data with respect to depth of 

 water table or moisture content of the soil, but unfortunately too little 

 is known of this factor to make such an attempt practicable. How- 

 ever, it is believed that, irrespective of the depth of the water table, 

 the character of the soil has a decided influence on the amount of 

 injury. 



