RESEARCH METHODS IN STUDY OF FOREST ENVIRONMENT. 27 



react mainly upon the very young seedling at the ground line, where 

 temperatures are usually highest. The proper investigation of this 

 subject certainly demands surface measurements, but temperatures 

 at a depth of a foot or more may give some indication of the sur- 

 face condition. The actual measurement of surface conditions pre- 

 sents great technical difficulties, which will be pointed out later. 



3. Determination of soil freezing, not as a directly operative tem- 

 perature condition, but in relation to the availability of soil mois- 

 ture. In this connection it must be borne in mind, of course, that 

 soil moisture may become essentially nonavailable at a temperature 

 as high as 34°; and, again, that it may not actually freeze until a 

 temperature of 30° or lower is reached. Soil temperatures for this 

 purpose must therefore be coordinated with some data on the soil 

 itself and on the plants involved. It is obvious that, to serve the 

 purpose, frequent observations may be necessary. Continuous ther- 

 mograph records are preferred because, while most forest trees are 

 not sensitive to freezing for short periods, if at all, in the considera- 

 tion of moisture even a short period of relief through thawing may 

 mean the beginning of a new cycle of observations on the effect of 

 drought. In the study of soil freezing, the surface or near-surface 

 temperatures are perhaps most important, but it is not entirely 

 certain that mere freezing of the surface soil will stop the movement 

 of water through the main root and stem. It is the part of caution, 

 therefore, to examine the entire root zone, and it may perhaps be 

 necessary to know the conditions of the tree itself as regards freez- 

 ing at a point near the ground line. 



Problems. 



The problems, then, to which soil temperatures are related, are 

 even more numerous than those concerning air temperatures and in- 

 volve more directly the relations with initiation, habitat extension, 

 and plant succession, rather than rates of growth. Some of the most 

 evident problems may be listed as follows : 



1. Optimum temperature of the soil as a seed bed in direct effect 

 on rate and amount of germination. 



2. Optimum temperature of the soil in stimulating osmosis in the 

 roots and hence rate of growth. 



3. Minimum temperature at which water is available or sutli- 

 ciently available to supply transpiration. 



4. Temperature at which the soil freezes and cuts off the plant 

 entirely from water, length of such periods, and atmospheric condi- 

 tions conducive to transpiration during such periods. 



5. Maximum temperatures of the soil or soil-surface which may be 

 tolerated without injury to root or stem of the young, shallow-rooted, 

 and barkless seedling. 



