RESEARCH METHODS IX STUDY OF FOREST ENVIRONMENT. 27 
mainly upon the very young seedling at the ground-line, where tem- 
peratures 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 surface condition. 
The actual measurement of surface conditions presents great tech- 
nical difficulties, which will be pointed out later. 
?>. Determination of soil freezing, hot 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 suffi- 
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. 
