RESEARCH METHODS IN STUDY OF FOREST ENVIRONMENT. 15 
difficult to measure with any great accuracy, especially as the humus 
layer is seldom the same on any two spots which might be selected, 
and may change in moisture content almost as rapidly as the atmos- 
phere. It is almost inevitable, therefore, that actual moisture meas- 
urements should be confined to the first layer of mineral soil and to 
greater depths if desired, and that the depth and character only of 
the humus should be noted, using some predetermined rule for esti- 
mating its moisture content at various times. For soil temperatures 
conditions at the surface are doubtless of the greatest importance; 
but here again the measurement of the actual and constantly chang- 
ing soil conditions presents a practical difficulty. Measurements be- 
low the surface may have considerable comparative value, even 
though they do not give the extremes which may have the most direct 
bearing on plant life; and it is therefore suggested that a depth of 
1 foot be taken in all such studies as furnishing a kind of control 
for other observations. 
Having considered the general arrangement of apparatus, the mat- 
ter of exact methods and instruments to be used in measuring each 
aerial and soil condition may now be taken up. 
AIR TEMPERATURES. 
Air temperatures are more readily measured than any other con- 
dition because of the simple equipment required, and they will prob- 
ably be most frequently considered at temporary stations. It is 
hardly to be questioned that air temperatures affect growth very 
directly, although this may not always be apparent if only periodic 
and annual mean temperatures are considered. It is also fairly ap- 
parent that the air temperature which is adequate for the growth 
of an individual plant receiving an abundance of light may be quite 
inadequate for one growing in competition with or in the shade of 
other plants. Then there are the maximum temperatures to be con- 
sidered, which it now seems may be more directly operative in pre- 
venting the extension of plant ranges than any other temperature 
condition. In this connection, the temperature of the soil surface may 
be most important, but that of the air layer just above the soil must 
not be overlooked. 
The following problems summarize briefly what are believed to be 
the most important temperature problems in relation to forestry. 
Problems. 
1. Temperature zones, as indicated by mean monthly, seasonal, and 
annual air temperatures, or length of frostless season, or temperature 
sums (hour degrees) above a fixed minimum (say, 40° F.), which 
furnish the conditions necessary for the existence of a given species. 
2. Actual rate of growth in height, diameter, volume, or weight of 
any species, within different temperature limits, should preferably be 
