RESEARCH METHODS IN STUDY OF EOREST 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 change 

 ing soil conditions present 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 he quite 

 inadequate for one growing in competition with or in the shade of 

 other plants. Then there are the maximum temperatures to he 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. 



Problkm^ 



1. Temperature zones, as indicated by mean monthly, seasonal, 

 and annual air temperatures, or length of frpstless season, or tem- 

 perature sums (hour-degrees) above a fixed minimum (say, to 1 

 which furnish the conditions necessary for the existence oi a given 

 species. 



