84 MISC. PUBLICATION 257, U. S. DEPT. OF AGRICULTURE 



extremely important to have a detailed correlation of the known 

 growth curves in the local and introduced tree species with the various 

 climatic data in as many localities as possible, if success in establishing 

 forest plantings is to be achieved. Furthermore, the growing impor- 

 tance attaching to the consumptive use of water by trees and other 

 natural vegetation in the economy of many farms and watersheds in 

 the drier portions of the country emphasizes the need for quantitative 

 data. Much more work on the water requirements of trees of all 

 ages and under varying site conditions is needed, since comparatively 

 very little work has been done in this field, and much of this has been 

 purely empirical — a method always wasteful and with little to recom- 

 mend it, although perhaps necessary in the development of a subject. 

 More comprehensive studies on large trees are especially desirable. 

 The work from small trees and cut twigs is very unreliable, as has 

 been shown. Work in large portable greenhouses should be planned 

 so that it would be possible to control the environmental conditions 

 and to learn the water requirements at different ages and in different 

 artificial sites which simulate the natural ones. This means apparatus 

 that will permit a careful measurement of both the intake and the out- 

 put. The fact that trees are large and not easy to handle in experi- 

 mental apparatus increases the difficulties, but they should not be 

 insurmountable; for no really satisfactory results can be expected 

 until more data are obtained on forest trees and stands under natural 

 conditions, supplemented by laboratory work on a scale comparable 

 with the size of the problem involved. 



