678 JOURNAL OF FORESTRY 



the prevailing height of young growth on the area. Saplings of this 

 height are about 25 years of age. 



Attempts to measure the effect of physical factors upon growth 

 under field conditions are usually complicated by the fact that several 

 more or less interdependent variables, some of which cannot be satis- 

 factorily measured, must be dealt with. It is often possible, however, 

 to select one controlling factor which, because of its extreme variabil- 

 ity or because it is prevailingly at a minimum, dominates the habitat as 

 far as the success or failure of certain plants is concerned. The study 

 at hand is simplified by the fact that we are not comparing different 

 sites, but conditions on the same site in different years. It is evident, 

 therefore, that one important factor — that of soil composition and 

 texture — remains constant and may thus be eliminated. The variables 

 demanding consideration are, in the probable order of importance, 

 moisture, heat, light, and biotic factors. 



MOISTURE 



An analysis of climatic conditions immediately suggests moisture as 

 the dominant factor in the problem before us. Moisture is the most 

 critical factor for yellow pine in this region, and it is also the factor 

 which is subject to the greatest fluctuations.. The moisture supply in 

 the locality of this study rarely falls so low as to endanger the exist- 

 ence of yellow pine, once it is well established ; but moisture may be 

 expected to exert a marked effect upon growth. Douglass^ has shown 

 that the width of the annual rings of yellow pine in the Southwest is a 

 very reliable index of rainfall. It is also a matter of common knowl- 

 edge among foresters that in young trees height growth is a more sensi- 

 tive indicator of site quality than is diameter growth. 



The most satisfactory expression of moisture conditions is to be 

 found in available soil moisture, or, still better, the ratio between evap- 

 oration and available soil moisture, as proposed by Fuller.^ Unfortu- 

 nately neither of these factors can be used, because continuous records 

 of soil moisture are not available. In the absence of better moisture 

 data, precipitation records have been employed. 



Annual height growth and precipitation for various periods of the 

 year are shown graphically in figure i. These graphs indicate spring 

 precipitation as the controlling factor. Contrary to what might be ex- 



* Douglass, A. E. : Bulletin of the American Geographical Society, Vol. XLVI, 

 No. 5, 1914. 



' Fuller, Geo. D. : "Evaporation arfd Soil Moisture in Relation to Succession 

 of Plant Associations." Botanical Gazette, 59, pp. 193-234, 19 14. 



