50 MISC. PUBLICATION 257, U. S. DEFT. OF AGRICULTURE 



It has been pointed out that trees are seldom, if ever, found in 

 genuinely arid situations, and they are included for the most part, 

 therefore, in the group of mesophytes. Some, however, are able to 

 endure greater extremes of drought than others, and, from a silvicul- 

 tural point of view, the reasons for these differences are extremely 

 important. While it is well known that certain species of trees thrive 

 on comparatively dry sites and others demand a high soil-moisture 

 content, the fundamental reasons for these differences have not yet 

 been thoroughly explained. We know that all trees reduce the water 

 content of a given soil to approximately the same wilting coefficient. 

 Also, we know that conifers, in general, transpire more slowly per unit 

 of leaf area than do hardwoods, and further that there is a i elation 

 between the osmotic pressure of the cell sap and both the water- 

 absorption and water-retention capacities. 



The distribution of species in relation to the moisture supply is one 

 of the fundamental problems of ecology and will continue for some 

 time to receive the attention of investigators in this field. Why are 

 plants distributed over the surface of the earth in the particular way 

 that they are found at the present time? Apparently, in the evolu- 

 tion of species, certain types have been able to survive in drier or 

 moister regions and in the course of time have become adapted to 

 these particular localities. In general, as has already been shown, 

 temperature and moisture (by which is meant not only soil moisture 

 but also the relative humidity) are the two most decisive factors in 

 determining the distribution of vegetation. 



Shreve (195) considered that the moisture ratio, or the ratio of the 

 evaporation rate, as determined by atmometers, to the soil moisture 



( ~^T? ) was an excen< ent single expression to show the combined effects 



of both air and soil moisture. The lower the ratio, the more favorable 

 are the conditions. In studying the distribution of species in the 

 Coast Range of California, he found that this ratio was 10 times as 

 great in chaparral as in redwood forests and about 5 times as great 

 at 5,000 feet as at sea level. The vegetation in this section was deter- 

 mined by (1) the seaward and landward exposure, (2) the altitude, 

 (3) slope exposure, (4) topographic irregularity, and (5) proximity to 

 the sea; but it will be noted that these factors are important mostly 

 because of their effect (either direct or indirect) upon the water 

 relations. 



Similarly Cooper (57) found that the distribution of oak, willow, 

 and chaparral associations near San Francisco Bay are determined 

 largely by the interrelations of these various factors, the most impor- 

 tant being topography, the soil, and the water table. 



Various methods have been employed by ecologists for determining 

 quickly and easily the type of vegetation which one may expect to 

 find in a given locality. Shreve settled on the soil moisture ratio as 

 a simple criterion. Others have worked in the other direction and 

 have tried by some simple plant measurement to tell what the 

 ecological conditions were. 



Huber (105) determined the degree of succulence of the vegetation 

 as the ratio of the surface of the leaves expressed in square centimeters 

 to the volume of the It f expressed in cubic centimeters. On this basis 



