58 REPORTS ON INVESTIGATIONS AND PROJECTS. 



Depth of Water Table as a Factor Limiting Distribution of Trees, by 



IV. A. Cannon. 



Preliminary comparison of the depth of the water-table and the distribu- 

 tion of the mesquite (Prosopis) indicates that the relation is probably an 

 important one. As shown in another place (The Root Habits of Desert 

 Plants, by W. A. Cannon, Carnegie Institution of Washington Publication 

 No. 131), the habit of the mesquite, both that of the root and the shoot, is a 

 very variable one. The shoot may be either a tree with a marked bole or a 

 true shrub with several short stems. The latter is characteristic of relatively 

 arid habitats, while the former has been found to indicate a water-table so 

 close to the surface as to be attainable by plant's roots. A survey of a lim- 

 ited area shows that where the species has the arboreal habit the water-table 

 does not usually lie deeper than 40 feet. In this area the soil is granitic sand 

 with an admixture of adobe clay. In areas adjacent to those thus inhabited 

 by the mesquite, but where the depth to the water-table is greater, the species 

 assumes the shrub habit. Observations in other regions indicate that the 

 root-water-table relation, to a certain degree and under certain conditions, 

 may be of importance in determining the presence or absence of trees and 

 probably of forests. 



The Precise Evaluation of Rainfall Records, by Forrest Shreve. 



Studies have been inaugurated looking toward a more precise evaluation 

 of records of desert rainfall in relation to vegetation, a subject of great 

 practical importance in its bearing on the principles of dry farming. The 

 moisture-content of a heavy clay soil — conservative in its moisture changes — 

 has been determined at three depths during a period of 12 months, and the 

 resulting data have been analyzed with respect to the influence of individual 

 falls of rain in restoring the supplies of soil-moisture. It has become pos- 

 sible to state the approximate limits of rainfall which is too slight to affect 

 the soil-moisture and the limit of falls which are so great as to produce a 

 run-off and no further increase of the soil supplies. The effect of the same 

 amount of rain is found to be different in the summer and in the winter 

 rainy seasons, due to differences in the rate of surface evaporation, and is 

 found to vary in relation to the pre-existing moistness of the soil. The 

 installation of an electrical recording rain-gage at the Desert Laboratory 

 and the development of improved methods of securing samples of soil for 

 moisture determination have given facilities for a continuation of this work 

 along more exact lines. 



Vital Statistics of Desert Plants, by Forrest Shreve. 



The work on vital statistics of desert plants has been continued on an 

 observational area for which a full record of germinations and deaths for 

 three years is now at hand. The restriction of all natural germinations of 

 desert perennials to a period of two weeks at the opening of the summer 



