March 14, 1913] 



SCIENCE 



421 



erate elevation, water can commonly be ob- 

 tained within 30-40 feet of the surface, while 

 in the valleys the water lies at a depth of 15 

 feet, or less.' In the states of the middle 

 west, as in Kansas and Nebraska, according 

 to various sources, mainly the Geological Sur- 

 vey, the depth to ground water on the flood 

 plains of streams varies from 10 to 40 feet, 

 but the depth on the benches, valley sides and 

 upland is from 60 to several hundred feet. 

 A like condition is to be found in the more 

 arid regions further west, while the more 

 humid regions of the extreme West are similar, 

 as regards depth to the water table, to the 

 humid east. In rough and mountainous re- 

 gions, the water reservoirs of whatever kind 

 may be regarded as the physical equivalents 

 of the water table of the more level country, 

 and provide such plants, especially the trees, 

 as penetrate to them, or to the soil moistened 

 by them, with a perennial supply of water. 



The various physical factors, climatic as 

 well as those pertaining to the soil, which in- 

 fluence the distance to ground water, are, in 

 the main, of significance in the physiological 

 activities of the trees occupying an area. And, 

 in addition to such factors, appropriate tem- 

 peratures being assumed, the specific responses 

 of trees to the water relation are to be consid- 

 ered. Chief among these are the water-re- 

 taining and water-absorbing capacities and 

 adjustments, of which the root-ground water 

 relation must be considered to be of great 

 • importance. 



The general relations of trees to perennially 

 moist soil, as indicated by the depth of the 

 water table and by the distribution of trees 

 and forests, and taken from a few widely 

 separated regions, may be illustrated by a few 

 examples. 



In southern Arizona, in the vicinity of the 

 Desert Laboratory, the distance to the water 

 table, or to perennial ground water, is various. 

 On the bajada, water is to be obtained at a 

 depth of 70 feet, or over, while on the flood 

 plains of the streams it lies from 15 to 35 



" Orider and Johnson, ' ' Water Resources of 

 Mississippi," TJ. S. Geological Survey, Water- 

 Supply and Irrigation Paper No. 159, 1906. 



feet beneath the surface. There is practically 

 no arboreal flora on the bajada, but along the 

 streams, and on their flood plains, occur ash, 

 Cottonwood and mesquite, the latter often 

 forming an open forest of trees ranging as 

 high as 40 feet, or more. The mesquite may 

 be taken to illustrate the relation between 

 trees of the vicinity and the depth to perennial 

 water supply. 



The mesquite is the most widely distributed 

 tree of the Tucson region, occurring not only 

 on the flood plains of streams, but on the 

 higher bajada as well. The form of the spe- 

 cies, however, when growing in such diverse 

 habitats is quite unlike, since apart from the 

 flood plains it assumes the form, not of a tree, 

 but of a shrub. There is a close association 

 between the dual habit of the mesquite as 

 noted and the depth to the water table, which 

 is also shown by a variation in the develop- 

 ment of its roots. 



The root-system of the mesquite is an ex- 

 tremely variable one. It may penetrate the 

 ground deeply, or it may extend widely and 

 lie not far beneath the surface of the ground, 

 or, again, it may be of rather limited extent 

 and of a generalized character. The first type 

 of root is probably most characteristic of the 

 tree form, and the last of the shrub form of 

 the species, while the second arrangement may 

 be connected either with the tree or the shrub 

 habit. On the flood plain, roots of the mes- 

 quite as a tree have been seen to penetrate to 

 a depth of 15-24 feet, or to the level of the 

 water table. Under especially favorable soil 

 conditions, as where it is fairly homogeneous 

 and easily penetrable, the roots may attain a 

 greater depth. 



A comparison of the distribution of the tree 

 form of the mesquite with maps which give 

 the water table depths indicate that the spe- 

 cies becomes a tree, soil conditions favoring, 

 where the ground water does not lie deeper 

 than 50 feet. On the other hand, where the 

 water table is at a greater distance, or is 

 otherwise not available, the shrub habit is 

 assumed, with characteristic generalized root- 

 system. 



