424 THE POPULAR SCIENCE MONTHLY 



The Relation of Ground Water to Forest Distribution 



It has been pointed out in another place 3 that the depth to peren- 

 nially moist soil, which is usually regulated by the depth of the water 

 table, undoubtedly plays an important role in the distribution of forests 

 as a whole. Thus, where the moist soil lies too deep to be tapped by 

 the roots of the trees, other factors being equal, forests are usually 

 wanting, but where the water table lies near enough to the surface, so 

 that the perennially moist soil above it can be reached by the roots Of 

 the trees, forests are present. In southern Arizona it has been found 

 that, given a practically homogeneous soil, the mesquite assumes a 

 tree habit with the water 35 feet, more or less, beneath the surface, but 

 that where the soil is not homogeneous, and is stratified, so that a 

 portion of the strata are dry, or where the water table is more than 

 35 to 40 feet beneath the surface, the species has a shrub-like habit. 

 An analogous condition obtains in the coastal plain of Texas and in the 

 treeless middle-west. In the latter region the deciduous forests are 

 almost wholly confined to the flood-plains of streams, while the adjacent 

 upland is treeless. Thus, over a Avide area where the climate is arid or 

 semi-arid, the depth to perennial water is an important factor in deter- 

 mining the presence of forests. Where the trees are unable to attain to 

 such moist soil, they usually develop xerophytic characters or special ad- 

 justments by which they are enabled to survive. How true this generali- 

 zation may be found to be can not at present be told, but that it 

 applies to such regions as central California, there can be no doubt. 

 Among the adjustments, as has been pointed out in this note, are to be 

 included those of the root-system by which they are especially adapted 

 to make use of the superficial waters. 



Coastal Laboratory, 

 Carmel, California 



s Science, N. S., Vol. XXXVII., p. 420, 1913. 



