Creosote Bush and the Lower Sonoran Zone of the Southwest

Abstract

Creosote Bush (Larrea divaricata Cav.)I is among most reliable of indicators of natural biologic areas in Southwest, delimiting Lower Sonoran Zone fairly sharply. The recent articles2 on effect of sustained subzero weather upon Larrea in Virgin River Valley of Utah suggest certain earlier studies made upon this shrub. For more than half a century this remarkable Sonoran species has commanded ecological, distributional, chemical, as well as systematic, studies from a score of workers. Among earliest of these investigations was that published by J. M. Stillman in 18803 upon chemical nature of gum secreted by Larrea stems. Recent digests of systematic and broad ecologic status of shrub have been drawn up by Shreve in 19314 and by Jepson in 1936.5 Volney M. Spalding treated relation of Creosote Bush to water supply with particular reference to Arizona.6 Shreve and Mallery have reported on work in progress upon controlling factors limiting distribution of shrub.7 They report substantial differences in environment exist within space occupied by a single individual. Accordingly, it is difficult to assign determining agency limiting distribution, even over a small portion of its range in Southwest. Osmotic values for leaves have been determined by Mallery8 and their relation to soluble salts in soils, mechanical and chemical nature of soil and influence of rainfall and soil moisture, both through day and seasonally. Material for these osmotic studies was collected over practically entire range of Creosote Bush in Southwest. As has been pointed out several times occurrence of Larrea over thousands of square miles of arid or desert country, where it is often a dominant species showing considerable physiological elasticity in its habitat requirements, is proof that it is one of most successful of desert perennials. The fact has been pointed out by Shreve9 that this small-leaved nonsucculent evergreen lacks anatomical characters common to desert plants in general that favor conservation of water. Shreve says the adjustments