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ABSTRACT

Since the l880's a series of changes has taken place in the hydrologlc regimes of the streams of the desert region of  Arizona and Sonora; the vegetation of the area has undergone equally pronounced modifications during the same period. Through the use of repeat photography the dissertation establishes the principal facts about the vegetative change. The  lower edge of the range of the oaks, Quercus emoryi, (Q. oblongifolia and arlzonica, has migrated upward. The range of  the paloverdes, Cercidlum microphyllum and C. floridum, has undergone a similar displacement. The desert grasslands  have generally deteriorated, and in some cases have vanished, being replaced by desert scrub vegetation. Mesquite (Prosopis jullflora var. velutina) and Acacia vernlcosa have been the principal woody invaders. Through the use of historical materials the study examines various hypotheses relating the changes to cultural factors, e.g., to man's influence on the ecology of the area through his introduction of cattle, his suppression of natural burning, and his control of  predators. It concludes that such factors are not sufficient in themselves to account for the observed phenomena; that the facts of settlement do not closely relate to the timing of the changes. On the basis of the climatologlcal records the study concludes that the region has participated in the world-wide "recent climatic fluctuation"; that trends toward decreased rainfall and increased temperature have been operative. The observed vegetative and hydrologic changes have taken place within this general framework of climatic variation. Cultural factors may have reinforced or supplemented
the effect of climate. [AUTHOR ABSTRACT]