The Botanical Review

, Volume 24, Issue 4, pp 193-252

First online:

The desert grassland a history of vegetational change and an analysis of causes

  • Robert R. HumphreyAffiliated withDept. of Agronomy and Range Management, Arizona Agricultural Experiment Station

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Extensive portions of the desert grassland of southern Arizona, New Mexico and southwestern Texas have been invaded by woody species. Mesquite, creosote bush, cacti of the genusOpuntia, burroweed and snakeweed are among the principal invaders. The prime factors commonly believed to have caused this change are reviewed and evaluated. These are (1) change of climate, (2) grazing by domestic livestock, (3) plant competition, (4) rodents, (5) fire. Of these various factors, change of climate seems to have had the least effect. Fires that were formerly frequent and widespread were the chief agency restricting shrub invasion. Since fires have been controlled, the introduction of domestic livestock, plant competition and rodents have been effective agents that have favored woody plants at the expense of grasses. Had fires continued to sweep the grasslands down through the years to the present with their original frequency, the desert grassland would probably occupy about the same area today as it did prior to the white settlement of the Southwest.