Baccharis pteronioides and Haplopappus tenuisectus are two shrubs that are increasing in the desert grasslands of Arizona, however, the cause of their increase is not known. In this study, Bock and Bock attempt to relate changes in populations of these shrubs to regional climate changes, fire frequency, and cattle use. Baccharis pteronioides density was not greatly affected by grazing or fire. Cattle use slightly reduced shrub density and regrowth following fire, but this shrub was greatly affected by variations in winter precipitation; increasing when winter precipitation is high and decreasing in drier periods. The effects of grazing and winter precipitation on H. tenuisectus density were similar to those for B. pteronioides, however, this species was also very susceptible to fire damage. Fire caused almost complete shrub mortality and recovery after fire was slow. This study provides evidence that grazing at moderate intensities may not promote encroachment by B. pteronioides and H. tenuisectus in Arizona desert grasslands.
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