Shrub Invasions of North American Semiarid Grasslands

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Abstract
The composition and structure of the semiarid or desert grasslands of southwestern North America have changed over the past 150 y. Brushy or woody species in these communities have increased in density and cover. This increase in density of woody species is called brush encroachment because most of these species have been present in these communities at lower densities for thousands of years. The brushy or woody species were not introduced from other continents or from great distances. They are indigenous species that have increased in density or cover because of changes in local abiotic or biotic conditions. The brushy and woody plants are not the cause of these changes, but their increase is the result of other factors. The causes of changes that have led to the present woody-brushy composition of these semiarid grasslands has been difficult to determine. Warming of the climate seems to be a background condition, but the driving force seems to be chronic, high levels of herbivory by domestic animals. This herbivory has reduced the aboveground grass biomass, leading to the reduction of fine fuel and a concomitant reduction or complete elimination of grassland fires. This combination of factors favors the encroachment, establishment, survival and growth of woody plants. Less competition from grasses, dispersal of seeds of woody plants by domestic animals, and changes in rodent, lagomorph, and insect populations seem to modify the rate of change. Elevated levels of atmospheric CO2 are not necessary to explain shrub encroachment in these semiarid grasslands. The direction of future change is difficult to predict. The density of brushy and woody plants will probably increase as will the stature and number of species. However, if soil nutrients increase, woody legumes may be replaced by other brushy or woody species. Reversing the changes that have been going on for 150 y will be a difficult, long-term, and perhaps impossible, task.