Mechanisms of range expansion and removal of mesquite in desert grasslands of the Southwestern United States

During the last 150 years, two species of mesquite trees in the Southwestern United States have become increasingly common in what formerly was desert grassland. These trees have spread from nearby watercourses onto relatively xeric upland areas, decreasing rangeland grass production. Management attempts to limit or reverse this spread have been largely unsuccessful. This paper reviews studies regarding mesquite natural history and management strategies, emphasizing studies published during the past decade. Mesquite possess a deep root system and are capable of fixing atmospheric N, rendering them capable of accessing resources unavailable to other plants in open rangeland. Their seeds, which remain viable for decades, have a hard exocarp and require scarification before germination. Consumption by cattle provides a means of scarification and seed dispersal, and is a major factor contributing to the spread of mesquite in open rangelands. Increases in atmospheric CO2 and winter precipitation during the past century also contribute to enhanced seed germination. Removal techniques have included herbicides, prescribed burning, grazing reduction, and mechanical removal. For increased effectiveness of these techniques, management goals must be clearly articulated; these goals include complete removal, no removal, and limited removal. Of these, limited removal appears the most feasible, using an initial herbicide application followed by periodic prescribed burning.