干旱区生态环境知识资源中心

We know more about vegetation change on the Santa Rita Experimental Range since 1903 than is known about any other 20,000-ha area in the world. This record is only possible because important techniques of measuring vegetation changes were developed on the Santa Rita, such as repeat photography and the line intercept, transect method, and because they were applied often and broadly. A 100-year record of experiments and systematic observations nourishes the interpretation of these changes. Together, they describe a steady increase of mesquite trees, four cycles of burroweed eruption and decline, one cholla cactus cycle, interannual and interdecadal variation in native grass composition, and the recent dominance of the normative Lehmann lovegrass. The most conspicuous change is the increase of mesquite, which began before 1903 when the spread of seed by livestock and cessation of fire led to the establishment of mesquite in the open grasslands. The growth of these plants and subsequent recruits transformed the grasslands into a mesquite-grass savanna, and neither the elimination of livestock grazing nor the occasional fire has reversed this change. Burroweed cycles appear to be more closely related to winter precipitation patterns and maximum plant longevity than land management activities. Similarly, the increase of Lehmann lovegrass is largely independent of livestock grazing management.