Vegetation change and surface erosion in desert grasslands of Otero Mesa, Southern New Mexico: 1982 to 1995

Desert grasslands that skirt mountain ranges in northern Mexico and the: so uthwestern United States were once common. These grasslands have largely be en replaced by shrublands and their soils have become eroded. The most freq uently cited causes of these changes are livestock overgrazing, fire and in creasing aridity. Studies have not separated grazing and fire from climate effects. Our aim was to determine whether desert grasses are being replaced by shrubs and how rapidly soil surfaces are eroding on unburnt Otero Mesa desert grasslands, where livestock overgrazing has not been a significant f actor in historic or recent times. In this article we describe how vegetati on and soil surface levels changed from 1982 to 1995 on six permanent trans ects. Vegetation was measured by charting canopy cover and stem bases of pe rennial plants in quadrats. Soil surface levels were surveyed in reference to benchmarks placed in nearby bedrock. Vegetation quadrats and surface lev els were remeasured in 1995. From 1982-1983 to 1995 the canopy cover of the desert shrubs, Larrea tridentata and Gutierrezia microcephala, declined by 1.5% and 5.8%, respectively. Canopy cover of the C4 xeric grass Bouteloua eriopoda declined by 5.3%. This decline in xeric grass cover was offset by a 1.2% increase in the C4 mesic grass Bouteloua curtipendula and a 4.0% inc rease in the C3 mesic grass Stipa neomexicana. From 1983-1984 to 1995 soil surfaces along transects eroded an average of 0.4 mm.y(-1). Thus, desert sh rubs were not replacing desert grasses, but mesic grass species were replac ing xeric species. These changes were associated with a 15-y period of rela tively wet cool-seasons and moist warm-seasons from 1981 through 1995. Thes e results document that, even in the absence of livestock grazing and fire, desert grassland vegetation is very responsive to precipitation change ove r relatively short time periods.