Long-term impacts of livestock grazing on Chihuahuan Desert rangelands

Abstract

Rangeland ecological condition was monitored over a 48 year period on 41 sites on Bureau of Land Management rangelands scattered across 6 counties in southwestern New Mexico. All sites were grazed by livestock during the study period. Sampling occurred in 1952, 1962, 1982, 1992, 1997, 1998, and 1999. A modified Parker 3 step method in conjunction with Dyksterhuis quantitative climax procedures were used to determine rangeland ecological condition. At the end of the 48 year study period (1952-1999), the average rangeland ecological condition score across study sites was the same (P > 0.05) as the beginning of the study (39% versus 41% remaining climax vegetation, respectively). Major changes (P > 0.05) in rangeland condition occurred within the study period due to annual fluctuations in precipitation. Ecological condition scores increased in the 1980s and early 1990s due to above average precipitation. However, drought in the early to mid 1950's and again in the mid to late 1990's caused rangeland condition scores to decline. At the end of the study (1997-1999), 38% of the sites were in late seral ecological condition, compared to an average of 25% in the 1952 to 1982 period. The amount of rangeland in late seral ecological condition increased while the amount of rangeland in mid seral and early seral condition decreased in the 1990s compared to the 1952-1962 period. The average percent cover of black grama (Bouteloua eriopoda Torr.) and tobosa (Hilaria mutica Buckley), the primary forage grasses in the Chihuahuan Desert, were the same (P > 0.05) in 1952 and 1999. Over the 48 year study period, the average cover of shrubs including honey mesquite (Prosopis glandulosa Torr.) showed no change (P > 0.05). However major increases in honey mesquite basal cover occurred on 1 site and creosote-bush (Larria tridentata [Pursh] Nutt.) increased on another. Grazing intensity was evaluated during the last 3 years of study (1997, 1998, 1999). Overall grazing use of forage across sites and years averaged 34% or conservative. Our research shows controlled livestock grazing is sustainable on Chihuahuan Desert rangelands receiving from 26-35 cm annual precipitation.