Native grassland declines and accelerated erosion in the ungrazed Paint Gap Hills of Southwest Texas

Background/Question/Methods: Livestock were removed after Big Bend National Park, Texas, was established in 1944. By 1981, desert grasslands in the Paint Gap Hills of Big Bend NP were dominated by a 60% canopy cover of native grasses, succulents, sub-shrubs and shrubs as evident from ten permanent transects we established in 1981. We resurveyed vegetation composition, canopy cover, and soil surface elevations on these transects in 1995 and 2014.

Results/Conclusions: After four progressively longer and hotter droughts occurred between 1985 and 2012 we found that by 2014 canopy covers of two dominant, native, warm-season perennial grasses, Bouteloua curtipendula and B. ramosa, were reduced to near zero, and two nonnative perennial grasses, Eragrostis lehmanniana and Pennisetum ciliare, had invaded. The succulent Agave lechuguilla was gone from all ten transects and the cover of many native shrubs and subshrubs had notably declined. Soil surfaces changed from accumulating sediments at a rate of +0.7 mm/y from 1983 to 1995 to eroding at -1.6 mm/y from 1995 to 2014. We suggest that these soil and vegetation changes indicate the vulnerability of ungrazed native desert grasslands to climate change.