Differences in population size structures drive grass response to long-term livestock removal

Questions: How does Desert Grassland vegetation respond to long-term grazing removal? Is grass response driven more by differences in the number or the size of plants? Does the response differ over time and in relation to precipitation patterns? Location: Santa Rita Experimental Range, southwestern United States. Methods: Four times between 2011 and 2020, we measured the cover of shrubs, cacti, and native and non-native perennial grasses and the density and biomass of individual perennial grasses on 40 permanent transects inside and outside 10 long-term (88-104 years old) livestock exclosures (0.3-2.9 ha) occurring on the same ecological site. We used Linear Mixed Models to compare vegetation variables in grazed vs. ungrazed transects through time and calculated the cumulative frequency distributions of grass plant diameters. Results: The cover of shrubs and cacti did not differ due to livestock removal. Instead, the exclosures had a greater cover, density, and biomass of native grasses and specifically the native Digitaria californica. Moreover, ungrazed populations of natives and D. californica showed a greater number and biomass of large plants (> 3 cm diameter) and a plant-size structure skewed to larger sizes. Non-native basal grass cover, density, and biomass showed no differences between grazing treatments. Patterns of inter-annual precipitation influenced shrubs and grass abundance, but not their response to livestock removal. Conclusions: Vegetation response to long-term grazing removal using exclosures was expressed only for native grasses. Absence of shrub and cacti response was expected because they are unpalatable and exclosures do not prevent their seed dispersal. Absence of non-native grass response was expected because the dominant Eragrostis lehmanniana is very tolerant of grazing. More interesting, the population size structure of larger native grasses with livestock removal suggested that grazing limits their longevity and vigor. These patterns persisted throughout ten years even though precipitation included wet and dry years.