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

Prepared by Ginevra Nota, Nicolò Anselmetto, Alessandra Gorlier & Mitchel P. McClaran

Photograph taken in 1922 at photo station 111 on the Santa Rita Experimental Range, Arizona (US). The view looks from grazed area in the foreground to the ungrazed exclosure (1A) beyond the fence. Photo credit: unknown photographer

Livestock exclosures are fenced areas which prevent livestock to graze inside. They are used worldwide to study the effects of grazing removal on rangeland vegetation by comparing the areas outside and inside the exclosures. In the desert grasslands of the southwestern United States, livestock exclosures were established in the early 20th century. Here, they provide a great opportunity to investigate the response of different plant growth forms, i.e., shrubs, cacti, and grasses, to more than 80 years of grazing removal and in relation to areas where grazing continued and to precipitation patterns. Moreover, exclosures support the investigation of the relative importance of plant number (does grazing removal affect plant number?) vs plant size (does grazing removal affect plant size?) in the response of grasses to long-term livestock removal.

To explore these topics, four times between 2011 and 2020, we surveyed the vegetation inside and outside of 10 long-term (>88 years) exclosures on the Santa Rita Experimental Range (Arizona, USA). We measured the cover of woody plants (mesquite, burroweed, and cacti) and recorded four measures of native and non-native grass abundance: cover, density, and individual plant size and biomass.

Photograph taken 100 years later (2022) at the same photo station 111 on the Santa Rita Experimental Range. Notice the increase in trees and shrubs in the grazed and ungrazed areas, which is consistent with our results. To see more photos from the repeated photography collection, visit https://cals.arizona.edu/srer/content/repeat-photography. Photo credit: Mitchel P. McClaran

Results showed that grazing removal had a different impact on woody plants and native and non-native grasses depending on the palatability, grazing resistance, and dispersal of seeds into the exclosures. Shrubs and cacti were not affected by grazing removal, and the reasons are mainly twofold. Firstly, woody plants are low to not palatable to cattle, thus they are not directly impacted by defoliation. Secondly, the fences that bound livestock exclosures do not prevent seeds from dispersing into the excluded area from outside. Native grasses (including the dominant Arizona cottontop) were more abundant in the ungrazed than grazed areas, whereas non-native grasses (including the dominant invasive Lehmann lovegrass) were equally abundant inside and outside the exclosures. Greater native abundance inside the exclosures is mainly due to differences in population size structures rather than in the number of plants: plants were bigger inside the exclosures. Possible mechanisms leading to larger plants include greater longevity, increased survival rates to environmental stresses (e.g., drought), increased growth rates, and greater vigor in the absence of livestock grazing. The contrasting lack of a response for non-native grasses to livestock removal is explained by their lower palatability and greater grazing resistance. Finally, the results highlighted that the vegetation response to precipitation patterns (increased plant abundance with above-average precipitation) was expressed in both grazed and ungrazed areas.

In conclusion, this study demonstrated that long-term livestock removal in desert grasslands leads to greater native grass size, density, cover and biomass, but there was no response of non-native grasses or woody species. From the management point of view, what’s critical for native grasses survival is to have sufficient recruitment to maintain the population, given that grazed native grasses appear to live shorter than ungrazed plants.

This is a plain language summary of the paper of Nota et al. published in Applied Vegetation Science (https://doi.org/10.1111/avsc.12696).