Livestock grazing and topographic site effects on grassland plant communities after long-term grazing cessation

Abstract

Ranchers are increasingly expected to manage grasslands for forage production and native biodiversity enhancement goals. However, longstanding relationships between grazing and plant species are often understudied because elucidating effects of grazing absence and presence often requires experimental opportunities that are difficult to establish, such as the introduction of grazing to long-term ungrazed pastures. Addressing this knowledge gap is critical for heterogeneous landscapes where site-specific properties might interact with grazing effects to ultimately structure plant communities. We conducted vegetation surveys for 3 years after grazing was reintroduced to an annual California grassland that was not grazed for more than 60 years. We investigated how grazing affected plant communities in terms of cover and richness of native and invasive species and how topographic sites of summit, backslope and toeslope altered these relationships. The plant communities were affected by the independent effects of grazing, site and year. Across years, native cover was 39% greater in grazed plots compared with ungrazed plots. Native species richness was slightly lower in ungrazed compared with grazed plots for toeslope sites relative to the other topographic positions. Invasive species cover was 17% lower in grazed plots compared with ungrazed plots and no predictors were found to contribute to significant differences across plots. Although we generally did not find expected relationships between site and plant response to grazing, this work demonstrates how managers can use livestock to quickly modify plant communities in areas with a long history of grazing absence.

Vernal pool wetlands respond to livestock grazing, exclusion and reintroduction

“…These studies, which span arid, mesic, and coastal grasslands, suggest that reintroducing large herbivores at low to moderate grazing levels in certain ecosystems can positively influence plant diversity and native cover. Our findings parallel previous results that reintroduced grazing can have significant effects on annual species in the first 2-5 years (Johnson and Cushman 2005) and can affect native cover more strongly than diversity (Gornish et al 2018).…”

“…Our study contributes to a small but growing body of literature that directly compares the effects of grazing reintroduction to systems that have been under prolonged rest from herbivores (Gornish et al, 2018;Groome & Shaw, 2015;Johnson & Cushman, 2007;Pykälä, 2004). These studies, which span arid, mesic and coastal grasslands, suggest that reintroducing large herbivores at low to moderate grazing levels in certain ecosystems can positively influence plant diversity and native cover.…”

“…These microdepressions likely allowed more native forb species to persist in areas where they would otherwise be outcompeted by exotic grasses. Trampling by livestock can increase microtopography (Stammel andKiel 2004, Allen-Diaz et al, 2004) that provides lower, moister refugia for native plants away from competitively dominant grasses, which cannot withstand prolonged soil moisture and/or inundation (Eriksson and Ehrlen 1992;Barry 1995;Robins and Vollmar 2002, Allen-Diaz et al 2004, Gornish et al 2018. Trampling may also aid in the germination of seeds after dispersal by increasing the light and moisture available to seedlings (Faust et al 2011).…”

“…Likewise, the complete removal of grazing disturbance from ecosystems that were previously grazed can result in biodiversity losses in a variety of ecosystems, from sagebrush steppe to subtropical wetlands (Barry, 1995;Boughton et al, 2016;Groome & Shaw, 2015;Marty, 2015;Porensky et al, 2020). This has led to increased interest in using livestock for conservation purposes, including the reintroduction of livestock to areas that have been under prolonged grazing rest (Gornish et al, 2018;Groome & Shaw, 2015).…”

“…However, there is less understanding of threshold dynamics in ecosystems which are adapted to low levels of grazing, and in which the degradation is caused by the complete removal of grazing. This is because the body of literature documenting the outcomes of grazing reintroduction at conservation stocking rates is relatively small (Boughton et al, 2016;Gornish et al, 2018;Stahlheber & D'Antonio, 2013). The reintroduction of grazing to lands that were previously excluded from grazing provides an excellent opportunity to study the thresholds at which these alternate states occur, and to understand the reversibility of these shifts.…”