Recovery of the herbaceous component of degraded sagebrush steppe is unimpeded by 75 years of moderate cattle grazing

Abstract

Understanding the effects of contemporary cattle grazing on herbaceous perennial communities in big sagebrush steppe is important for managing for wildlife habitat, plant diversity, and productivity, yet potentially complicated by legacy impacts of historic, often higher intensity, livestock grazing. Here, we evaluate whether recovery of herbaceous communities in eastern Oregon, USA, after the cessation of intense spring sheep grazing (1935) was affected by moderate cattle grazing in paired plots with or without grazing over the past 75 yr (1936-2011). We tested for the effects of cattle grazing on herbaceous community recovery, as indicated by changes over time in plant density, and composition, as measured by Bray-Curtis dissimilarity. We also included current and prior to sampling year precipitation anomalies, to account for the weather effects, and a random term for pasture location of plot pairs to include potential subtle differences in abiotic environment and grazing management. We further tested whether time since cessation of intense sheep grazing and moderate cattle grazing were associated with convergence or divergence in community composition indicated by changes in evenness, richness, species relative abundance (rank order), and turnover or species appearance or disappearance. Total perennial herbaceous, forb, and grass density increased over time in sites grazed and ungrazed by cattle, though species varied in the direction of their response to contemporary cattle grazing. Community composition metrics indicated convergence over time including increasing evenness, decreasing Bray-Curtis dissimilarity, decreasing shifts in species relative abundance (rank order), and lower rates of species turnover (and gain and loss). Contemporary cattle grazing was not associated with convergence or divergence in composition. Precipitation anomalies for the current or prior water year were only occasionally significant in herbaceous density and community composition change models. Our results indicate similar long-term recovery trajectories occurred in sites with moderate cattle grazing or removal of all livestock following cessation of intense sheep grazing. Management planning and resource assessment focused on herbaceous perennial communities in sagebrush steppe should seek to separate the impacts of historic from contemporary livestock grazing practices.

Small effects of livestock grazing intensification on diversity, abundance, and composition in a dryland plant community

“…In some studies, heavy grazing intensities have been shown to homogenize vegetation through selective pressure (Adler & Lauenroth, 2000; Adler et al, 2001; Anderson & Briske, 1995), and we expected that we would find greater similarity in species composition between plots at the heavily grazed end of our gradient. However, other grazing studies in semiarid rangelands have reported small or no effects of grazing on species composition, consistent with our study (Copeland et al, 2021; Courtois et al, 2004; Davies et al, 2016; Fernandez‐Gimenez & Allen‐Diaz, 2001). In addition, a global meta‐analysis of grazing effects on plant community composition showed little difference in composition when grazing pressure switched from moderate to low levels or even to no grazing (Herrero‐Jáuregui & Oesterheld, 2018).…”

“…Livestock grazing did not dramatically alter plant community structure of the big sagebrush stands we sampled. Cattle grazing can modify dryland plant community structure (Davies et al, 2018; Eldridge et al, 2016; Veblen et al, 2015), but over the range of big sagebrush there is varying sensitivity to modern grazing practices, which are likely to be driven by the interactions of climate, historical grazing pressure, and modern plant community composition (Copeland et al, 2021; Davies et al, 2018). Collectively, our findings support existing evidence that the balance between grasses and shrubs in big sagebrush ecosystems is relatively stable with current grazing practices (Davies et al, 2018).…”

“…Because the rangelands we sampled are grazed only at moderate levels (0.09-0.5 AUMs/ha), our grazing intensity gradients may not capture an intensity high enough to cause changes in plant composition. Furthermore, these areas probably experienced widespread overgrazing by sheep prior to the passage of the Taylor Grazing Act in 1934 (Morris & Rowe, 2014) and, as a result of this history of heavy grazing, the composition of the modern plant communities we sampled may have changed over time and now have a low sensitivity to modern cattle grazing practices (Copeland et al, 2021).…”

“…Other studies have found that grazing can increase species richness (Manier & Hobbs, 2006), forb cover (Manier & Hobbs, 2006), and shrub cover (Veblen et al, 2015) or have little or no effects on big sagebrush plant community structure and composition (Courtois et al, 2004; Holechek & Stephenson, 1983; Stohlgren et al, 1999). Recent studies have shown that grazing exclusion over multiple decades produces only minor differences from moderate grazing (Copeland et al, 2021), or even decrease diversity and increase exotic species abundance (Porensky et al, 2020). Far fewer studies have examined plant community responses along grazing intensity gradients in these systems (but please refer to Adler et al, 2005; Tueller & Platou, 1991).…”

Plant community trajectories following livestock exclusion for conservation vary and hinge on initial invasion and soil‐biocrust conditions in shrub steppe

“…In spite of the relatively strong evidence that inappropriate grazing has contributed to less perennial herbs and more exotic annual grasses (Pyke et al, 2016), findings from grazing exclosures have provided mixed evidence for grazing impacts, even though these studies are expected to provide the strongest inference owing to their greater experimental control of confounding factors (Davies et al, 2009, 2021; Davies, Bates, & Boyd, 2016; Davies, Bates, Boyd, & Svejcar, 2016; Porensky et al, 2020; Veblen et al, 2015; Yeo, 2005). The difference in correlative compared to exclosure‐experiment findings may relate to a tendency of grazing‐exclosure studies in sagebrush steppe to be located in areas in relatively good ecological condition and/or relatively low abundances of cheatgrass (Anderson & Inouye, 2001; Condon et al, 2020; Copeland et al, 2021; Courtois et al, 2004; Davies et al, 2012, 2018; Davies, Bates, & Boyd, 2016; Davies, Bates, Boyd, & Svejcar, 2016; Porensky et al, 2020). Assessments of whole‐community responses to grazing exclusion in the presence of invasive threats are thus needed.…”