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

Abstract

Adjustments or complete withdrawal of livestock grazing are among the most common conservation actions in semiarid uplands, but outcomes can vary considerably with ecological context. Invasion by exotic annual grasses and the excessive wildfire they promote are increasing threats to semiarid shrub‐steppe, and plant‐community response to livestock exclusion in these areas may be complicated by the rapid colonization ability of invaders. We evaluated vegetation‐community changes over 14‐year interval (2007–2021) in a shrub‐steppe landscape where a >100‐year history of livestock grazing had been terminated in 1996. Field surveys revealed that bare‐soil exposure decreased >20% over the 14 years owing to biomass accumulation, but this was primarily due to large increases in exotic annual “cheatgrass” (Bromus tectorum, +1.8‐fold) and the litter it produces (+1.5‐fold). Soil biocrusts increased 11.9% and perennial bunchgrasses increased 3% over the 14 years. These community changes varied at the patch scale and entailed inverse relationships of (1) both cheatgrass and biocrusts to plant‐community basal cover, (2) cheatgrass to both biocrusts and perennial grasses, and (3) biocrusts to cheatgrass and litter. The spatiotemporal variability in vegetation constituted changes in plant‐community states, according to cluster analysis. The modeled probability of a community transitioning to a cheatgrass state was (1) strongly and positively related to the initial (2007) cover of cheatgrass in hotspots where initial cheatgrass cover was >20%, and (2) negatively related to biocrust cover where initial biocrust cover was >4% of ground area. The decision space for this landscape can be framed as a shifting from acceptance towards resisting further degradation by removing livestock and their trampling of soil surfaces and utilization of perennial herbs. However, cheatgrass appears to be the most impactful agent of change and continued invasion appears imminent. Active restoration may help resist further degradation and direct change towards tolerable conditions.

A nontarget, disturbance‐resilient native species influences post‐fire recovery and multiphasic herbicide‐seeding outcomes in drylands threatened by exotic annual grasses

“…In burned but untreated areas, mean exotic annual grass cover varied among years, but remained >20% of ground area in the first 5 year post‐fire, which is considered to be a level conferring additional invasion in future years and thus more difficult restoration interventions (Germino et al 2022). Although mean cover was high, more than one‐fourth of untreated plots had <20% exotic annual grass, and these relatively uninvaded plots also had >18% P. secunda cover.…”

“…This deferred treatment sequence could increase the synergistic benefits of both allowing residual perennial vegetation to reestablish before herbicide application with minimal impact to P. secunda, while still effectively reducing exotic annual grasses that would otherwise compete with seeded vegetation (Davies & Sheley 2011). Moreover, this deferred treatment sequence would avoid hazards of off-site herbicide drift that can occur when bare, freshly burned soils erode (Germino et al 2016).…”

“…Invasive species that colonize disturbed areas may have these same effects but are also commonly recognized to interfere with natural or assisted successional recovery. A prominent example of such an exotic, invasive species is cheatgrass (Bromus tectorum L.), a widespread exotic annual grass originating from Eurasia that is now prevalent in many semiarid ecosystems across Western North America and the sagebrush biome in particular (Mack 1981;Germino et al 2016). Cheatgrass has greatly altered fire regimes by increasing fuel continuity and combustibility, leading to increased fire frequency, which in turn favors more cheatgrass establishment in the post-fire environment (D'Antonio & Vitousek 1992;Brooks et al 2004).…”

“…Cheatgrass has greatly altered fire regimes by increasing fuel continuity and combustibility, leading to increased fire frequency, which in turn favors more cheatgrass establishment in the post-fire environment (D'Antonio & Vitousek 1992;Brooks et al 2004). This creates a positive feedback loop of invasion and disturbance that can cause replacement of native grass stands with annual grasslands (Knapp 1996;Germino et al 2016). Community changes in sagebrush steppe following fire can result in variable stable plantcommunity outcomes that vary with resilience of the predisturbance plant-community and the residual post-disturbance plantcommunity composition, resulting in a postdisturbance mosaic of "climax" community states ranging from exotic to nativedominated (Allen 1989;Bates et al 2020).…”

“…However, in terms of ecological transformation, P. secunda seems to be a more benign component of the plant communities it inhabits. In contrast, exotic annual grasses aggressively compete with indigenous species and promote increased wildfire activity on the landscape, often leading to conversion of diverse perennial stands to annual grasslands (Germino et al 2016). Surprisingly little is known about (1) how restoration treatments may affect P. secunda abundance, or (2) how P. secunda may influence the effectiveness of treatments, or (3) the likelihood of P. secunda playing an important intermediary role in assisting transition of the ecological community from a post-fire to later-seral states.…”