Cross-scale analysis reveals interacting predictors of annual and perennial cover in Northern Great Basin rangelands

Exotic annual grass invasion is a widespread threat to the integrity of
sagebrush ecosystems in Western North America. Although many predictors of
annual grass prevalence and native perennial vegetation have been
identified, there remains substantial uncertainty about how regional-scale
and local-scale predictors interact to determine vegetation heterogeneity,
and how associations between vegetation and cattle grazing vary with
environmental context. Here, we conducted a regionally extensive,
one-season field survey across burned and unburned, grazed, public lands
in Oregon and Idaho, with plots stratified by aspect and distance to water
within pastures to capture variation in environmental context and grazing
intensity. We analyzed regional and local-scale patterns of annual grass,
perennial grass, and shrub cover, and examined to what extent plot-level
variation was contingent on pasture-level predictions of site
favorability. Annual grasses were widespread at burned and unburned sites
alike, contrary to assumptions of annual grasses depending on fire, and
more common at lower elevations and higher temperatures regionally, as
well as on warmer slopes locally. Pasture-level grazing pressure
interacted with temperature such that annual grass cover was associated
positively with grazing pressure at higher temperatures but associated
negatively with grazing pressure at lower temperatures. This suggests that
pasture-level temperature and grazing relationships with annual grass
abundance are complex and context-dependent, though the causality of this
relationship deserves further examination. At the plot-level within
pastures, annual grass cover did not vary with grazing metrics, but
perennial cover did; perennial grasses, for example, had lower cover
closer to water sources, but higher cover at higher dung counts within a
pasture, suggesting contrasting interpretations of these two grazing
proxies. Importantly for predictions of ecosystem response to temperature
change, we found that pasture-level and plot-level favorability
interacted: perennial grasses had higher plot-level cover on cooler
slopes, and this difference across topography was starkest in pastures
that were less favorable for perennial grasses regionally. Understanding
the mechanisms behind cross-scale interactions and contingent responses of
vegetation to grazing in these increasingly invaded ecosystems will be
critical to land management in a changing world.