New indicators of ecological resilience and invasion resistance to support prioritization and management in the sagebrush biome, United States

Ecosystem transformations to altered or novel ecological states are
accelerating across the globe. Indicators of ecological resilience to
disturbance and resistance to invasion can aid in assessing risks and
prioritizing areas for conservation and restoration. The sagebrush biome
encompasses parts of 11 western states and is experiencing rapid
transformations due to human population growth, invasive species, altered
disturbance regimes, and climate change. We built on prior use of static
soil moisture and temperature regimes to develop new, ecologically
relevant and climate-responsive indicators of both resilience and
resistance. Our new indicators were based on climate and soil water
availability variables derived from process-based ecohydrological models
that allow predictions of future conditions. We asked: (1) Which variables
best indicate resilience and resistance? (2) What are the relationships
among the indicator variables and resilience and resistance categories?
(3) How do patterns of resilience and resistance vary across the area? We
assembled a large database (n = 24,045) of vegetation sample plots from
regional monitoring programs and derived multiple climate and soil water
availability variables for each plot from ecohydrological simulations. We
used USDA Natural Resources Conservation Service National Soils Survey
Information, Ecological Site Descriptions, and expert knowledge to develop
and assign ecological types and resilience and resistance categories to
each plot. We used random forest models to derive a set of 19 climate and
water availability variables that best predicted resilience and resistance
categories. Our models had relatively high multiclass accuracy (80% for
resilience; 75% for resistance). Top indicator variables for both
resilience and resistance included mean temperature, coldest month
temperature, climatic water deficit, and summer and driest month
precipitation. Variable relationships and patterns differed among
ecoregions but reflected environmental gradients; low resilience and
resistance were indicated by warm and dry conditions with high climatic
water deficits, and moderately high to high resilience and resistance were
characterized by cooler and moister conditions with low climatic water
deficits. The new, ecologically-relevant indicators provide information on
the vulnerability of resources and likely success of management actions
and can be used to develop new approaches and tools for prioritizing areas
for conservation and restoration actions.