Semi-arid plant communities of the Southern Colorado Plateau in relation to regional climate context and local topoedaphic conditions: Comparing species abundance patterns across a network of sites to identify climate-driven vulnerabilities and inform hypotheses about future composition shifts
Authors: Thomas Lisa , Calvo Christopher , Gareis Jolie , Thomas Lisa , Calvo Christopher , Gareis Jolie 
Published: December 2024

With climate projections for increased warming and greater drought severity, the semi-arid grasslands, shrublands and pinyon-juniper woodlands occurring at lower elevations on the Colorado Plateau are likely to change in coming decades. Using shrub and herbaceous data from seven long-term study areas, I describe across-ecosite compositional gradients within the current regional climate context and compare spatial variability among assemblages. At the local scale, I model the relationship between composition and topoedaphic conditions. I use these results to develop hypotheses regarding the types of community change that may occur.

From the across-ecosite ordination, I found high similarity among three of four grassland/shrubland assemblages, and low to moderate similarity among woodland assemblages. Beta diversity is significantly higher in two of the five assemblages, denoting spatially heterogenous composition patterns. Analysis of the relationship between the species abundance matrix and climate variables found that the three primary axes of variation accounted for 48.7% of total variation. Axis 1 separates grassland/shrubland assemblages from woodland assemblages, aligning with a gradient in annual precipitation. Axis 2, reflecting a gradient in perennial grass dominance, aligns with precipitation seasonality. Axis 3 aligns with seasonal temperature differences and Vapor Pressure Deficit.

Because topoedaphic conditions mediate soil water availability in semi-arid ecosystems, understanding their relationship to local composition patterns is essential to predicting how these communities will respond to climate change. Within-ecosite compositional gradients aligned with one to five environmental variables per ecosite. Elevation correlated with compositional gradients in all ecosites, and edaphic variables correlated with compositional gradients in four ecosites.

Compositional dominance also influences community stability. Dominance strength varied among ecosites, with abundance of the top two species ranging from 66% of total abundance for the strongest pattern to 35% for the weakest. From a functional group perspective, dominance was shared between perennial grass and shrub species for three woodland ecosites and one grassland/shrubland ecosite. Perennial grasses dominated in the other ecosites with several shrub species displaying moderate abundance.

These results contribute to comparing vulnerabilities and sources of resilience across ecosites. By considering both the strength of dominance patterns and dominant species characteristics against the backdrop of projected climate impacts, I offer hypotheses concerning future assemblage stability and propose comparisons of community change trajectories between ecosite pairs. Such comparisons complement ongoing trend analyses and may provide earlier warning of changes in dominance strength or species rank shifts than would be apparent by comparing a single assemblage to itself through time.