Vegetation in dryland ecosystems is highly sensitive to changes in precipitation amount and timing. Globally, forecasted increases in climate aridity and variability in these regions are likely to alter vegetation dramatically. The Colorado Plateau (CP) is an important dryland ecoregion in the southwest United States for which drier and more variable conditions are forecast. The CP is characterized by moderate to high elevations, cold winters, and shallow soils. Vegetation across much of the CP has already undergone significant changes during the past century that have been largely attributed to grazing intensity and multidecadal shifts in precipitation regime. Long-lived native and non-native tree and shrub species have filled previously barren streambeds while upland pinyon-juniper woodlands have seen widespread regional dieoff. Plant functional type dominance has shifted across the CP, and remote areas have been colonized by invasive annual plant species. These large shifts in plant functional type have changed leaf area, phenology, and net primary productivity in many habitats, consequently altering the magnitude and timing of evapotranspiration and soil drying. We combine 100 years of historic repeat photography with remote sensing and field observations from across southeastern Utah, USA, to model the mechanisms of state changes in vegetation, climate, and land use. This model will aid in understanding how vegetation changes influence water cycling, and how land use history and climatic and edaphic factors limit the scope of vegetation change.
Results/Conclusions
We found widely differing responses of Colorado Plateau vegetation to 100 years of warming, variability in timing and amount of precipitation, and cessation of grazing. Most sites showed little change in species composition and vegetation cover. Some of the warmest and driest sites showed loss of large-statured evergreen species, with replacement by drought-deciduous and smaller-statured shrubs. Some cooler and wetter sites showed substantial recovery from overgrazing, transforming to conditions likely similar to pre-domestic grazing. Overall, we found that CP vegetation has been resilient to multidecadal changes in precipitation, warming, and cessation of grazing. However, vegetation types growing at their physiological limits may be experiencing rapid functional type shifts with a changing climate. Understanding the mechanisms behind these shifts is critical for developing successful adaptive management strategies for the Colorado Plateau.