Using high-resolution future climate scenarios to forecast Bromus tectorum invasion in Rocky Mountain National Park

Abstract

National Parks are hallmarks of ecosystem preservation in the United States. The introduction of alien invasive plant species threatens protection of these areas. Bromus tectorum L. (commonly called downy brome or cheatgrass), which is found in Rocky Mountain National Park (hereafter, the Park), Colorado, USA, has been implicated in early spring competition with native grasses, decreased soil nitrogen, altered nutrient and hydrologic regimes, and increased fire intensity. We estimated the potential distribution of B. tectorum in the Park based on occurrence records (n = 211), current and future climate, and distance to roads and trails. An ensemble of six future climate scenarios indicated the habitable area of B. tectorum may increase from approximately 5.5% currently to 20.4% of the Park by the year 2050. Using ordination methods we evaluated the climatic space occupied by B. tectorum in the Park and how this space may shift given future climate change. Modeling climate change at a small extent (1,076 km2) and at a fine spatial resolution (90 m) is a novel approach in species distribution modeling, and may provide inference for microclimates not captured in coarse-scale models. Maps from our models serve as high-resolution hypotheses that can be improved over time by land managers to set priorities for surveys and removal of invasive species such as B. tectorum.

Fig 1. Comparison of mean annual temperature (MAT) at three spatial resolutions (4 km, 1 km, and 90 m) for climate normals 1981–2010 in Rocky Mountain National Park, Colorado.

Data Sources: PRISM Climate Group (http://www.prism.oregonstate.edu/) WorldClim (http://www.worldclim.org/) and ClimateWNA (http://climatewna.com/).

Fig 2. Variable contribution to training gain (a) and area under curve (b).

Gray bars indicate how well the model performs with only that variable, versus a full model. Values shown are averaged over 100 replicate MaxEnt model runs.

Fig 3. Bromus tectorum current suitable habitat in Rocky Mountain National Park (a) and increasing, decreasing, stable, and unsuitable habitat for the year 2050 (b) based on MaxEnt model outputs.

Elevation has been included for reference (coordinate system NAD 1983 UTM Zone 13 N).

Fig 4. Principle component analysis (PCA) of niche overlap (blue) for B. tectorum in Rocky Mountain National Park based on current (green) and future (red) climate space.

Averages for five climatic variables from six global circulation models (GCMs) were included in this analysis: mean annual temperature, spring degree days below 18°C, beginning of frost-free period, mean summer (May-Sept.) precipitation, and continentality (see methods for description of GCMs).