Understanding the Anomalies in Exotic Annual Grass Cover in Precipitation Scenario Maps of Rangelands in the Western United States

In rangeland ecosystems of the western United States, invasion by exotic annual grass (EAG) poses a substantial threat to native biodiversity. Studies have shown that weather, especially precipitation, can greatly influence the rate of invasion and EAG cover in arid and semi-arid rangeland ecosystems. In a previous effort to help inform timely decisions of local and regional land managers, the U.S. Geological Survey released multiple EAG cover maps that were driven by varying precipitation scenarios for rangeland ecosystems of the western United States. In those modelled maps, we found a positive correlation of EAG cover to precipitation in most areas as expected. However, in certain anomalous areas (less than 10% of the landscape) precipitation had no or negative correlation with EAG cover. In this study, we set out to understand what causes these anomalies. We identified variables, such as edaphic, topographic, and coexisting vegetation that may influence EAG cover in different precipitation scenarios. We implemented a thresholding approach to assess the influence of these variables on EAG cover. We found that soils with low clay content have a higher likelihood of positive EAG anomalies (higher EAG cover with less precipitation) and that increasing perennial herbaceous and decreasing shrub vegetation cover results in a higher likelihood of negative EAG anomalies (lower EAG cover with more precipitation). We also found a higher likelihood of negative EAG anomalies in higher solar radiation areas, and high frequency of positive EAG anomalies in mid and low solar radiation areas. Multiple factors play significant roles in EAG cover in the arid and semi-arid rangelands of the United States. Understanding these factors can help to better forecast EAG cover and therefore better plan for fire risk and management strategies.