Fire Invasive Response Metric (FIRM) and Habitat Invasive Risk (HIR) for Western U.S. Rangelands

Short-term Fire Invasive Response Metric (FIRM) (2016-2024), Long-term FIRM (1985-2024), and Habitat Invasive Risk (HIR) quantify wildfire-driven invasive annual grass responses across western U.S. rangelands. Context - Understanding the relationship between Exotic Annual Grass (EAG) expansion, drought, and fire frequency across western US rangelands is complex, but critical for maintaining ecological integrity. Objectives - We quantified the impacts of wildfire on EAGs, with emphasis on understanding the factors driving the spatial and temporal variability of those impacts, highlighting the role of drought. Methods - We investigate two study periods using 1985-2024 and 2016-2024 EAG cover datasets to better understand EAG dynamics in relation to fire. We, 1) leverage mapped EAG cover responses to fire in the context of various biophysical variables to model expected change in EAG cover (ΔG) given a fire in a specific location, even if no fire has occurred in that location in the period of record; 2) evaluate the influence of drought and other biophysical variables on the ΔG; 3) combine our ΔG values with burn probability and ecological integrity data to evaluate the profile of EAG risk due to fire in sagebrush habitats. Results - Our results show that fire impacts on EAG are often delayed, with limited effects in the first post-fire year followed by increasing cover over subsequent years. Importantly, areas with low pre-fire EAG cover exhibited the greatest potential for post-fire increases, whereas heavily invaded sites showed lower incremental responses, suggesting saturation or carrying-capacity constraints. Conclusions - Understanding the connections among EAGs, drought, and wildfire is critical for maintaining the ecological integrity of western rangelands. Identifying the magnitude of fire impacts to EAG cover provides critical insight for land managers.

We provide short-term (based on EAG ) and long-term (based on RCMAP [Rangeland Condition Monitoring Assessment and Projection]) FIRM data depicting the modelled change in EAG cover by post-fire year, given a fire in a specific location. The short‑term FIRM dataset characterizes post‑fire invasive annual grass cover response during the first five years after wildfire. The long‑term FIRM dataset captures multi‑decadal trajectories of invasive annual grass response following fire. It uses the RCMAP annual herbaceous cover dataset (1985–2024), allowing estimation of ΔG for up to 30 years after fire. Additionally, we provide HIR data which ranks the EAG risk to site ecological integrity based on a) risk of fire occurrence (likelihood of impact), b) existing ecological integrity (i.e. value), and c) maximum FIRM based on the long-term, RCMAP dataset. By combining these datasets, HIR provides a more complete view of the ecological integrity risk due to the EAG-fire cycle.