Projections of Rangeland Fractional Component Cover Across the Sagebrush Biome for Representative Concentration Pathways (RCP) 4.5 and 8.5 Scenarios for the 2020s, 2050s, and 2080s Time-Periods (ver. 1.1, April 2022)

Climate change over the past century has altered vegetation community composition and species distributions across rangelands in the western United States. The scale and magnitude of climatic influences are unknown. While a number of studies have projected the impacts of climate change using several modeling approaches, none has evaluated impacts to fractional component cover at a 30-m resolution across the full sagebrush (Artemisia spp.) biome. We used fractional component cover data for rangeland functional groups and weather data from the 1985 to 2018 reference period in conjunction with soils and topography data to develop empirical models describing the spatio-temporal variation in component cover. To investigate the ramifications of future change across the western US, we extended models based on historical relationships over the reference period to model landscape effects based on future weather conditions from two emissions scenarios (Representative Concentration Pathways [RCP] 4.5 and 8.5) and three time periods (2020s, 2050s, and 2080s). We tested both Generalized Additive Models (GAMs) and regression tree models, finding that the former led to superior spatial and statistical results. Our results suggest more xeric vegetation across most of the study area, with an increasing dominance of non-sagebrush shrubs, annual herbaceous, and bare ground over herbaceous and sagebrush cover in both RCP scenarios. In general, both scenarios yielded similar results, but RCP 8.5 tended to be more extreme, with greater change relative to the reference period. Results demonstrate that in cool sites some degree of warming to growing season maximum temperature or non-growing season minimum temperature could be beneficial to sagebrush and shrub growth. This is not the case, regardless of temperature, for non-growing season maximum temperature. This information can be used to inform management to prepare for future vegetation composition and cover through the prioritization of conservation and restoration and shed light on species’ range shifts. 
*Please note that some suspect increases in sagebrush cover are projected in the Southern Great Basin and Southern Colorado Plateau, which we interpret as model error. We have developed a mask (see below) to apply to the future sagebrush cover layers. Masked areas meet three criteria: 1) they are within the sagebrush biome (Jeffries and Finn 2019), 2) within the following EPA level 3 ecoregions: Arizona/New Mexico Mountains, Arizona/New Mexico Plateau (excluding the San Luis Valley from the mask), Colorado Plateaus (excluding the Uinta Basin from the mask), Mojave Basin and Range, and Sonoran Basin and Range, and 3) have a maximum of 0% sagebrush cover observed over the 1985-2020 RCMAP time-series.

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Organization: Department of the Interior