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Increasing temperature seasonality may overwhelm shifts in soil moisture to favor shrub over grass dominance in Colorado Plateau drylands

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Ecosystems in the southwestern U.S. are predicted to experience continued warming and drying trends of the early twenty-first century. Climate change can shift the balance between grass and woody plant abundance in these water-limited systems, which has large implications for biodiversity and ecosystem processes. However, variability in topo-edaphic conditions, notably soil texture and depth, confound efforts to quantify specific climatic controls over grass vs. shrub dominance. Here, we utilized weather records and a mechanistic soil water model to identify the timing and depth at which soil moisture related most strongly to the balance between grass and shrub dominance in the southern Colorado Plateau. Shrubs dominate where there is high soil moisture availability during winter, and where temperature is more seasonally variable, while grasses are favored where moisture is available during summer. Climate change projections indicate consistent increases in mean temperature and seasonal temperature variability for all sites, but predictions for summer and winter soil moisture vary across sites. Together, these changes in temperature and soil moisture are expected to shift the balance towards increasing shrub dominance across the region. These patterns are strongly driven by changes in temperature, which either enhance or overwhelm effects of changes in soil moisture across sites. This approach, which incorporates local, edaphic factors at sites protected from disturbance, improves understanding of climate change impacts on grass vs. shrub abundance and may be useful in other dryland regions with high edaphic and climatic heterogeneity.

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This research was funded by the USGS National Park Monitoring Program and made possible by the National Park Service Inventory and Monitoring Division and the Southern Colorado Plateau Network (SCPN). We would like to thank Megan Swan and Jim DeCoster who manage the upland vegetation monitoring project for SCPN, and data managers Lee McCoy and Cindy Parker for their assistance with compiling vegetation, climate, and soils data as well as providing general consultation about details of the ecosystems in the study. Thanks also to James Allen and the Northern Arizona University field crews who collaborated with NPS to collect field data. Finally, we would like to thank four anonymous reviewers for valuable comments. Any use of trade, product, or firm names in this paper is for descriptive purposes only and does not imply endorsement by the U.S. Government.

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JBB, JRG, LPT, JRN, SMM, and MCD developed research ideas and approach, JRG, JBB, and CA developed and conducted analyses. JRG wrote the manuscript with assistance and editing from all co-authors.

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Correspondence to Jennifer R. Gremer.

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Communicated by Heather Throop.

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Gremer, J.R., Andrews, C., Norris, J.R. et al. Increasing temperature seasonality may overwhelm shifts in soil moisture to favor shrub over grass dominance in Colorado Plateau drylands. Oecologia 188, 1195–1207 (2018).

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