Positive Drought Feedbacks Increase Tree Mortality Risk in Dry Woodlands of the US Southwest.

Global changes in temperature and aridity are increasing the frequency of extreme drought events. Such changes can have pronounced impacts on dryland ecosystems, which exist at the margins of plant physiological tolerances. Pinyon-juniper (PJ) woodlands-a dryland vegetation type spanning 40 million ha in western North America-are a model system for the impacts of drought, where recurrent short-interval drought events may trigger feedback mechanisms that influence future drought resistance. Leveraging a long-term monitoring network in PJ woodlands of the United States (US) Southwest, we sought to understand how interactions between recurrent drought events influence tree mortality risk. We developed generalized linear mixed models to predict patterns of recent (i.e., 2014-2023) tree mortality based on biophysical variables, tree size, and prior drought-driven changes (ca. 1998-2014) in forest conditions. We then used these models to quantify how mortality risk has shifted over time. Tree density and stand basal area declined substantially throughout our 1998-2023 monitoring period. Since 2014, tree mortality was more common and spatially extensive than new tree recruitment, and nearly half of the surviving trees experienced crown dieback. Tree size influenced biotic interactions and responses to environmental conditions, and soil organic matter and mycorrhizal fungi communities buffered individuals against drought. Shifts in woodland demographics (e.g., reduced stand densities, crown dieback) led to a 28.2% increase in mortality risk between 2014 and 2023 for trees that survived this period, a pattern that was consistent across species. Recent drought events have triggered widespread tree mortality and dieback in PJ woodlands of the US Southwest. These events also increase future tree mortality risk, overcoming system inertia created by local edaphic conditions and compensatory responses.