Climate warming in recent decades has negatively impacted forest health in the western United States. Here, we report on potential early warning signals (EWS) for drought-related mortality derived from measurements of tree-ring growth (ring width index; RWI) and carbon isotope discrimination (∆¹³C), primarily focused on ponderosa pine (Pinus ponderosa). Sampling was conducted in the southern Sierra Nevada Mountains, near the epicenter of drought severity and mortality associated with the 2012–2015 California drought and concurrent outbreak of western pine beetle (Dendroctonus brevicomis). At this site, we found that widespread mortality was presaged by five decades of increasing sensitivity (i.e., increased explained variation) of both tree growth and ∆¹³C to Palmer Drought Severity Index (PDSI). We hypothesized that increasing sensitivity of tree growth and ∆¹³C to hydroclimate constitute EWS that indicate an increased likelihood of widespread forest mortality caused by direct and indirect effects of drought. We then tested these EWS in additional ponderosa pine-dominated forests that experienced varying mortality rates associated with the same California drought event. In general, drier sites showed increasing sensitivity of RWI to PDSI over the last century, as well as higher mortality following the California drought event compared to wetter sites. Two sites displayed evidence that thinning or fire events that reduced stand basal area effectively reversed the trend of increasing hydroclimate sensitivity. These comparisons indicate that reducing competition for soil water and/or decreasing bark beetle host tree density via forest management—particularly in drier regions—may buffer these forests against drought stress and associated mortality risk. EWS such as these could provide land managers more time to mitigate the extent or severity of forest mortality in advance of droughts. Substantial efforts at deploying additional dendrochronological research in concert with remote sensing and forest modeling will aid in forecasting of forest responses to continued climate warming.

中文翻译：

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树木干旱敏感性的变化为加州干旱和森林死亡事件提供了预警信号

近几十年来的气候变暖对美国西部的森林健康产生了负面影响。在这里，我们报告了与干旱相关的死亡率的潜在早期预警信号 (EWS)，这些信号来源于对树木年轮生长（年轮宽度指数；RWI）和碳同位素鉴别（Δ ¹³ C）的测量，主要集中在黄松（Pinus ponderosa）。采样是在内华达山脉南部进行的，靠近与 2012-2015 年加利福尼亚干旱和西部松甲虫（Dendroctonus brevicomis）同时爆发相关的干旱严重程度和死亡率的中心。在这个地点，我们发现树木生长和 Δ 的敏感性增加（即解释变化增加）的五个十年预示着广泛的死亡率¹³ C 到 Palmer 干旱严重程度指数 (PDSI)。我们假设树木生长的敏感性增加和 Δ ¹³C to hydroclimate 构成 EWS，表明干旱的直接和间接影响导致广泛森林死亡的可能性增加。然后，我们在其他以黄松为主的森林中测试了这些 EWS，这些森林经历了与同一加州干旱事件相关的不同死亡率。总体而言，上个世纪，较干燥的地点显示出 RWI 对 PDSI 的敏感性增加，并且与较潮湿的地点相比，加州干旱事件后的死亡率更高。两个地点的证据表明，减少林分基底面积的变薄或火灾事件有效地扭转了水文气候敏感性增加的趋势。这些比较表明，通过森林管理减少对土壤水的竞争和/或降低树皮甲虫寄主树的密度 - 特别是在干旱地区 - 可以缓冲这些森林免受干旱压力和相关的死亡风险。诸如此类的 EWS 可以为土地管理者提供更多时间，以在干旱之前减轻森林死亡的程度或严重程度。在部署额外的树木年代学研究与遥感和森林建模相结合的大量努力将有助于预测森林对持续气候变暖的反应。