Increasing aridity is a challenge for forest managers and reducing stand density to minimize competition is a recognized strategy to mitigate drought impacts on growth. In many dry forests, the most widespread and common forest management programs currently being implemented focus on restoration of historical stand structures, primarily to minimize fire risk and enhance watershed function. The implications of these restoration projects for drought vulnerability are not well understood. Here, we examined how planned restoration treatments in the Four Forests Restoration Initiative, the largest forest restoration project in the United States, would alter landscape‐scale patterns of forest growth and drought vulnerability throughout the 21st century. Using drought–growth relationships developed within the landscape, we considered a suite of climate and treatment scenarios and estimated average forest growth and the proportion of years with extremely low growth as a measure of vulnerability to long‐term decline. Climatic shifts projected for this landscape include higher temperatures and shifting seasonal precipitation that promotes lower soil moisture availability in the early growing season and greater hot‐dry stress, conditions negatively associated with tree growth. However, drought severity and the magnitude of future growth declines were moderated by the thinning treatments. Compared to historical conditions, proportional growth in mid‐century declines by ~40% if thinning ceases or continues at the status quo pace. By comparison, proportional growth declines by only 20% if the Four Forest Restoration Initiative treatments are fully implemented, and <10% if stands are thinned even more intensively than currently planned. Furthermore, restoration treatments resulted in dramatically fewer years with extremely low growth in the future, a recognized precursor to forest decline and eventual tree mortality. Benefits from density reduction for mitigating drought‐induced growth declines are more apparent in mid‐century and under RCP4.5 than under RCP8.5 at the end of the century. Future climate is inherently uncertain, and our results only reflect the climate projections from the representative suite of models examined. Nevertheless, these results indicate that forest restoration projects designed for other objectives also have substantial benefits for minimizing future drought vulnerability in dry forests and provide additional incentive to accelerate the pace of restoration.

中文翻译：

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景观尺度的恢复将树木生长对干旱森林中21世纪干旱的脆弱性降至最低

干旱增加是森林经营者的挑战，降低林分密度以最大程度地减少竞争是减轻干旱对生长的影响的公认策略。在许多干旱森林中，目前正在实施的最广泛，最常见的森林管理计划集中在恢复历史林分结构上，主要是为了最大程度地降低火灾风险和增强流域功能。这些恢复项目对干旱脆弱性的影响尚不清楚。在这里，我们研究了美国最大的森林恢复项目“四森林恢复倡议”中计划的恢复处理如何改变整个21世纪森林生长和干旱脆弱性的景观格局。利用景观中发展的干旱与增长的关系，我们考虑了一套气候和处理方案，并估计了平均森林生长以及极低生长的年所占比例，以衡量其对长期下降的脆弱性。预计该景观的气候变化包括较高的温度和季节性降水的变化，这会导致早期生长季节土壤水分的利用率降低，以及干热胁迫加剧，这些条件与树木的生长负相关。但是，通过稀疏处理减轻了干旱的严重程度和未来增长下降的幅度。与历史情况相比，如果减薄以现状速度停止或继续下去，本世纪中叶的比例增长将下降约40％。相比之下，如果全面实施“四项森林恢复倡议”的措施，则比例增长率仅下降20％，并且< 如果林分比目前计划的集约化程度更大，则减少10％。此外，恢复处理导致的年数大大减少，未来的增长极低，这是公认的森林退化和最终树木死亡的先兆。在本世纪中叶，RCP4.5之下，减少密度以减轻干旱导致的增长下降的好处比本世纪末的RCP8.5更为明显。未来的气候具有内在的不确定性，我们的结果仅反映了所研究的代表性模型对气候的预测。然而，这些结果表明，为其他目标而设计的森林恢复项目也具有极大的好处，可以最大程度地减少干旱森林未来的干旱脆弱性，并为加快恢复速度提供了额外的动力。