Data_Sheet_1_Modeling Climate Impacts on Tree Growth to Assess Tree Vulnerability to Drought During Forest Dieback.docx

Forest dieback because of drought is a global phenomenon threatening particular tree populations. Particularly vulnerable stands are usually located in climatically stressing locations such as xeric sites subjected to seasonal drought. These tree populations show a pronounced loss of vitality, growth decline, and high mortality in response to extreme climate events such as heat waves and droughts. However, dieback events do not uniformly affect stands, with some trees showing higher symptoms of drought vulnerability than other neighboring conspecifics. In this study, we investigated if trees showing different vulnerabilities to dieback showed lower growth rates (Grs) and higher sensitivities to the climate in the past using dendroecology and the Vaganov-Shashkin (VS) process-based growth model. We studied two Pinus pinaster stands with contrasting Grs showing recent dieback in the Iberian System, north-eastern Spain. We compared coexisting declining (D) and non-declining (ND) trees with crown defoliation values above and below the 50% threshold, respectively. The mean growth rate was lower in D than in ND trees in the two stands. The two vigor classes showed a growth divergence prior to the dieback onset and different responsiveness to climate. The ND trees were more responsive to changes in spring water balance and soil moisture than D trees, indicating a loss of growth responsiveness to the climate in stressed trees. Such an interaction between water availability and vigor was reflected by the VS-model simulations, which provided evidence for the observation that growth was mainly limited by low soil moisture in both sites. Such an interaction between water availability and vigor was reflected by the VS-model simulations, which provided evidence for the observation that growth was mainly limited by low soil moisture in both sites. The presented comparisons indicated different stand vulnerabilities to drought contingent on-site conditions. Further research should investigate the role played by environmental conditions and individual features such as access to soil water or hydraulic traits and implement them in process-based growth models to better forecast dieback.

干旱引发的森林衰退是一种全球性现象，正威胁着特定的树木种群。尤其易受影响的林分通常分布在气候胁迫生境中，例如经历季节性干旱的旱生生境。这些树木种群在遭遇热浪、干旱等极端气候事件时，会表现出显著的活力下降、生长衰退以及较高的死亡率。但森林衰退事件对林分的影响并不均匀，部分树木比周边同种个体表现出更严重的干旱脆弱性症状。本研究采用树木年代生态学方法与瓦加诺夫-沙什金（Vaganov-Shashkin，VS）过程性生长模型，探究了对衰退表现出不同脆弱性的树木，在过去是否具有更低的生长速率（Grs）以及更高的气候敏感性。我们以西班牙东北部伊比利亚山系中两处近期发生衰退、生长速率差异显著的海岸松（Pinus pinaster）林分为研究对象。我们分别以冠层脱叶率高于和低于50%阈值的衰退木（D）与非衰退木（ND）作为共存对照对象进行比较。两处林分的衰退木平均生长速率均低于非衰退木。两类活力等级的树木在衰退发生前就已出现生长分化，且对气候的响应模式存在差异。非衰退木比衰退木对春季水分平衡与土壤湿度变化的响应更为强烈，这表明受胁迫树木的生长对气候的响应能力出现了丧失。水分可利用性与树木活力间的这种相互作用在VS模型模拟结果中得到了体现，该模拟结果证实了两处林分的生长主要受土壤水分不足限制的观测结果。水分可利用性与树木活力间的这种相互作用在VS模型模拟结果中得到了体现，该模拟结果证实了两处林分的生长主要受土壤水分不足限制的观测结果。本次对比研究表明，林分对干旱的脆弱性因立地条件不同而存在差异。未来研究应探究环境条件与个体特征（如土壤水分获取能力、水力性状）所发挥的作用，并将其纳入基于过程的生长模型，以更精准地预测森林衰退事件。