Valeriano Cristina, Gazol Antonio, Colangelo Michele, González de Andrés Ester, Camarero J Julio
Instituto Pirenaico de Ecología (IPE-CSIC), Zaragoza, Spain.
Departamento de Sistemas Naturales e Historia Forestal, Universidad Politécnica de Madrid, Madrid, Spain.
Front Plant Sci. 2021 Aug 26;12:672855. doi: 10.3389/fpls.2021.672855. eCollection 2021.
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 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.
干旱导致的森林衰退是一种威胁特定树木种群的全球现象。特别脆弱的林分通常位于气候条件恶劣的地区，如遭受季节性干旱的干旱地区。这些树木种群在应对热浪和干旱等极端气候事件时，会明显丧失活力、生长减缓且死亡率高。然而，衰退事件并非均匀地影响林分，一些树木比其他相邻同种树木表现出更高的干旱脆弱症状。在本研究中，我们利用树木年代学和基于瓦加诺夫-沙什金（VS）过程的生长模型，调查了对衰退表现出不同脆弱性的树木过去是否生长速率较低（Grs）且对气候更敏感。我们研究了西班牙东北部伊比利亚山脉近期出现衰退、生长速率对比鲜明的两个林分。我们比较了共存的衰退（D）树和非衰退（ND）树，其树冠落叶值分别高于和低于50%阈值。在这两个林分中，D树的平均生长速率低于ND树。这两个活力等级在衰退开始前就出现了生长差异，且对气候的响应不同。ND树比D树对春季水分平衡和土壤湿度的变化更敏感，这表明受胁迫树木对气候的生长响应丧失。水分可利用性与活力之间的这种相互作用在VS模型模拟中得到体现，该模拟为两个地点生长主要受低土壤湿度限制这一观察结果提供了证据。所呈现的比较表明，不同林分对干旱的脆弱性取决于现场条件。进一步的研究应调查环境条件和个体特征（如获取土壤水分或水力特性）所起的作用，并将其纳入基于过程的生长模型，以更好地预测衰退。
