Dipartimento Territorio e Sistemi Agro-Forestali (TESAF), Universitá degli Studi di Padova, Legnaro, PD, Italy.
Instituto Pirenaico de Ecología (IPE-CSIC), Zaragoza, Spain.
Glob Chang Biol. 2021 Dec;27(24):6394-6408. doi: 10.1111/gcb.15881. Epub 2021 Sep 23.
In 2010-2018, Northern Patagonia featured the longest severe drought of the last millennium. This extreme dry spell triggered widespread growth decline and forest dieback. Nonetheless, the roles played by the two major mechanisms driving dieback, hydraulic failure and carbon starvation, are still not clear and understudied in this seasonally dry region. Here, for the 1800-2017 period, we apply a retrospective analysis of radial growth, wood anatomical traits (lumen area, cell-wall thickness) and δ C and δ O stable isotopes to assess dieback causes of the iconic conifer Araucaria araucana. We selected three stands where declining (defoliated) and nondeclining (not defoliated) trees coexisted along a precipitation gradient from the warm-dry Coastal Range to the cool-wet Andes. At all sites declining trees showed lower radial growth and lower theoretical hydraulic conductivity, suggesting a long-lasting process of hydraulic deterioration in their water transport system compared to nondeclining, coexisting trees. Wood anatomical traits evidenced that this divergence between declining and nondeclining trees started at least seven decades before canopy dieback. In the drier stands, declining trees showed higher water-use efficiency (WUE) throughout the whole period, which we attributed to early stomatal closure, suggesting a greater carbon starvation risk consistent with thinner cell walls. In the wettest stand, we found the opposite pattern. Here, a reduction in WUE coupled with thicker cell walls suggested increased carbon assimilation rates and exposure to drought-induced hydraulic failure. The δ O values indicated different strategies of gas exchange between sites, which are likely a consequence of microsite conditions and water sources. Multiproxy, retrospective quantifications of xylem anatomical traits and tree-ring isotopes provide a robust tool to identify and forecast, which stands or trees will show dieback or, on the contrary, which will likely withstand and be more resilient to future hotter droughts.
在 2010-2018 年，巴塔哥尼亚北部经历了上千年以来最长的严重干旱。这一极端干旱期引发了广泛的生长衰退和森林枯败。然而，在这个季节性干旱地区，驱动枯败的两个主要机制——水力衰竭和碳饥饿的作用仍不清楚，也尚未得到充分研究。在这里，我们对径向生长、木材解剖特征（腔面积、细胞壁厚度）和 δC 和 δO 稳定同位素进行了回顾性分析，以评估标志性针叶树南洋杉在 1800-2017 年间的枯败原因。我们选择了三个林分，在从温暖干燥的海岸山脉到凉爽潮湿的安第斯山脉的降水梯度上，有衰退（落叶）和非衰退（无落叶）的树木共存。在所有的地点，衰退的树木表现出较低的径向生长和较低的理论水力传导率，这表明与非衰退、共存的树木相比，它们的水分运输系统的水力恶化过程持续时间较长。木材解剖特征表明，这种衰退树木与非衰退树木之间的差异至少在树冠枯败前 70 年就开始了。在较干燥的林分中，衰退树木在整个时期表现出较高的水分利用效率（WUE），我们认为这是由于早期的气孔关闭，表明存在更大的碳饥饿风险，与较薄的细胞壁一致。在最潮湿的林分中，我们发现了相反的模式。在这里，WUE 的降低伴随着细胞壁的增厚，这表明碳同化率增加，并暴露于干旱引起的水力衰竭中。δO 值表明了不同站点之间气体交换的不同策略，这可能是微站点条件和水源的结果。木质部解剖特征和树木年轮同位素的多指标、回顾性量化为识别和预测哪些林分或树木将出现枯败提供了一个强有力的工具，或者相反，哪些树木将更有可能承受并对未来更热的干旱更具弹性。
