School of Biological Sciences, University of Utah, Salt Lake City, Utah.
School of Public and Environmental Affairs, Indiana University, Bloomington, Indiana.
Glob Chang Biol. 2019 Sep;25(9):2978-2992. doi: 10.1111/gcb.14710. Epub 2019 Jun 30.
Severe drought can cause lagged effects on tree physiology that negatively impact forest functioning for years. These "drought legacy effects" have been widely documented in tree-ring records and could have important implications for our understanding of broader scale forest carbon cycling. However, legacy effects in tree-ring increments may be decoupled from ecosystem fluxes due to (a) postdrought alterations in carbon allocation patterns; (b) temporal asynchrony between radial growth and carbon uptake; and (c) dendrochronological sampling biases. In order to link legacy effects from tree rings to whole forests, we leveraged a rich dataset from a Midwestern US forest that was severely impacted by a drought in 2012. At this site, we compiled tree-ring records, leaf-level gas exchange, eddy flux measurements, dendrometer band data, and satellite remote sensing estimates of greenness and leaf area before, during, and after the 2012 drought. After accounting for the relative abundance of tree species in the stand, we estimate that legacy effects led to ~10% reductions in tree-ring width increments in the year following the severe drought. Despite this stand-scale reduction in radial growth, we found that leaf-level photosynthesis, gross primary productivity (GPP), and vegetation greenness were not suppressed in the year following the 2012 drought. Neither temporal asynchrony between radial growth and carbon uptake nor sampling biases could explain our observations of legacy effects in tree rings but not in GPP. Instead, elevated leaf-level photosynthesis co-occurred with reduced leaf area in early 2013, indicating that resources may have been allocated away from radial growth in conjunction with postdrought upregulation of photosynthesis and repair of canopy damage. Collectively, our results indicate that tree-ring legacy effects were not observed in other canopy processes, and that postdrought canopy allocation could be an important mechanism that decouples tree-ring signals from GPP.
严重干旱会对树木的生理机能产生滞后影响，从而导致森林功能在数年内受到负面影响。这些“干旱遗留效应”在树木年轮记录中已有广泛记载，可能对我们理解更广泛的森林碳循环具有重要意义。然而，由于（a）干旱后碳分配模式的改变；（b）径向生长和碳吸收之间的时间不同步；以及（c）树木年代学采样偏差，年轮增量中的遗留效应可能与生态系统通量脱钩。为了将年轮中的遗留效应与整个森林联系起来，我们利用了美国中西部一个森林的丰富数据集，该森林在 2012 年的干旱中受到了严重影响。在该地点，我们编译了树木年轮记录、叶片气体交换、涡度通量测量、树木测径带数据以及卫星遥感的绿色度和叶面积在 2012 年干旱之前、期间和之后的数据。在考虑了林分中树种的相对丰度后，我们估计，在严重干旱后的一年中，遗留效应导致树木年轮宽度增量减少了约 10%。尽管树木的径向生长有这种程度的减少，但我们发现，在 2012 年干旱后的一年中，叶片水平的光合作用、总初级生产力（GPP）和植被绿色度并没有受到抑制。径向生长和碳吸收之间的时间不同步以及采样偏差都不能解释我们在年轮中观察到的遗留效应，但不能解释 GPP 中的遗留效应。相反，在 2013 年初，较高的叶片水平光合作用与叶片面积减少同时发生，这表明资源可能被重新分配，而不是用于径向生长，同时伴随着干旱后的光合作用上调和冠层损伤的修复。总的来说，我们的研究结果表明，其他冠层过程中没有观察到树木年轮的遗留效应，干旱后的冠层分配可能是将树木年轮信号与 GPP 解耦的一个重要机制。
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