Threshold Response to Extreme Drought Shifts Inter-Tree Growth Dominance in Pinus sylvestris
Threshold Response to Extreme Drought Shifts Inter-Tree Growth Dominance in Pinus sylvestris
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DOI：
10.3389/ffgc.2021.737342
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发表时间：
2021-12
期刊：
影响因子：
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通讯作者：
Thomas S. Ovenden;M. Perks;Toni‐Kim Clarke;Maurizio Mencuccini;A. Jump
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文献类型：
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作者：
Thomas S. Ovenden;M. Perks;Toni‐Kim Clarke;Maurizio Mencuccini;A. Jump
许多研究量化了相对于干旱前生长平均值的短期干旱对树木生长的影响。然而，很少有研究考察不同严重程度的干旱对树木生长或形状林分动态的不同影响程度。针对苏格兰樟子松高密度林分和低密度林分的三次干旱，利用气候标准化的前期生长年数计算干旱前的生长平均值，以百分比生长变化量(PGC)来评估树木水平的干旱和干旱后的生长表现。然后，我们使用混合效应模型来了解不同严重程度的干旱对树木生长的影响，并计算生长优势度(Gd)、大小不均匀(Si)和大小不对称(Sa)指数来检测林分结构的变化。混合效应模型的结果表明，在更极端的干旱期间和之后，生长减少的幅度和持续时间明显大于不那么严重的干旱，我们发现干旱影响的证据有限。虽然在任何干旱后没有注意到Si或Sa的变化，但我们发现在最极端干旱后两种林分密度的Gd差异表明了阈值反应，较小的树木相对于其大小对林分生长的贡献比例更大。在不太严重的干旱情况下，树间变异可能部分地缓冲了林分水平的生长变化，然而，干旱严重程度的小幅增加与平均树木生长显著减少、生长在2SD的树木数量增加到干旱前水平以下以及Gd向较小的树木转移有关，这表明可能已经超过了小叶松的干旱严重程度阈值。
Many studies quantify short-term drought impact on tree growth relative to pre-drought growth averages. However, fewer studies examine the extent to which droughts of differing severity differentially impact tree growth or shape stand dynamics. Focusing on three droughts in high and low density stands of Pinus sylvestris in Scotland, we calculated pre-drought growth averages using climatically standardized antecedent growth years to assess tree level drought and post-drought growth performance as percentage growth change (PGC). We then used mixed-effects models to understand how droughts of differing severity impact tree growth and calculated indices of growth dominance (Gd), size inequality (Si), and size asymmetry (Sa) to detect changes in stand structure. Mixed-effects model results indicate that the magnitude and duration of the growth reduction during and following the more extreme drought was significantly larger compared to less severe droughts, for which we found limited evidence of drought impact. While no changes in Si or Sa were noted following any drought, we found evidence of a difference in Gd after the most extreme drought in both stand densities indicative of a threshold response, with smaller trees contributing proportionally more to stand growth relative to their size. Under less severe droughts, inter-tree variability may have partially buffered against stand-level growth change, however, a small increase in drought severity was associated with a significant reduction in average tree growth, an increase in the number of trees growing at >2SD below pre-drought levels and a shift in Gd toward smaller trees, indicating that a drought severity threshold in P. sylvestris may have been exceeded.