all_mod_coefs from Interactive effects of tree size, crown exposure and logging on drought-induced mortality

Large trees in the tropics are reportedly more vulnerable to droughts than their smaller neighbours. This pattern is of interest due to what it portends for forest structure, timber production, carbon sequestration and multiple other values given that intensified El Niño Southern Oscillation (ENSO) events are expected to increase the frequency and intensity of droughts in the Amazon region. What remains unclear is what characteristics of large trees renders them especially vulnerable to drought-induced mortality and how this vulnerability changes with forest degradation. Using a large-scale, long-term silvicultural experiment in a transitional Amazonian forest in Bolivia, we disentangle the effects of stem diameter, tree height, crown exposure and logging-induced degradation on risks of drought-induced mortality during the 2004/2005 ENSO event. Overall, tree mortality increased in response to drought in both logged and unlogged plots. Tree height was a much stronger predictor of mortality than stem diameter. In unlogged plots, tree height but not crown exposure was positively associated with drought-induced mortality, whereas in logged plots, neither tree height nor crown exposure was associated with drought-induced mortality. Our results suggest that at the scale of a site, hydraulic factors related to tree height, not air humidity, are a cause of elevated drought-induced mortality of large trees in unlogged plots.This article is part of a discussion meeting issue ‘The impact of the 2015/2016 El Nino on the terrestrial tropical carbon cycle: patterns, mechanisms and implications'.据报道，相较于周边小型乔木，热带大型乔木对干旱的脆弱性更高。鉴于增强型厄尔尼诺南方涛动（El Niño Southern Oscillation，ENSO）事件预计将提升亚马逊地区干旱的发生频率与强度，该格局对森林结构、木材生产、碳汇（carbon sequestration）及其他多项生态系统服务价值的潜在影响引发了学界关注。目前尚未明确的是，大型乔木具备哪些特征使其格外易受干旱诱导死亡（drought-induced mortality）的威胁，且这种脆弱性如何随森林退化进程发生变化。本研究依托玻利维亚境内亚马逊过渡带森林中的一项大规模长期营林试验（silvicultural experiment），解析了2004/2005年厄尔尼诺事件期间，茎干直径（stem diameter）、树高、冠层暴露度（crown exposure）以及采伐诱导的森林退化（logging-induced degradation）对干旱诱导死亡风险的影响。整体而言，无论采伐样地还是未采伐样地，树木死亡率均因干旱出现显著上升。相较于茎干直径，树高对死亡率的预测能力更强。在未采伐样地中，树高与干旱诱导死亡呈显著正相关，而冠层暴露度则无此关联；与之相反，在采伐样地中，树高与冠层暴露度均与干旱诱导死亡无显著相关性。研究结果表明，在样地尺度下，未采伐样地内大型乔木干旱诱导死亡风险升高的驱动因子是与树高相关的水力特性，而非空气湿度。本文属于‘2015/2016年厄尔尼诺对热带陆地碳循环的影响：格局、机制与启示’专题讨论会议论文集的一部分。