NOAA/WDS Paleoclimatology - Boyd - Interior Alaska Aspen Site SA1104 - PPTR - ITRDB AK184

Climate change-driven droughts and insect outbreaks are becoming more frequent and widespread, increasing forest vulnerability to mortality. By addressing the impacts of climate and insects on tree growth preceding death, we can better understand tree mortality risk under a changing climate. Here, we used tree stature and interannual growth (basal area increment; BAI) to assess processes leading to trembling aspen (Populus tremuloides) survival or mortality during an unprecedented leaf miner (Phyllocnistis populiella) outbreak in boreal North America. We identified eight sites (22 plots) in the longest running forest monitoring network in Alaska, spanning ~350 km of latitude, that experienced >=0.25 Mg ha-1 yr-1 aspen mortality during the outbreak. We compared the size and canopy position, growth patterns, and sensitivity to climate and leaf mining of aspen that survived (living; n = 84) vs. died (dying; n = 76) and linked the normalized difference vegetation index (NDVI) to plot-level aspen growth and stand biomass recruitment, growth, and mortality. Dying aspen were in the subcanopy, smaller in diameter, and after a drought in 1957 had lower growth than living aspen until death. Before the outbreak, growth of all trees was positively influenced by moisture and negatively by temperature, but only living trees maintained this climate response during the outbreak. Leaf mining reduced growth of both groups, exerting at least a twofold greater impact than climate. The NDVI captured plot-level tree growth and stand biomass growth and mortality, yet it was nearly two times more strongly associated with living than dying tree growth and 12 times more strongly associated with biomass growth than mortality. These differences suggest that NDVI may inadequately detect insect-driven dieback and dispersed mortality of aspen across the boreal biome. Our findings reveal that a historic drought triggered a multi-decadal growth decline that predisposed aspen to mortality during the leaf miner outbreak and that while aspen growth is influenced by moisture and temperature, it is more strongly affected by P. populiella. We conclude that as the climate warms and insect outbreaks increase in frequency and magnitude at high latitudes, we should expect to see persistent and greater declines in aspen growth and increases in mortality.

受气候变化驱动的干旱与昆虫暴发愈发频繁、分布范围愈发广泛，加剧了森林面临死亡的脆弱性。通过厘清气候与昆虫对树木死亡前生长过程的影响，我们能够更好地理解气候变化背景下的树木死亡风险。
本研究依托北美北方寒温带地区一场史无前例的杨潜叶旋蛾（*Phyllocnistis populiella*）暴发事件，以树木树体特征和年际生长量（断面积增量，basal area increment; BAI）为评估指标，分析了颤杨（*Populus tremuloides*）存活与死亡的相关过程。
我们在阿拉斯加运行时间最长的森林监测网络中选定了8个样地（共22个样方），样地横跨约350公里纬度范围，且在该虫害暴发期间均出现了≥0.25 Mg·ha⁻¹·yr⁻¹的颤杨死亡率。
我们对比了存活（存活个体；n=84）与死亡（濒死个体；n=76）颤杨的个体大小、冠层位置、生长模式，以及对气候与潜叶虫害的响应敏感性，并将归一化植被指数（normalized difference vegetation index; NDVI）与样地水平的颤杨生长、林分生物量更新、生长及死亡情况进行关联分析。
研究发现，濒死颤杨多处于林冠下层，直径更小；且在1957年干旱事件后，其生长速率便持续低于存活颤杨，直至最终死亡。
在虫害暴发前，所有树木的生长均与水分条件呈正相关、与温度呈负相关，但仅存活个体在虫害暴发期间仍维持了这种气候响应特征。
潜叶虫害降低了两组树木的生长速率，其影响强度至少是气候因素的两倍。
归一化植被指数能够反映样地水平的树木生长、林分生物量生长与死亡情况，但其与存活树木生长的关联强度近乎是濒死树木的两倍，且与生物量生长的关联强度是死亡率的12倍。
上述差异表明，归一化植被指数可能无法充分检测北方生物群区内由昆虫驱动的颤杨衰退与分散性死亡事件。
本研究结果揭示，一场历史性干旱触发了长达数十年的生长衰退，使颤杨在后续的潜叶蛾暴发中更易发生死亡；同时，尽管颤杨生长受水分与温度影响，但*P. populiella*的作用更为显著。
我们据此得出结论：随着气候变暖，高纬度地区的昆虫暴发频率与强度持续提升，颤杨的生长将持续大幅下降，死亡率也将进一步升高。