The response of understory trees to climate variability is key to understanding current and future forest dynamics. However, analyses of climatic effects on tree growth have primarily focused on the upper canopy, leaving understory dynamics unresolved. We analyzed differences in climate sensitivity based on canopy position of four common tree species (Acer rubrum, Fagus grandifolia, Quercus rubra, and Tsuga canadensis) using growth information from 1084 trees across eight sites in the northeastern United States. Effects of canopy position on climate response varied, but were significant and often non-linear, for all four species. Compared to overstory trees, understory trees showed stronger reductions in growth at high temperatures and varied shifts in precipitation response. This contradicts the prevailing assumption that climate responses, particularly to temperature, of understory trees are buffered by the overstory. Forest growth trajectories are uncertain in compositionally and structurally complex forests, and future demography and regeneration dynamics may be misinferred if not all canopy levels are represented in future forecasts.

中文翻译：

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林下林木的气候敏感性与温带中生林中林下林木的气候敏感性不同

林下树木对气候变化的反应是了解当前和未来森林动态的关键。然而，气候对树木生长的影响分析主要集中在上层树冠上，而未解决林下动态。我们使用来自美国东北部八个地点的 1084 棵树的生长信息，分析了基于四种常见树种（红紫杉、山毛榉、栎属和加拿大杉）的冠层位置的气候敏感性差异。冠层位置对气候响应的影响各不相同，但对所有四个物种来说都是显着的，而且通常是非线性的。与林下乔木相比，林下乔木在高温下表现出更强的生长减少和降水响应的不同变化。这与普遍的假设相矛盾，即气候响应，特别是温度，林下树木被林上层缓冲。在组成和结构复杂的森林中，森林生长轨迹是不确定的，如果未来的预测中没有代表所有的冠层水平，那么未来的人口统计学和更新动态可能会被错误地推断出来。