The intensity of extreme heat and drought events has drastically risen in recent decades and will likely continue throughout the century. Northern forests have already seen increases in tree mortality and a lack of new recruitment, which is partially attributed to these extreme events. Boreal species, such as paper birch (Betula papyrifera) and white spruce (Picea glauca), appear to be more sensitive to these changes than lower-latitude species. Our objectives were to investigate the effects of repeated heatwaves and drought on young paper birch and white spruce trees by examining 1) responses in leaf gas exchange and plant growth, and 2) thermal acclimation of photosynthetic and respiratory traits to compare ecophysiological responses of two co-occurring, yet functionally dissimilar species. To address these objectives, we subjected greenhouse-grown seedlings to two consecutive summers of three 8-day long, +10°C heatwaves in elevated atmospheric CO₂ conditions with and without water restriction. The data show that heatwave stress reduced net photosynthesis, stomatal conductance, and growth—more severely so when combined with drought. Acclimation of both photosynthesis and respiration did not occur in either species. The combination of heat and drought stress had a similar total effect on both species, but each species adjusted traits differently to the combined stress. Birch experienced greater declines in gas exchange across both years and showed moderate respiratory, but not photosynthetic acclimation to heatwaves. In spruce, heatwave stress reduced the increase in basal area in both experimental years and had a minor effect on photosynthetic acclimation. The data suggest these species lack the ability to physiologically adjust to extreme heat events, which may limit their future distributions, thereby altering the composition of boreal forests.

中文翻译：

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有限的生理适应性对两个北方树种的反复热浪。

近几十年来，极端高温和干旱事件的强度急剧上升，并且可能会持续整个世纪。北部森林已经看到树木死亡率增加和缺乏新的征聘，这部分归因于这些极端事件。北方物种，​​例如桦木（Betula papyrifera）和白云杉（Picea glauca），似乎比低纬度物种对这些变化更敏感。我们的目标是通过研究以下方面来研究反复的热浪和干旱对桦木和白云杉树的影响：1）叶片气体交换和植物生长的响应，以及2）光合和呼吸性状的热适应，以比较两种玉米的生态生理响应。 -发生但功能上不同的物种。为了实现这些目标，我们将温室生长的幼苗在大气CO ₂升高且无水限制的条件下，连续两个夏天进行了三个连续8天，+ 10°C的热浪。数据显示，热浪胁迫会降低净光合作用，气孔导度和生长，而与干旱相结合则更为严重。两者的适应两种物种均未发生光合作用和呼吸作用。高温和干旱胁迫的组合对这两个物种的总影响相似，但每种物种对性状的调节均不同于组合胁迫。在过去的两年中，桦树的气体交换量下降幅度更大，并表现出中等呼吸作用，但对热波没有光合适应。在云杉中，热浪胁迫在两个实验年中均减少了基面积的增加，并且对光合作用的适应性影响较小。数据表明这些物种缺乏对极端高温事件进行生理适应的能力，这可能会限制其未来的分布，从而改变北方森林的组成。