Potential land–climate feedbacks in subarctic regions, where rapid warming is driving forest expansion into the tundra, may be mediated by differences in transpiration of different plant functional types. Here, we assess the environmental controls of overstorey transpiration and its relevance for ecosystem evapotranspiration in subarctic deciduous woodlands. We measured overstorey transpiration of mountain birch canopies and ecosystem evapotranspiration in two locations in northern Fennoscandia, having dense (Abisko) and sparse (Kevo) overstories. For Kevo, we also upscale chamber‐measured understorey evapotranspiration from shrubs and lichen using a detailed land cover map. Subdaily evaporative fluxes were not affected by soil moisture and showed similar controls by vapour pressure deficit and radiation across sites. At the daily timescale, increases in evaporative demand led to proportionally higher contributions of overstorey transpiration to ecosystem evapotranspiration. For the entire growing season, the overstorey transpired 33% of ecosystem evapotranspiration in Abisko and only 16% in Kevo. At this latter site, the understorey had a higher leaf area index and contributed more to ecosystem evapotranspiration compared with the overstorey birch canopy. In Abisko, growing season evapotranspiration was 27% higher than precipitation, consistent with a gradual soil moisture depletion over the summer. Our results show that overstorey canopy transpiration in subarctic deciduous woodlands is not the dominant evaporative flux. However, given the observed environmental sensitivity of evapotranspiration components, the role of deciduous trees in driving ecosystem evapotranspiration may increase with the predicted increases in tree cover and evaporative demand across subarctic regions.

中文翻译：

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北极北极落叶林地的蒸腾作用：环境控制和对生态系统蒸散的贡献

在北极地区，由于快速变暖推动森林向苔原扩张的潜在土地-气候反馈可能是由不同植物功能类型的蒸腾作用差异引起的。在这里，我们评估了南极落叶林地中超高蒸腾量的环境控制及其与生态系统蒸散的相关性。我们在芬诺斯坎迪亚北部的两个地方测量了山桦冠层的蒸腾作用和生态系统的蒸散量，这些地方有浓密的（Abisko）和稀疏的（Kevo）故事。对于Kevo，我们还使用详细的土地覆盖图，对由灌木和地衣引起的地下室蒸散量进行了高级测量。次日蒸发通量不受土壤水分的影响，并且显示出通过蒸气压不足和跨站点辐射的类似控制。在每天的时间范围内，蒸发需求的增加导致过高蒸腾作用对生态系统蒸散量的贡献更大。在整个生长季节中，高层建筑在阿比斯库蒸发了33％的生态系统蒸散量，而在科沃仅蒸发了16％。在下一个站点，与下层白桦林冠层相比，下层林叶面积指数更高，对生态系统蒸散量的贡献更大。在阿比斯库，生长季的蒸散量比降水量高27％，这与夏季土壤水分的逐渐消耗一致。我们的研究结果表明，北极亚落叶林中的冠层蒸腾作用不是主要的蒸发通量。但是，考虑到蒸散成分对环境的敏感性，