Forest-floor evapotranspiration (ETff) is a major pathway for water loss in terrestrial ecosystems, often accounting for more than half of ecosystem evapotranspiration. However, our understanding of the environmental and stand structural controls on the spatio-temporal dynamics of ETff across the managed boreal forest landscape remains limited. In this study, we conducted chamber-based flux measurements of ETff and its components, i.e., soil evaporation (Es) and forest-floor understory transpiration (Tu), on natural and vegetation removal plots across 50 diverse forest stands (ranging 5–211 years old) in Northern Sweden over two contrasting growing seasons. We found manifold variations in the growing season means of ETff, Es, and Tu, ranging from 0.008 to 0.048 mm h−1, 0.004 to 0.034 mm h−1, and 0.002 to 0.030 mm h−1, respectively, across the 50 forest stands. The contribution of Es and Tu to ETff ranged from 19 to 83 % and 38 to 85 %, respectively, with the average Es:Tu ratio shifting from 0.84 in 2017 to 0.63 during 2018, the latter experiencing an exceptional summer drought. Seasonal variations in ETff and its component fluxes were mainly controlled by below-canopy air temperature, while radiation was the main driver of their spatial variations across the forest stands. At the landscape-level, stand age was the dominant control of ETff by modifying overstory tree characteristics such as biomass and leaf area index. In contrast, neither tree species nor soil type had any effect on ETff or Tu. However, Es was higher in sediment compared to till soils. Thus, our results suggest that environmental and stand structural factors jointly control the spatio-temporal dynamics of ETff across the managed boreal forest landscape. Our study furthermore highlights the need for an in-depth understanding of ETff and its components when assessing the water cycle feedbacks of the boreal forest to changes in forest management and climate.

中文翻译：

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北方森林管理景观中森林地面蒸散的时空动力学和控制


森林地面蒸散 （ETff） 是陆地生态系统中水分流失的主要途径，通常占生态系统蒸散的一半以上。然而，我们对整个管理北方森林景观中 ETff 时空动态的环境和林分结构控制的理解仍然有限。在这项研究中，我们在瑞典北部 50 个不同林分（年龄从 5-211 年不等）的自然和植被移除地块上对 ETff 及其组成部分，即土壤蒸发 （Es） 和森林地面林下蒸腾 （Tu） 进行了基于室的通量测量两个对比鲜明的生长季节。我们发现 ETff、Es 和 Tu 的生长季节平均值存在多种变化，范围分别为 0.008 至 0.048 mm h-1、0.004 至 0.034 mm h-1 和 0.002 至 0.030 mm h-1，在 50 个林分中。Es 和 Tu 对 ETff 的贡献分别为 19% 至 83% 和 38% 至 85%，平均 Es：Tu 比率从 2017 年的 0.84 变为 2018 年的 0.63，后者经历了异常的夏季干旱。ETff 及其组成通量的季节变化主要受冠层以下气温控制，而辐射是其在整个林分空间变化的主要驱动因素。在景观水平上，林龄通过改变生物量和叶面积指数等上层树木特征成为 ETff 的主要控制因素。相比之下，树种和土壤类型对 ETff 或 Tu 均无影响。然而，与耕作土壤相比，沉积物中的 Es 含量更高。因此，我们的结果表明，环境和林分结构因素共同控制着整个管理北方森林景观中 ETff 的时空动态。 我们的研究进一步强调了在评估北方森林对森林管理和气候变化的水循环反馈时，需要深入了解 ETff 及其组成部分。