The circumpolar forest-tundra ecotone is experiencing rapid changes in vegetation composition and structure. Collectively, these changes modify surface-atmosphere energy exchanges and thus characteristics of the atmospheric boundary layer (ABL). Here, we characterize differences in bulk surface properties and resulting energy balance components using multi-year eddy covariance and supporting measurements made at a mineral upland tundra and a nearby subarctic woodland site between 2013 and 2022. The two sites are characteristic of the forest-tundra ecotone of northwestern Canada. A mixed-layer slab model, in combination with radiosonde observations, was used to gain more insights into differences in ABL characteristics. Compared to the tundra, the tree cover of the woodland led to an enhanced ability to transfer heat into the atmosphere, a higher resistance to evapotranspiration and a stronger coupling between surface and atmosphere. Sensible heat flux (H) was generally higher at the woodland than at the tundra. The largest difference in daily mean H was observed in late winter and spring when the albedo of the snow-covered landscape at the woodland was reduced by 45% compared to the tundra. Both sites experienced similar latent heat flux throughout the year. At the woodland, modelled afternoon air temperature in the mixed layer was up to 9 °C higher in spring and up to 3 °C higher in summer compared to the tundra. In accordance with model results, radiosonde observations indicated a deeper ABL and higher air temperature at the woodland. The presence of trees in the southern part of the forest-tundra ecotone increases air temperature throughout the year and has a drying effect in spring. Consequently, vegetation shifts in the forest-tundra ecotone can be expected to modify local surface climate change patterns, which must be considered when assessing climate change impacts in the region.

中文翻译：

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加拿大西北部森林-苔原交错带地表-大气能量交换及其对地表气候和大气边界层特征的影响

环极地森林-苔原交错带的植被组成和结构正在经历快速变化。总的来说，这些变化改变了表面-大气能量交换，从而改变了大气边界层（ABL）的特征。在这里，我们使用多年涡度协方差和 2013 年至 2022 年间在矿物高地苔原和附近亚北极林地地点进行的支持测量来描述总体表面特性的差异以及由此产生的能量平衡成分。这两个地点具有森林苔原的特征加拿大西北部生态交错带。混合层板模型与无线电探空仪观测相结合，用于更深入地了解 ABL 特性的差异。与苔原相比，林地的树木覆盖导致向大气传递热量的能力增强，对蒸散的抵抗力更高，地表与大气之间的耦合更强。林地的感热通量 (H) 通常高于苔原。日平均 H 值的最大差异出现在冬末和春季，此时林地积雪景观的反照率比苔原降低了 45%。两个地点全年都经历了相似的潜热通量。在林地中，与苔原相比，模拟的混合层午后气温在春季高出 9°C，在夏季高出 3°C。根据模型结果，无线电探空仪观测表明林地的 ABL 更深，气温更高。森林苔原交错带南部的树木使全年气温升高，并在春季产生干燥效应。因此，森林-苔原交错带的植被变化预计会改变当地地表气候变化模式，在评估该地区气候变化影响时必须考虑这一点。