Water-use efficiency (WUE) represents the coupling of forest carbon and water. Little is known about the responses of WUE to thinning at multiple spatial scales. The objective of this research was to use field measurements to understand short-term effects of two thinning treatments (T1: 4500 stems ha−1; and T2: 1100 stems ha−1) and the control (NT: 27,000 stems ha−1) on WUE at the three spatial scales (leaf level: the ratio of leaf photosynthesis to leaf transpiration; tree-level: tree growth to tree transpiration; and stand level: net primary production (NPP) to stand transpiration) and intrinsic WUEi (the ratio of leaf photosynthesis to stomatal conductance at leaf-level; and NPP to canopy conductance at stand-level) in a 16-year old natural lodgepole pine forest. Leaf-level measurements were conducted in 2017, while tree- and stand-level measurements were conducted in both 2016 (the normal precipitation year) and 2017 (the drought year). The thinning treatments did not significantly affect the tree- and stand-level WUE in the normal year of 2016. However, the thinning significantly affected WUE in the drought year of 2017: T2 exhibited significantly higher tree-level WUE (0.49 mm2 kg−1) than NT (0.08 mm2 kg−1), and compared to NT, the stand-level WUE values in the thinned stands (T1 and T2) were significantly higher, with means of 0.31, 0.56 and 0.70 kg m−3, respectively. However, the leaf-level and stand-level WUEi in the thinned stands in the drought year were significantly lower than those in the unthinned stands. No significant differences in the leaf-level WUE were found among the treatments in 2017. In addition, the thinning did not significantly change the WUE-VPD relationships at any studied spatial scale. The thinning treatments did not cause significant changes in all studied WUE metrics in a normal year. However, their effects were significantly promoted under the drought conditions probably due to the decrease in soil water availability, demonstrating that thinning can improve WUE and consequently support forests to cope with the drought effects. The inconsistent results on the effects of the thinning on forest carbon and water coupling at the spatial scales and the lack of the consistent WUE metrics constraint across-scale comparison and transferring of WUE.

中文翻译：

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山地松林中森林碳水耦合对叶片稀疏处理的响应

用水效率（WUE）代表森林碳与水的耦合。关于WUE在多个空间尺度上变薄的反应知之甚少。这项研究的目的是使用田间测量来了解两种间伐处理（T1：4500茎ha-1； T2：1100茎ha-1）和对照（NT：27,000茎ha-1）的短期效果。 WUE在三个空间尺度上（叶水平：叶片光合作用与叶片蒸腾的比率；树水平：树木生长与树木蒸腾的比率；林分等级：净初级生产力（NPP）与林分蒸腾的比率）和内在WUEi（比率在16岁的天然山毛榉松林中，叶片光合作用对叶片水平的气孔导度；以及NPP对站立水平的冠层电导率的影响。在2017年进行了叶级测量，在2016年（正常降水年）和2017年（干旱年）都进行了树木和林分水平的测量。在2016年的正常年份，间伐处理不会显着影响树木和林分的WUE。但是，在2017年的干旱年份，间伐处理显着影响了WUE：T2表现出明显更高的树状WUE（0.49 mm2 kg-1 ）比NT（0.08 mm2 kg-1）和与NT相比，稀疏林分（T1和T2）中的林分水平WUE值显着更高，分别为0.31，0.56和0.70 kg m-3。然而，干旱年份间伐林分的叶片水平和林分水分利用效率明显低于未间伐林分。在2017年的处理之间，未发现叶片水平WUE的显着差异。此外，在任何研究的空间尺度上，间伐并没有显着改变WUE-VPD关系。在正常年份中，细化处理不会在所有研究的WUE指标中引起显着变化。然而，在干旱条件下，其影响得到了显着促进，这可能是由于土壤水分的减少所致，这表明间伐可以改善WUE，从而支持森林应对干旱的影响。在空间尺度上，稀疏对森林碳水耦合的影响不一致，并且缺乏一致的WUE指标约束了WUE的跨尺度比较和转移。在干旱条件下，它们的作用得到了显着促进，这可能是由于土壤可用水量减少所致，表明间伐可以改善WUE，从而支持森林应对干旱的影响。在空间尺度上，稀疏对森林碳水耦合的影响不一致，并且缺乏一致的WUE指标约束了WUE的跨尺度比较和转移。在干旱条件下，它们的作用得到了显着促进，这可能是由于土壤可用水量减少所致，表明间伐可以改善WUE，从而支持森林应对干旱的影响。在空间尺度上，稀疏对森林碳水耦合的影响不一致，并且缺乏一致的WUE指标约束了WUE的跨尺度比较和转移。