Gross primary production (GPP) is a key component of the forest carbon cycle. However, our knowledge of GPP at the stand scale remains uncertain, because estimates derived from eddy covariance (EC) rely on semi‐empirical modelling and the assumptions of the EC technique are sometimes not fully met. We propose using the sap flux/isotope method as an alternative way to estimate canopy GPP, termed GPP_iso/SF, at the stand scale and at daily resolution. It is based on canopy conductance inferred from sap flux and intrinsic water‐use efficiency estimated from the stable carbon isotope composition of phloem contents. The GPP_iso/SF estimate was further corrected for seasonal variations in photosynthetic capacity and mesophyll conductance. We compared our estimate of GPP_iso/SF to the GPP derived from PRELES, a model parameterized with EC data. The comparisons were performed in a highly instrumented, boreal Scots pine forest in northern Sweden, including a nitrogen fertilized and a reference plot. The resulting annual and daily GPP_iso/SF estimates agreed well with PRELES, in the fertilized plot and the reference plot. We discuss the GPP_iso/SF method as an alternative which can be widely applied without terrain restrictions, where the assumptions of EC are not met.

中文翻译：

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通过将韧皮部稳定同位素与冠层和叶肉电导相结合，估算冠层的总初级生产力。

初级生产总值（GPP）是森林碳循环的关键组成部分。但是，我们对展位规模的GPP知识仍然不确定，因为从涡动协方差（EC）得出的估计值依赖于半经验模型，并且有时无法完全满足EC技术的假设。我们建议使用树液通量/同位素方法作为一种替代方法，以林分规模和日分辨率估算被称为GPP _{iso / SF的}冠层GPP 。它基于从树液通量推断的冠层电导和根据韧皮部含量的稳定碳同位素组成估算的内在用水效率。GPP _{iso / SF}估计值针对光合作用能力和叶肉电导的季节性变化进行了进一步校正。我们比较了我们对GPP _{iso / SF的}估计源自从PRELES（使用EC数据参数化的模型）的GPP。比较是在瑞典北部一个高度仪器化的北方苏格兰松林中进行的，包括施氮肥和参考地。在受精图和参考图中，得出的年度和每日GPP _{iso / SF}估计值与PRELES非常吻合。我们讨论了GPP _{iso / SF}方法作为一种替代方法，该方法可以在不满足EC假设的情况下不受地形限制地广泛应用。