Vertical gradients in the canopy represent a major challenge for scaling from foliar photosynthesis to ecosystem-level CO₂ fluxes. We tested whether accounting for independent gradients of carboxylation capacity (V_cmax) and photosynthetic electron transport (J_max) improves estimates of forest net ecosystem CO₂ exchange (NEE).

We modified the process-based Soil-Plant-Atmosphere (SPA) model to represent different gradients of foliar N allocation to V_cmax and J_max in the canopy, and inversely calibrated the model via Bayesian inference using eddy covariance measurements of NEE and evapotranspiration from a mixed-deciduous forest.

Inversely calibrated N allocation resulted in highest V_cmax at the top and highest J_max at the bottom of the canopy, which is similar to N allocation gradients from measured foliar traits. These vertical gradients resulted in the best fit of simulated CO₂ fluxes to measured NEE compared to alternative N allocation schemes, due to a higher, more realistic foliar CO₂ uptake in the lower canopy.

Canopy gradients of V_cmax and J_max are thus important drivers of NEE and need to be considered to improve simulations of ecosystem-level CO₂ fluxes of forests.

树冠中的垂直梯度代表了从叶面光合作用到生态系统水平 CO ₂通量的缩放的主要挑战。我们测试了考虑羧化能力 (V _{c max} ) 和光合电子传递 (J _max ) 的独立梯度是否能改善森林净生态系统 CO ₂交换 (NEE) 的估计值。

我们修改了基于过程的土壤-植物-大气 (SPA) 模型，以表示冠层中叶面 N 分配到 V _{c max}和 J _{max 的}不同梯度，并使用 NEE 和蒸散量的涡度协方差测量值通过贝叶斯推理对模型进行逆校准来自混合落叶林。

反向校准的 N 分配导致顶部的V _{c max}最高，而冠层底部的J _max最高，这与测量的叶性状的 N 分配梯度相似。与替代的氮分配方案相比，这些垂直梯度导致模拟的 CO ₂通量与测量的 NEE的最佳拟合，这是由于较低冠层中更高、更现实的叶面 CO ₂吸收。

因此，V _{c max}和 J _{max 的}冠层梯度是NEE 的重要驱动因素，需要考虑以改进森林生态系统级 CO ₂通量的模拟。