Vegetation carbon stock (C_veg) in global forests, which is important for C cycle‐climate feedbacks, commonly increases with forest age. Due to the allometric growth of plants, the nonlinear increase in C_veg with woody fraction (f_w) is expected across space. However, it remains unclear whether such a nonlinear relationship between C_veg and f_w can be constrained by observations and further used to benchmark Earth system models (ESMs). Here, based on the in situ measurements at 1,145 forest sites, we found that the nonlinear relationship between C_veg and f_w followed an exponential equation (i.e., ). Then, we showed that such an exponential dependence of C_veg on f_w also exists in ESMs of CMIP5 and CMIP6 (all P < 0.01), even though age‐dependent processes have not been incorporated in most models. However, the exponential C_veg‐f_w relationship varied greatly among the models, and the coefficient b was systematically lower in the ESMs (0.08 ± 0.11; mean ± SD) than the observations (0.28). Based on a compiled forest age data set, we further found that the observed nonlinear increase of C_veg with forest age across the Northern Hemisphere (>30°N) was not captured by ESMs. These findings reveal a high disagreement on the spatially nonlinear relationship between vegetation carbon stock and woody fraction in current ESMs. The exponential relationship based on observations provides one useful benchmark for ESMs when they implement the age‐dependent processes in the future.

中文翻译：

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老化森林中植被碳储量的非线性增加及其对地球系统模型的影响

全球森林中的植被碳储量（C _veg）对于碳循环气候反馈很重要，通常随森林年龄而增加。由于植物的异速生长，预计整个空间中C _veg随木本分数（f _w）的非线性增加。但是，尚不清楚C _veg和f _w之间的这种非线性关系是否可以通过观测来约束并进一步用于基准地球系统模型（ESM）。在此基础上，中-原位1145个森林地点的测量，我们发现之间的非线性关系Ç_蔬菜和f _w遵循指数方程（即）。然后，我们表明，即使大多数模型中都没有依赖于年龄的过程，CMeg5和CMIP6的ESM中也存在C _veg对f _w的指数依赖关系（所有P <0.01）。但是，模型之间的指数C _veg – f _w关系变化很大，ESM中的系数b系统地低于观测值（0.28）（0.08±0.11;平均值±SD）。基于已编译的森林年龄数据集，我们进一步发现观察到的C _veg非线性增加ESM未捕获到整个北半球的森林年龄（> 30°N）。这些发现揭示了在当前ESM中植被碳储量与木质成分之间的空间非线性关系存在高度分歧。基于观察的指数关系为ESM在将来实施与年龄有关的过程时提供了一种有用的基准。