Transitioning across biological scales is a central challenge in land surface models. Processes that operate at the scale of individual leaves must be scaled to canopies, and this is done using dedicated sub‐models. Here, we focus on a sub‐model that prescribes how light and nitrogen are distributed through plant canopies. We found a mathematical inconsistency in a sub‐model implemented in the Community and Energy Land Models (CLM and ELM), that incorporates twigs, branches, stems, and dead leaves in nitrogen scaling from leaf to canopy. The inconsistency leads to unrealistic (physically impossible) values of the nitrogen scaling coefficient. The mathematical inconsistency is a general mistake, i.e. would occur in any model adopting this particular sub‐model. We resolve the inconsistency by allowing distinct profiles of stems and branches versus living leaves. We implemented the updated scheme in the Energy Land Model (ELM) and find that the correction reduces global mean gross primary production (GPP) by 3.9 Pg C (3%). Further, when stems and branches are removed from the canopy in the updated model (akin to models that ignore shading from stems), global GPP increases by 4.1 Pg C (3.2%), because of reduced shading. Hence, models that entirely ignore stem shading also introduce errors in the global spatial distribution of GPP estimates, with a strong signal in the tropics, increasing GPP there by over 200 gC m^‐2 yr^‐1. Appropriately incorporating stems and other non‐photosynthesizing material into the light and nitrogen scaling routines of global land models, will improve their biological realism and accuracy.

中文翻译：

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查看树枝的冠层：在冠层氮素叶尺度上得到改善

跨生物尺度的过渡是陆地表面模型的主要挑战。以单个叶子为尺度运行的过程必须按比例缩放到树冠，这是使用专用子模型完成的。在这里，我们关注一个子模型，该子模型规定了植物冠层中光和氮的分布方式。我们在“社区和能源土地模型”（CLM和ELM）中实现的子模型中发现了数学上的不一致，该子模型将枝条，树枝，茎和枯叶合并成从叶到冠层的氮比例。不一致导致氮缩放系数的值不切实际（在物理上是不可能的）。数学上的不一致是一个普遍的错误，即在采用该特定子模型的任何模型中都可能发生。我们通过允许茎和枝与活叶的不同轮廓来解决不一致问题。我们在能源土地模型（ELM）中实施了更新后的方案，发现该修正使全球平均初级生产总值（GPP）降低了3.9 Pg C（3％）。此外，当在更新的模型中从树冠上移除茎和分支时（类似于忽略茎的阴影的模型），由于减少了阴影，全局GPP增加4.1 Pg C（3.2％）。因此，完全忽略茎阴影的模型也会在GPP估计值的全球空间分布中引入误差，在热带地区会有较强的信号，从而使GPP增加200 gC m 当在更新的模型中将茎和枝从树冠上移除时（类似于忽略茎的阴影的模型），由于减少了阴影，全局GPP增加4.1 Pg C（3.2％）。因此，完全忽略茎阴影的模型也会在GPP估计值的全球空间分布中引入误差，在热带地区会有较强的信号，从而使GPP增加200 gC m 当在更新的模型中将茎和枝从树冠上移除时（类似于忽略茎的阴影的模型），由于减少了阴影，全局GPP增加4.1 Pg C（3.2％）。因此，完全忽略茎阴影的模型还会在GPP估计值的全球空间分布中引入误差，在热带地区会有很强的信号，那里的GPP会增加200 gC m^‐2年^‐1。将茎和其他非光合作用材料适当地纳入全球土地模型的光和氮比例缩放程序，将改善其生物现实性和准确性。