Seeing the Canopy for the Branches: Improved Within Canopy Scaling of Leaf Nitrogen. (26th October 2020) - Butler, Ethan E.
Abstract: 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 submodels. Here, we focus on a submodel that prescribes how light and nitrogen are distributed through plant canopies. We found a mathematical inconsistency in a submodel implemented in the Community and Energy Land Models (CLM and ELM), which 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, that is, would occur in any model adopting this particular submodel. We resolve the inconsistency by allowing distinct profiles of stems and branches versus living leaves. We implemented the updated scheme in the 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 g C m −2 yr −1 . Appropriately incorporating stems and other nonphotosynthesizing material into the light and nitrogen scaling routines of global land models, will improve their biological realism and accuracy. Plain Language Summary: Land surface models that estimate the flow of chemical elements, water, and energy into and out of terrestrial vegetation rely on many submodels. We found a mathematical inconsistency in a widely used submodel that controls how light and nitrogen are distributed through the plant canopy. The inconsistency is centered around how twigs, branches, stems, and dead leaves influence the distribution of nitrogen, and thus photosynthesis, throughout the canopy. When we corrected the inconsistency, we found that the modeled global carbon uptake of terrestrial vegetation decreased by 3%. We further show that completely ignoring twigs, branches, stems, and dead leaves in canopy upscaling inappropriately increases estimated global gross primary production, by failing to account for the shade they cast, with the greatest increases in the tropics. Key Points: We identified a mathematical inconsistency in a submodel scaling nitrogen from leaf to canopy as used in some global land surface models Correcting the inconsistency appreciably reduces predicted global gross primary production (GPP) We also show that ignoring twigs, branches, stems, and dead leaves in canopy shading schemes introduces errors in global GPP estimates