Scaling Photosynthesis From Leaf to Canopy: A Synthesis of Optimization Theories, Vertical Structure, and Leaf Turnover Across Timescales

Abstract

This paper introduces the Global Multilayer Canopy OPTimization (GMC-OPT) model, designed to scale sub-daily leaf-level carbon fluxes to the canopy level. The model integrates three core components: canopy radiative transfer, optimization-based physiology, and energy balance. This study highlights the model's simulation of gross primary productivity (GPP), with a novel focus on vertically resolved radiation fields, photosynthetic capacity, and associated physiological processes. Specifically, the model accounts for: (a) sub-daily stomatal conductance optimization; (b) sub-daily timing of photosynthetic capacity optimization, and (c) vertical positioning of seasonal leaf turnover. After benchmarking against flux tower GPP data, GMC-OPT achieves high agreement with the tower GPP at annual, monthly, and hourly scales. Model calibration suggests a lower-than-expected efficiency in converting absorbed photosynthetically active radiation (APAR), primarily because not all APAR reaches chlorophyll. In addition, the model predicts a decrease in photosynthetic capacity from the top to the bottom of the canopy, with vertical acclimation becoming less responsive under high light intensities at the upper canopy. The model further reveals distinct plant functional type strategies on seasonal acclimation due to vertical leaf turnover. Tree-dominated biomes such as needleleaf and mixed forests tend to prioritize light harvesting, while non-tree biomes do not. Deciduous broadleaf forests maintain a relatively constant leaf and canopy photosynthetic capacity through leaf turnover, whereas biomes such as evergreen broadleaf forests and woody savannas often reallocate nutrients within existing leaves through acclimation. GMC-OPT promises a powerful diagnostic tool for exploring interactions among carbon, water, nutrients, and energy in a changing environment.