Evaluation of leaf‐to‐canopy upscaling methodologies against carbon flux data in North America

Despite the wide acceptance of the “big‐leaf” upscaling strategy in evapotranspiration modeling (e.g., the Penman‐Monteith model), its usefulness in simulating canopy photosynthesis may be limited by the underlying assumption of homogeneous response of carbon assimilation light‐response kinetics through the canopy. While previous studies have shown that the separation of the canopy into sunlit and shaded parts (i.e., two‐leaf model) is typically more effective than big‐leaf models for upscaling photosynthesis from leaf to canopy, a systematic comparison between these two upscaling strategies among multiple ecosystems has not been presented. In this study, gross primary productivity was modeled using two‐leaf and big‐leaf upscaling approaches in the Boreal Ecosystem Productivity Simulator for shrublands, broadleaf, and conifer forest types. When given the same leaf‐level photosynthetic parameters, the big‐leaf approach significantly underestimated canopy‐level GPP while the two‐leaf approach more closely predicted both the magnitude and day‐to‐day variability in eddy covariance measurements. The underestimation by the big‐leaf approach is mostly caused by its exclusion of the photosynthetic contributions of shaded leaves. Tests of the model sensitivity to a foliage clumping index revealed that the contribution of shaded leaves to the total simulated productivity can be as high as 70% for highly clumped stands and seldom decreases below ∼40% for less‐clumped canopies. Our results indicate that accurate upscaling of photosynthesis across a broad array of ecosystems requires an accurate description of canopy structure in ecosystem models. Key Points The big‐leaf approach significantly underestimated the canopy‐level GPP The underestimation is caused by the exclusion of the contributions of shaded leaves Shaded leaves contribute 40%‐70% to the total simulated productivity