Simple scaling of photosynthesis from leaves to canopies without the errors of big‐leaf models

TLDR

Big‑leaf canopy photosynthesis models sum leaf‑area activities, leading to overestimation of photosynthesis and requiring empirical curvature factors, whereas this study uses a scaled leaf model distinct from integrative approaches. The study aims to develop a simpler single‑layer sun/shade model that avoids the errors inherent in big‑leaf models. The authors construct a single‑layer sun/shade model by separately integrating sunlit and shaded leaf fractions, an approach that matches the accuracy of multi‑layer models while being simpler. The study finds that curvature factors vary with canopy leaf area index and leaf nitrogen, driven by differences between time‑averaged and instantaneous irradiance from sunflecks and leaf angles, and that the single‑layer sun/shade model achieves the same accuracy as multi‑layer models.

Abstract

ABSTRACT In big‐leaf models of canopy photosynthesis, the Rubisco activity per unit ground area is taken as the sum of activities per unit leaf area within the canopy, and electron transport capacity is similarly summed. Such models overestimate rates of photosynthesis and require empirical curvature factors in the response to irradiance. We show that, with any distribution of leaf nitrogen within the canopy (including optimal), the required curvature factors are not constant but vary with canopy leaf area index and leaf nitrogen content. We further show that the underlying reason is the difference between the time‐averaged and instantaneous distributions of absorbed irradiance, caused by penetration of sunflecks and the range of leaf angles in canopies. These errors are avoided in models that treat the canopy in terms of a number of layers – the multi‐layer models. We present an alternative to the multi‐layer model: by separately integrating the sunlit and shaded leaf fractions of the canopy, a single layered sun/shade model is obtained, which is as accurate and simpler. The model is a scaled version of a leaf model as distinct from an integrative approach.