Optimal nitrogen allocation controls tree responses to elevated CO 2

Abstract

• Despite the abundance of experimental data, understanding of forest responses to elevated CO2 is limited. Here I show that a key to previously unexplained production and leaf area responses lies in the interplay between whole-plant nitrogen (N) allocation and leaf photosynthesis. • A simple tree growth model, controlled by net growth maximization through optimization of leaf area index (LAI) and plant N, is used to analyse CO 2 responses in both young, expanding and closed, steady-state canopies. The responses are sensitive to only two independent parameters, the photosynthetic capacity per leaf N (a) and the fine-root N:leaf N ratio. • The model explains observed CO2 responses of photosynthesis, production and LAI in four forest free air CO2 enrichment (FACE) experiments. Insensitivity of LAI except at low LAI, increase in light-use efficiency, and photosynthetic down-regulation (as a result of reduced leaf N per area) at elevated CO2 are all explained through the combined effects on a and leaf quantum efficiency. • The model bridges the gap between the understanding of leaf-level and plant-level responses and provides a transparent framework for interpreting and linking structural (LAI) and functional (net primary production (NPP) : gross primary production (GPP) ratio, light-use efficiency, photosynthetic down-regulation) responses to elevated CO2. © The Authors (2007).