Optimal nitrogen allocation controls tree responses to elevated CO2
2007
Franklin, Oskar
• 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 CO₂ 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.
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