An eco-physiological model of forest photosynthesis and transpiration under combined nitrogen and water limitation

Abstract

SummaryForest growth is strongly affected by climate and soil conditions and reliable modelling these responses are needed to project forest growth for future climate scenarios. Although the separate effects of water and nitrogen limitations are well known, understanding how trees manage their combined requirements remains a challenge. Here we address this challenge based on a new eco-physiological model that accounts for plasticity in stomatal conductance and leaf nitrogen concentration.Based on an optimality principle, our model determines stomatal conductance and leaf nitrogen concentration by balancing C uptake maximization, hydraulic risk and cost of maintaining photosynthetic capacity.We demonstrate the accuracy of the model predictions compared to weekly GPP estimates from eddy flux measurements and canopy transpiration in long-term fertilized and unfertilizedPinus sylvestrisforest in northern Sweden. The model also explains the response to increased soil N availability as a consequence of reduced carbon cost of N uptake and reduced hydraulic conductance per leaf area (linked to increased leaf area per sapwood area).The results suggest that leaves optimally coordinate N concentration and stomatal conductance both on short (weekly) time scales in response to weather conditions and on longer time scales in response to water and N availability in the soil.