Abstract
� Considerable experimental evidence indicates that stomatal conductance and leaf-nitrogen concentration ([N]) decline under CO2-enrichment, and that the percentage growth response of plants to elevated CO2 is amplified under water limitation but reduced under nitrogen limitation. In this paper we advance simple explanations for these responses based on an optimisation hypothesis.
� We explore this hypothesis using a simple model of the annual carbon - nitrogen - water economy of deciduous trees growing at a ten-year duration CO2-enrichment field experiment at Oak Ridge, Tennessee. The model is shown to have an optimum for leaf [N], stomatal conductance and leaf-area index (LAI), where annual plant productivity is maximised.
� The model is used to evaluate the optimum in years with contrasting rainfall and N fertility. If annual rainfall is increased, the optimum shifts to increased stomatal conductance and LAI and reduced leaf [N], whereas if N supply is increased, the optimum shifts to increased leaf [N] and LAI and reduced stomatal conductance.
� When atmospheric CO2 concentration ([CO2]) is increased, the optimum shifts to reduced stomatal conductance and leaf [N] and enhanced LAI. The model is used to predict maximum net primary productivity (NPP) at current and elevated [CO2] in years with contrasting rainfall and plant N uptake. The predicted CO2 response of maximum NPP is greatest in a dry, high-N year and least in a wet, low-N year.
� The underlying physiological explanation for this contrast in the effects of water versus nitrogen limitation is that leaf photosynthesis is more sensitive to [CO2] at lower stomatal conductance whereas it is less sensitive to [CO2] at lower leaf [N].
