Water and nitrogen are two limiting resources for biomass production of terrestrial vegetation. Palmroth et al. employ optimization theory to quantify interactions between marginal water and nitrogen resource use efficiencies of Pinus taeda needles grown under elevated atmospheric CO2 concentration and soil nitrogen amendments in a free-air CO2 enrichment (FACE) experiment. They show that when foliar nitrogen content varies on time scales much longer than stomatal conductance, marginal nitrogen use efficiency is complementary to the square-root of marginal water use efficiency. The novel analytical expression can be employed not only to scale-up carbon and water fluxes from leaf to ecosystem, but also to derive transpiration estimates from those of marginal nitrogen use efficiency, and hence can assist in predicting how increasing atmospheric [CO2] influences ecosystem water use.