A paradox of leaf-trait convergence: Why is leaf nitrogen concentration higher in species with higher photosynthetic capacity?

It is well known that leaf photosynthesis per unit dry mass (A _mass) is positively correlated with nitrogen concentration (N _mass) across naturally growing plants. In this article we show that this relationship is paradoxical because, if other traits are identical among species, plants with a higher A_mass should have a lower N _mass, because of dilution by the assimilated carbon. To find a factor to overcome the dilution effect, we analyze the N_mass-A _mass relationship using simple mathematical models and literature data. We propose two equations derived from plant-growth models. Model prediction is compared with the data set of leaf trait spectrum obtained on a global scale. The model predicts that plants with a higher A_mass should have a higher specific nitrogen absorption rate in roots (SAR), less biomass allocation to leaves, and/or greater nitrogen allocation to leaves. From the literature survey, SAR is suggested as the most likely factor. If SAR is the sole factor maintaining the positive relationship between N_mass and A_mass, the variation in SAR is predicted to be much greater than that in A_mass; given that A_mass varies 130-fold, SAR may vary more than 2000-fold. We predict that there is coordination between leaf and root activities among species on a global scale.