Leaf economics explained by optimality principles

The worldwide leaf economics spectrum relates leaf lifespan (LL) to leaf dry mass per unit area (LMA)1. By combining three well-supported principles2-4, we show that an isometric relationship between these two quantities maximizes the leafs net carbon gain. This theory predicts a spectrum of equally competent LMA-LL combinations in any given environment, and how their optimal ratio varies across environments. By analysing two large, independent leaf-trait datasets for woody species1,5, we provide quantitative empirical support for the predicted dependencies of LL on LMA and environment in evergreen plants, and for the distinct predicted dependencies of LMA on light, temperature, growing-season length and aridity in evergreen and deciduous plants. We thereby resolve the long-standing question of why deciduous LMA tends to increase (with increasing LL) towards the equator, while evergreen LMA and LL decrease6. We also show how the statistical distribution of LMA within communities can be modelled as an outcome of environmental selection on the global pool of species with diverse values of LMA and LL.