Optimal Biomass Allocation to Leaves Across Global Terrestrial Ecosystems

ABSTRACT Leaves influence global climate and water resources by regulating the flows of CO 2 , energy and water between the atmosphere and land. Here we developed a theoretical framework that predicts the optimal annual fraction of biomass production allocated to the production of new leaves ( η f ) as the product of the optimal leaf area index (LAI) and the optimal ratio of leaf mass–per–area to the leaf life cycle (LMA/LC), divided by annual total biomass production (BP). All quantities depend on local environmental variables, with different solutions (embedding a climatic aridity effect on optimal LMA/LC) for evergreen and deciduous plants. Using a global compilation of η f data, we show that this theory predicts the emergent responses of η f to growing–season temperature and aridity. These responses arise from the trade‐offs among the environmental sensitivities of LMA/LC, LAI and BP. It is predicted that the rising CO 2 is steadily reducing the fraction of biomass production allocated to leaves.