How much carbon is allocated to leaves?

Carbon allocation is a critical process that helps to optimize plant growth and significantly impacts ecosystem structure and function, with immediate implications for the global carbon cycle. Since leaves are the primary organs regulating the exchange of CO₂, energy, and water between terrestrial ecosystems and the atmosphere, accurate simulation of leaf carbon allocation is important. However, most land surface models (LSMs) lack detailed consideration of the leaf economics spectrum, which is represented by the coordination of leaf mass per area (LMA) and leaf longevity (LL) and how their relationship varies with the growth environment. Instead, LSMs commonly predict the proportion of biomass allocated to leaves either directly from the environment based on resource limitation theories, or from their functional relationships with other organs. A new theory based on eco-evolutionary optimality principles successfully predicts changes in LMA and LL with the environment by maximizing the average net carbon gain over the leaf life cycle. In addition, a prognostic, globally applicable Leaf Area Index (LAI) model has been developed recently, using climate data alone to capture LAI dynamics across biomes on the principle that the annual cycle of leaf display is closely related to the cycle of potential primary production by those leaves. Here, combining these two theoretical developments, we provide a universal expression for the proportion of biomass allocated to leaves. We successfully predict foliar carbon allocation as measured on a site basis and capture the plasticity of foliar carbon allocation with environmental change. The global average fraction of biomass production allocated to leaves is estimated as 0.31. The model also accounts for the different regulatory mechanisms of leaf carbon allocation by deciduous and evergreen plants.