A leaf-carbon economy model to explain the adaptive value of low vs. high leaf mass per area as a function of habitat productivity

Plant traits interact with environmental conditions to determine plant fitness. In particular, plant species with low leaf mass per area (LMA) are more common in high fertility soils and forest gaps, while plant species with high LMA are more common in low fertility soils and in shaded forest understories. It is not yet known why the adaptive value of LMA changes as a function of environmental productivity. We present a model for plant growth as a function of LMA and productivity. Our model relies on an empirically derived relationship between LMA and leaf lifespan. We are able to explain why species with lower LMA are successful in high productivity environments including environments with high availability of either light or soil nutrients, by showing that the LMA which maximizes growth rate decreases with increasing rate of carbon assimilation. We show that low LMA species are better at taking advantage of the 'high interest' environment of high productivity habitats by reinvesting assimilated carbon in new leaves and earning compound interest, while high LMA species grow more efficiently in the 'low interest' environment of low productivity habitats. This tradeoff sheds light on the adaptive value of LMA as it relates to environmental productivity.