A general framework explaining variation in plant economics traits with environment and through ontogeny

Plant economics traits, such as leaf mass per unit leaf area (LMA) and stem specific density (SSD), capture diversity among plant species in how common tissues (leaf, wood, root) are constructed. These traits are key descriptors of plant strategy, yet it has proven difficult to explain this variation with theory and process-based models. Here we reveal a general explanation on why these economics traits vary with environment, through ontogeny, and with other plant traits. This explanation relies on three core assumptions: 1) plants seek to maximise growth rate, 2) growth rate can be decomposed into a product, and 3) there is a tradeoff between the efficiency of tissue construction and tissue turnover rate. Formulation of growth as a product is essential, as it causes the optimal value of an economics trait to vary with the plants biomass production rate, which means economics traits will naturally covary with the abiotic environment, the competitive context, and other strategical features of the plant. Finally, we show how a modification of the trait into plastic and non-plastic components alters the magnitude of intra-specific responses, aligning model responses with empirical trends. Broadly, our results help explain how plant form and function for a wide diversity of species is shaped to suit their environment and, moreover, they reveal insight into a general fast-slow spectrum (Reich 2014) with coordinated shifts among organs (leaf & stem) through tradeoffs between efficient tissue construction and turnover.