The leaf economics spectrum and its underlying physiological and anatomical principles

Large variations are found in leaf morphology and physiology across species in nature, reflecting diversity in carbon fixation and growth strategies. These variations in leaf traits are not random; rather, they are tightly coordinated with each other. Leaf traits can be expressed per leaf dry mass or per leaf area. A leaf-mass basis reflects leaf economics, i.e., revenues and expenditures per unit investment of biomass, while a leaf-area basis reflects fluxes in relation to surfaces. Leaf N and P concentrations, and photosynthetic and respiration rates" all considered on a mass basis, are negatively correlated with leaf mass per area (LMA) whilst leaf lifespan is positively correlated with LMA. These correlations are summarized into a single major axis called the "leaf economics spectrum" that runs from "quick-return" to "slow-return" species. On the other hand, correlations among area-based traits are less consistent and less understood in relation to leaf economy. LMA was positively correlated with leaf N content but mostly independent from photosynthetic rates per unit leaf area. Given that N is an essential element in photosynthetic proteins and thus photosynthesis, clarifying the mechanisms why the efficiency of photosynthesis (photosynthesis per unit N) decreases with LMA is a major concern in understanding the correlations among area-based traits in relation to leaf economy. Currently available data suggest that greater amounts of cell wall are required for long-lived leaves, which reduces the efficiency of photosynthesis by lowering (1) the fraction of leaf N invested in photosynthetic proteins and (2) CO2 diffusion rates through thicker and denser mesophyll cell walls. These physiological and structural constraints are a fundamental mechanism underpinning the general correlations among leaf economic traits.