Global leaf trait estimates biased due to plasticity in the shade

The study of leaf functional trait relationships, the so-called leaf economics spectrum1,2, is based on the assumption of high-light conditions (as experienced by sunlit leaves). Owing to the exponential decrease of light availability through canopies, however, the vast majority of the world's vegetation exists in at least partial shade. Plant functional traits vary in direct dependence of light availability3, with different traits varying to different degrees, sometimes in conflict with expectations from the economic spectrum3. This means that the derived trait relationships of the global leaf economic spectrum are probably dependent on the extent to which observed data in existing large-scale plant databases represent high-light conditions. Here, using an extensive worldwide database of within-canopy gradients of key physiological, structural and chemical traits3, along with three different global trait databases4,5, we show that: (1) accounting for light-driven trait plasticity can reveal novel trait relationships, particularly for highly plastic traits (for example, the relationship between net assimilation rate per area (Aa) and leaf mass per area (LMA)); and (2) a large proportion of leaf traits in current global plant databases reported as measured in full sun were probably measured in the shade. The results show that even though the majority of leaves exist in the shade, along with a large proportion of observations, our current understanding is too focused on conditions in the sun

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