Constraints on Global Photosynthetic Capacity (Vcmax) Within Canopies

Maximum rubisco carboxylation rate (V_cmax) varies with daily integrated quantum flux density (Q_int) along vertical transects within forest canopies. In this study, we investigated what drives the patterns of V_cmax plasticity observed within forest canopies, where gradient in light availability from canopy top to bottom can be >50-fold. We conducted correlation analyses of V_cmax plasticity against both environmental (light: PAR, temperature, vapor pressure deficit: VPD, elevation, latitude & longitude) and soil (soil water availability: a, silt content, clay content, pH, C:N & cation exchange capacity: CECS, water content: WC) parameters to explore their influence on V_cmax plasticity within forest canopies and different vegetation types (Deciduous broadleaved forests: Decid. BL, Evergreen temperate: Evergr. temp, Evergreen needle leaf: Evergr. NL, & Evergreen tropical: Evergr. trop). We calculated V_cmax plasticity between Q_int step changes: 1-3, 3-6, 6-12,12-20, 20-30, 30-40. For environmental and soil parameters, the most commonly correlated V_cmax plasticity step changes included: plasticity changes 30-40 and 6-12. Our results revealed that V_cmax plasticity varied within plant functional types (PFTs) and for environmental and soil parameters. Two out of seven environmental parameters -VPD and PAR-, and three out of six soil parameters -a, WC & C:N- showed strong correlations across PFTs for V_cmax plasticity step change 12-20. Evergr. NL forests showed strong correlation between V_cmax plasticity step change 12-20 and VPD (R² = 0.61); PAR was also strongly correlated with this plasticity step change (R² = 0.56); a was negatively correlated within Evergr. NL (R² = -0.77); WC was negatively correlated within Decid. BL (R² = -0.33); C:N was negatively correlated within Evergr. trop (R² = -0.31). PFTs also showed strong correlations between plasticity step change 30-40 for individual environmental and soil parameters. For example, within Decid. BL V_cmax plasticity step change 30-40, results showed elevation (R² = 0.92), VPD (R² = 0.79) and PAR (R² = 0.82) were the strongly correlated. Our study offers an analysis of the relative influence of environmental vs soil parameters on within canopy V_cmax plasticity that can help to better represent global photosynthetic capacity in Earth system models (ESMs).