Constraints on Global Photosynthetic Capacity (Vcmax) Within Canopies

Friday, December 13, 2019 - 13:40
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Maximum rubisco carboxylation rate (Vcmax) varies with daily integrated quantum flux density (Qint) along vertical transects within forest canopies. In this study, we investigated what drives the patterns of Vcmax plasticity observed within forest canopies, where gradient in light availability from canopy top to bottom can be >50-fold. We conducted correlation analyses of Vcmax 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 Vcmax 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 Vcmax plasticity between Qint step changes: 1-3, 3-6, 6-12,12-20, 20-30, 30-40. For environmental and soil parameters, the most commonly correlated Vcmax plasticity step changes included: plasticity changes 30-40 and 6-12. Our results revealed that Vcmax 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 Vcmax plasticity step change 12-20. Evergr. NL forests showed strong correlation between Vcmax 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 Vcmax 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 Vcmax plasticity that can help to better represent global photosynthetic capacity in Earth system models (ESMs).

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