Is photosynthetic enhancement sustained through three years of elevated CO₂ exposure in 175-year old Quercus robur?

Abstract

Current carbon cycle models attribute rising atmospheric CO₂ as the major driver of the increased terrestrial carbon sink, but with substantial uncertainties. The photosynthetic response of trees to elevated atmospheric CO₂ is a necessary step, but not the only one, for sustaining the terrestrial carbon uptake, but can vary diurnally, seasonally and with duration of CO₂ exposure. Hence we sought to quantify the photosynthetic response of the canopy-dominant species, Quercus robur, in a mature deciduous forest to elevated CO₂ (eCO₂) (+150 μmol mol^-1 CO₂) over the first three years of a long-term free air CO₂ enrichment facility at the Birmingham Institute of Forest Research in central England (BIFoR FACE). Over three thousand measurements of leaf gas exchange and related biochemical parameters were conducted in the upper canopy to assess the diurnal and seasonal responses of photosynthesis during the 2^nd and 3^rd year of eCO₂ exposure. Measurements of photosynthetic capacity via biochemical parameters, derived from CO₂ response curves, (V_cmax and J_max) together with leaf nitrogen concentrations from the pre-treatment year to the 3 ^rd year of eCO₂ exposure, were examined. We hypothesized an initial enhancement in light-saturated net photosynthetic rates (A_sat) with CO₂ enrichment of ≈37% based on theory but also expected photosynthetic capacity would fall over the duration of the study. Over the three-year period, A_sat of upper-canopy leaves was 33 ± 8 % higher (mean and standard error) in trees grown in eCO₂ compared with ambient CO₂ (aCO₂), and photosynthetic enhancement decreased with decreasing light. There were no significant effects of CO₂ treatment on V_cmax or J_max, nor leaf nitrogen. Our results suggest that mature Q. robur may exhibit a sustained, positive response to eCO₂ without photosynthetic downregulation, suggesting that, with adequate nutrients, there will be sustained enhancement in C assimilated by these mature trees. Further research will be required to understand the location and role of the additionally assimilated carbon.