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 atmospheric CO₂ is central to sustaining the terrestrial carbon sink, but can vary diurnally, seasonally and with duration of CO₂ exposure. Hence we sought to quantify responses of 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 (FACE) facility in central England. Over three thousand measurements of leaf gas exchange and related biochemical parameters were conducted at the top of the canopy to assess the diurnal and seasonal responses of photosynthesis during the 2^nd and 3^rd year of eCO exposure at the Birmingham Institute of Forest Research (BIFoR) FACE facility. Measurements of photosynthetic capacity and biochemical parameters derived from CO₂ response curves together with leaf nitrogen concentrations from the pre-treatment year to the 3^rd year of CO exposure were examined to assess changes in Q. robur photosynthetic capacity. We expected an enhancement in light-saturated net photosynthetic rates (A_sat) consistent with CO₂ enrichment (≈37%) and that photosynthetic capacity may reduce across over the time of the project. Over the three-year period, A_sat of upper-canopy leaves was 33 ± 8% higher in trees grown in eCO₂ compared with ambient CO₂ (aCO₂), and this enhancement decreased with decreasing light levels. There were also no significant CO₂ treatment effects on photosynthetic capacity measures, nor area- and mass-weighted leaf nitrogen. These results suggest that mature oak trees may exhibit a sustained, positive response to eCO₂ without photosynthetic downregulation, suggesting that, with adequate nutrients, there may be increases in C storage in elevated CO₂.