TY - JOUR T1 - Is photosynthetic enhancement sustained through three years of elevated CO<sub>2</sub> exposure in 175-year old <em>Quercus robur</em>? JF - bioRxiv DO - 10.1101/2020.12.16.416255 SP - 2020.12.16.416255 AU - A Gardner AU - DS Ellsworth AU - KY Crous AU - J Pritchard AU - AR MacKenzie Y1 - 2021/01/01 UR - http://biorxiv.org/content/early/2021/07/09/2020.12.16.416255.abstract N2 - 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 2nd and 3rd year of eCO2 exposure. Measurements of photosynthetic capacity via biochemical parameters, derived from CO2 response curves, (Vcmax and Jmax) 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 (Asat) 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, Asat 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 Vcmax or Jmax, 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.Competing Interest StatementThe authors have declared no competing interest.[CO2] CO2concentration of the atmosphereAphotosynthesisA-Cicurve Photosynthetic CO2 response curveaCO2CO2 at ambient Ca (~405 ppm)Anetphotosynthetic rates.AsatLight-saturated net photosynthesisCCarbonCASCanopy access systemCiCO2 concentration of the intercellular leaf spaceeCO2CO2 at elevated Ca (+150 ppm ambient)FACEFree air carbon dioxide enrichmentJmaxMaximal photosynthetic electron transport rate (a proxy for ribulose-1,5-bisphosphate regeneration)NNitrogenNaArea-based foliar NitrogenNmMass-based foliar NitrogenQphoton flux densityRHrelative humidityTtemperatureTairAir temperatureTleafLeaf temperatureSEStandard error of the meanVcmaxMaximal carboxylation rate of RubiscoDvapour pressure deficit of the atmosphereδ13Cratio of 13C to 12C stable carbon isotopesδ N ratioof 15N to 14N stable carbon isotopes ER -