RT Journal Article
SR Electronic
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
FD Cold Spring Harbor Laboratory
SP 2020.12.16.416255
DO 10.1101/2020.12.16.416255
A1 Gardner, A
A1 Ellsworth, DS
A1 Crous, KY
A1 Pritchard, J
A1 MacKenzie, AR
YR 2020
UL http://biorxiv.org/content/early/2020/12/16/2020.12.16.416255.abstract
AB 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 2nd and 3rd 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 3rd year of CO exposure were examined to assess changes in Q. robur photosynthetic capacity. We expected an enhancement in light-saturated net photosynthetic rates (Asat) consistent with CO2 enrichment (≈37%) and that photosynthetic capacity may reduce across over the time of the project. Over the three-year period, Asat 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.Competing Interest StatementThe authors have declared no competing interest.[CO2]CO2 concentration of the atmosphereAphotosynthesisA–Cicurve Photosynthetic CO2 response curveaCO2CO2 at ambient Ca (∼405 ppm)AnetNet photosynthetic rates.AsatLight-saturated net photosynthesisCCarbonCASCanopy access systemCiCO2 concentration of the intercellular leaf spaceDWdry weighteCO2CO2 at elevated Ca (+150 ppm ambient)FACEFree air carbon dioxide enrichmentFWfresh weightJmaxMaximal photosynthetic electron transport rate (a proxy for ribulose-1,5-bisphosphate regeneration)LMALeaf mass per unit areaMAPmean annual precipitationMATmean annual temperatureNNitrogenNaArea-based foliar NitrogenNmMass-based foliar NitrogenNSCnon-structural carbohydratesPARphotosynthetically active radiationPFDphoton flux densityRHrelative humidityTtemperatureTairAir temperatureTleafLeaf temperatureSEStandard error of the meanVcmaxMaximal carboxylation rate of RubiscoVPDvapour pressure deficit of the atmosphereδ13Cratio of 13C to 12C stable carbon isotopesδ15Nratio of 15N to14 N stable carbon isotopes