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Current carbon cycle models attribute rising atmospheric CO 2  as the major driver of the increased terrestrial carbon sink, but with substantial uncertainties. The photosynthetic response of trees to elevated atmospheric CO 2  is a necessary step, but not the only one, for sustaining the terrestrial carbon uptake, but can vary diurnally, seasonally and with duration of CO 2  exposure. Hence, we sought to quantify the photosynthetic response of the canopy-dominant species,  Quercus robur , in a mature deciduous forest to elevated CO 2  (eCO 2 ) (+150 μmol mol −1  CO 2 ) over the first 3 years of a long-term free air CO 2  enrichment facility at the Birmingham Institute of Forest Research in central England (BIFoR FACE). Over 3000 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 eCO 2  exposure. Measurements of photosynthetic capacity via biochemical parameters, derived from CO 2  response curves, ( V  cmax  and  J  max ) together with leaf nitrogen concentrations from the pre-treatment year to the 3rd year of eCO 2  exposure, were examined. We hypothesized an initial enhancement in light-saturated net photosynthetic rates (A sat ) with CO 2  enrichment of ≈37% based on theory but also expected photosynthetic capacity would fall over the duration of the study. Over the 3-year period, A sat  of upper-canopy leaves was 33 ± 8% higher (mean and standard error) in trees grown in eCO 2  compared with ambient CO 2  (aCO 2 ), and photosynthetic enhancement decreased with decreasing light. There were no significant effects of CO 2  treatment on  V  cmax  or  J  max , nor leaf nitrogen. Our results suggest that mature  Q. robur  may exhibit a sustained, positive response to eCO 2  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.

Is photosynthetic enhancement sustained through three years of elevated CO 2 exposure in 175-year-old Quercus robur ?