Tree Growth in an Oak Woodland Exposed to Elevated Atmospheric CO2

Aboveground tree growth is an essential component of forest net primary productivity and must be part of any assessment of forest response to CO2 enrichment of the atmosphere. The task of measuring the growth response to elevated CO2 is especially difficult in structurally complex, mature, mixed-deciduous forests. Such is the case at the Birmingham Institute for Forest Research (BIFoR) free air CO2 enrichment (FACE) experiment in central England (52483.6 N, 2180 W). The forest is dominated by 25-meter tall, 170-year-old Quercus robur (English oak) trees comprising the upper canopy, and Acer pseudoplatanus (sycamore), coppiced Corylus avellana (hazel), and several other deciduous hardwood species in the understory. Three 30-m plots have been receiving elevated CO2 (+ 150 ppm) through FACE apparatus since 2017, and three other plots with FACE apparatus are exposed to ambient CO2. To measure aboveground growth we installed dendrobands on all stems with dbh > 10 cm within the inner 25-m diameter area of the plots. In 2020 we measured initial stem circumference at 1.3 m height and the monthly change in circumference recorded by the dendrobands. Published species-specific allometric equations were used to estimate dry mass and dry mass increment (DMI), and the individual tree data were summed over the 491 m2 plot. The oaks comprised 91% of the stand basal area and 98% of the stand level DMI. DMI of plots in elevated CO2 was 11% greater than plots in ambient CO2 in 2020 and ranged from 4% greater in 2019 to 50% greater in 2017, but these differences cannot necessarily be ascribed to an effect of the CO2 enrichment given the native variability among plots. In 2021 increment cores were collected from oaks growing within the FACE plots from which we will be able to assess recent wood formation within the longer-term context and growth history of the tree. Analysis of the tree ring chronologies prior to CO2 treatment is expected to provide a more definitive assessment of pre-treatment forest growth and hence the effect of elevated CO2 in this stand.