Root Dynamics Under Elevated CO2 in a Free Air Carbon Enrichment (FACE) Experiment

Evidence supporting a carbon fertilisation effect, where increasing levels of carbon dioxide (CO2) in the atmosphere lead to photosynthetic enhancement in trees, suggests that forests can sequester more carbon under elevated CO2 (eCO2). However, it remains largely unclear where and for how long this carbon is stored within the forest ecosystem. To sustain photosynthetic enhancement under eCO2 concentrations, trees are likely to require higher intake of nutrients from the soil, which should stimulate root growth. This ongoing study (2022-2026) investigates the hypothesis that fine root biomass and turnover rates will increase, and proliferation will be higher at greater depths, because of eCO2. It is vital that the consequences of increased atmospheric CO2 on plant carbon allocation are understood to improve the accuracy of models projecting the future of forests as global carbon sinks. This study is carried out at the Birmingham Institute of Forest Research Free Air Carbon Enrichment (BIFoR FACE) experiment, the only FACE experiment in a mature, temperate forest simulating atmospheric CO2 concentrations to those predicted to be the mid-century planetary norm. For ambient and elevated CO2 treatments, dry biomass of fine roots, specific root length (SRL) and depth distribution were assessed. 1m soil cores, deeper than the standard 30cm, were used to investigate changes in fine root depth distribution. Changes in fine root growth rates are calculated from minirhizotron images taken at monthly intervals over a 2-year period, with 15 replicates per treatment. Average fine root biomass was >30% higher under eCO2 in all depths down to 70cm. As expected, fine root biomass declined approximately exponentially with depth under both elevated and ambient CO2 conditions, but this slope of decline was lower under eCO2. Other than in the O horizon, average SRL was also higher under eCO2 with depth, meaning roots were on average longer per unit biomass. This implies that trees adapt root proliferation and morphology to increase the volume of soil exploited under eCO2, particularly at greater depths.