Informing Mature Temperate Forests as Natural Climate Solutions: Changed Fine Root Biomass and Morphology under Elevated CO2

• Nature based solutions to climate change must incorporate mitigation strategies that sustain and enhance forest carbon sequestration, requiring comprehensive accounting of forest carbon budgets, including carbon stored in roots and soils. Forests’ capacity to remain as carbon sinks under elevated CO2 (eCO2) may depend on tree root systems adjusting to overcome nutrient and water limitation. It remains uncertain whether and how root systems can change across depth under eCO2 in mature forests.
• We assessed fine root biomass, morphology, depth distribution and C:N ratio, using 1 m deep soil cores from years five and seven of the Birmingham Institute of Forest Research Free-Air CO2 Enrichment experiment (BIFoR FACE), a mature, deciduous forest subject to eCO2 (+150 µl/L i.e. mid-21st century projected atmospheric CO2 concentration).
• Fine root biomass was ~40% greater under eCO2, concentrated in the top 50 cm and equivalent to ~36% more root carbon standing stock. Contrary to expectations, the distribution of fine root biomass did not shift to greater depths. Changes in morphology were variable but, on average, there was a positive interaction between eCO2 and depth on specific root length. Under eCO2, greater fine root biomass and changes in morphology result in higher fine root surface area, and thereby a greater potential for resource acquisition across the soil profile.
• This mature temperate forest has the potential to adjust its root systems to eCO2. Belowground carbon in models predicting the future terrestrial carbon sink will benefit from the improved characterisation of fine roots under eCO2.