Changed Fine Root Biomass and Morphology in a Mature Temperate Forest Under Elevated CO2

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, chemical composition and depth distribution 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, but changes in morphology showed higher specific root length with increasing depth. Under eCO2, greater fine root biomass and changes in morphology may result in a larger fine root surface area and greater potential for resource acquisition across the soil profile. Mature temperate forests have the capacity to adapt their root systems to eCO2. This is critical information for models predicting the future terrestrial carbon sink.