Informing the potential of mature temperate forests as natural climate solutions: Changed fine root biomass and morphology under elevated CO₂

Societal Impact Statement
The land carbon sink absorbs approximately 25% of anthropogenic CO2 emissions, with forests accounting for most. Managing forests as Natural Climate Solutions is therefore a societal imperative, requiring models of where and how long carbon resides within these ecosystems. We investigated the effects of elevated CO2 on fine roots, the primary source of soil carbon, in a mature temperate forest, reporting greater biomass and changes in morphology. Improved characterisation of fine roots under elevated CO2 can reduce uncertainties in modelled root function, addressing the reliance on aboveground observations and poorly constrained fine root representations in global forest carbon sink assessments.

Summary
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 5 and 7 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 previous results, the distribution of fine root biomass did not shift to greater depths. Changes in morphology were variable, but, on average, there was greater fine root length under eCO2 and, with depth, higher 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. Better characterisation of fine roots under eCO2 can benefit belowground carbon modelling, improving predictions of forest carbon sinks and refining estimates of forests as natural climate solutions for climate policy.