Quantification and uncertainty of root growth stimulation by elevated CO₂ in a mature temperate deciduous forest

Increasing CO₂ levels are a major global challenge, and the potential mitigation of anthropogenic CO₂ emissions by natural carbon sinks remains poorly understood. The uptake of elevated CO₂ (eCO₂) by the terrestrial biosphere, and subsequent sequestration as biomass in ecosystems, remain hard to quantify in natural ecosystems. Here, we combine field observations of fine root stocks and flows, derived from belowground imaging and soil cores, with image analysis, stochastic modelling, and statistical inference, to elucidate belowground root dynamics in a mature temperate deciduous forest under free-air eCO₂ to 150 ppm above ambient levels. eCO₂ led to relatively faster root production (a peak volume fold change of 4.52 ± 0.44 eCO₂ versus 2.58 ± 0.21 control), with increased root elongation relative to decay the likely causal mechanism for this acceleration. Physical analysis of 552 root systems from soil cores support this picture, with lengths and widths of fine roots significantly increasing under eCO₂. Estimated fine root contributions to belowground net primary productivity increase under eCO₂ (mean annual 204 ± 93 g dw m^-2 yr^-1 eCO₂ versus 140 ± 60 g dw m^-2 yr^-1 control). This multi-faceted approach thus sheds quantitative light on the challenging characterisation of the eCO₂ response of root biomass in mature temperate forests.