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

Increasing CO2 levels are a major global challenge, and the extent to which increasing anthropogenic CO2 emissions can be mitigated by natural carbon sinks remains poorly understood. The uptake of elevated CO2 (eCO2) by the terrestrial biosphere, and subsequent sequestration as biomass in ecosystems, may act as a negative feedback in the carbon budget, but remains hard to quantify in natural ecosystems. Here, we combine large-scale 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 CO2 enrichment to 150ppm above ambient levels. Using over 67k frames of belowground observation, we observe that eCO2 leads to relatively faster root production (a peak volume fold change of 4.52 {+/-} 0.44 eCO2 versus 2.58 {+/-} 0.21 control). We identify an increase in existing root elongation relative to root mass decay as the likely causal mechanism for this acceleration. Direct physical analysis of biomass and width measurements from 552 root systems recovered from soil cores support this picture, with lengths and widths of fine roots significantly increasing under eCO2. We use dynamic measurements to estimate fine root contributions to net primary productivity, finding an increase under eCO2, with an estimated mean annual 204 {+/-} 93 g dw m-2yr-1 eCO2 versus 140 {+/-} 60 g dw m-2 yr-1 control. We also quantify and discuss the uncertainties in such productivity measurements. This multi-faceted approach thus sheds quantitative light on the challenging characterisation of the eCO2 response of root biomass in mature temperate forests.