Respiratory fluxes and fine root responses in mature Picea abies trees exposed to elevated atmospheric CO2 concentrations

With their dominant share in global plant biomass carbon (C), forests and their responses to atmospheric CO2 enrichment are key to the global C balance. In this free air CO2 enrichment (FACE) study, we assessed respiratory losses from stems and soil, and fine root growth of ca. 110-year-old Picea abies growing in a near-natural forest in NW Switzerland. We anticipated a stimulation of all three variables in response to a ca. 150 ppm higher CO2 concentration in the tree canopies. During the first 2.5 years of the experiment, stem CO2 efflux (R stem) remained unresponsive to CO2 enrichment. This indicates that there is no enhancement of metabolic activity in phloem and xylem of these mature trees. Soil CO2 efflux (R soil) beneath trees experiencing elevated CO2 (eCO2) showed a slight but significant reduction compared to R soil under control trees. High CO2 trees did not increase their fine root biomass in in-growth cores after 20 months under FACE relative to the fine root fractions collected in undisturbed soil. Tree growth (stem radial increment, not shown here) remained completely unchanged although earlier experiments showed largest responses (if any) during the early years after a step increase in atmospheric CO2 concentration. The data presented here suggest C saturation of the study trees at the current close to 400 ppm CO2 ambient concentrations. Together with the high local atmospheric N-deposition rates (ca. 20 kg N ha−1 a−1), our findings imply that factors other that C and N supply appear to constrain growth and metabolism of these mature P. abies trees under eCO2.