Response of mature Norway spruce (Picea abies) to elevated atmospheric CO2

Background/Question/Methods

Considerable effort has been invested during the last three decades using long-term Free Air CO₂ Enrichment (FACE) systems to trace the fate of carbon (C) in trees under elevated atmospheric CO₂. Mainly for logistic reasons however, few systems have looked at mature, steady state forest systems. Starting in 2009, we subjected five mature, 35m tall Picea abies trees in a near-natural mixed forest to twice-industrial atmospheric CO₂. Here we present a synthesis of tree responses over three seasons on carbon allocation (using δ¹³C), growth, soil respiration, leaf gas exchange, and water relations (sap flow, leaf water potential, soil moisture). By tracing the stable isotope signal of the additional CO₂, we also offer a detailed picture of the time constants of carbon transfer in tall Norway spruce trees. The results are compared to the output of a global dynamic vegetation model (DGVM) that was run for the same site.

Results/Conclusions

Only 9-12 days after the start of the experiment, newly assimilated carbon traced by ¹³C signals was transferred through the entire tree from needles to stem wood, stem air, and soil air. After two years, needles of treated trees showed a stable carbon isotope depletion of 4.4‰ compared to control trees. The isotopic signal in the xylem of treated trees has not yet arrived at the steady state by year three. This isotopic depletion of respiratory CO₂ in stem tissue and soil persisted during the entire survey except for CO₂ released from soils during the first winter and spring. Until now, aboveground biomass (tree rings) does not show a detectable signal in response to elevated CO₂. Soil respiration under trees subjected to elevated CO₂ increased during the first season, a signal that disappeared in the following seasons. Water use was not affected by elevated CO₂, neither in terms of sap flow, leaf-water potential, stomatal conductance, or soil moisture which contrasts the results obtained by a DGVM model which assumes general physiological principles. However, the lack of a transpirational response to elevated CO₂ lines up to earlier findings that conifers show weak or no stomatal sensitivity to atmospheric CO₂ enrichment. Overall, we see minor consequences of elevated CO₂ in both carbon usage and water relations at the whole-tree level and a fast transfer of newly labeled carbon across the plant-soil continuum.