Stem CO2 efflux of a Populus nigra stand: effects of elevated CO2, fertilization,and shoot size

To determine whether long-term growth in elevated atmospheric CO₂ concentration [CO₂] and nitrogen fertilization affects woody tissue CO₂ efflux, we measured stem CO₂ efflux as a function of temperature in three different size classes of shoots of Populus nigra L. (clone Jean Pourtet) on two occasions in 2004. Trees were growing in a short rotation coppice in ambient (370 μmol mol⁻¹) and elevated (550 μmol mol⁻¹, realised by a Free Air Carbon dioxide Enrichment system) [CO₂], and measurements were performed during the third growing season of the second rotation. Elevated CO₂ did not affect Q₁₀ or specific stem CO₂ efflux (E₁₀) of overall poplar shoots. The lack of any effect of N on stem CO₂ efflux indicated that nutrients were sufficient. Specific stem CO₂ efflux differed significantly between shoot sizes, emphasizing the importance of tree size when scaling-up respiration measurements to the stand level. Variation in stem CO₂ efflux could not be satisfactorily explained by temperature as the only driving variable. We hypothesize that transport of CO₂ with the sapflow might have confounded our results and could explain the high Q₁₀ values reported here. Predicting the respiratory carbon loss in a future elevated [CO₂] world must therefore move beyond the single-factor temperature dependent respiration model and involve multiple factors affecting stem CO₂ efflux rate.