Stem CO<sub>2</sub> efflux of a Populus nigra stand: effects of elevated CO<sub>2</sub>, fertilization, and shoot size

Abstract

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.