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 CO(2) concentration [CO(2)] and nitrogen fertilization affects woody tissue CO(2) efflux, we measured stem CO(2) 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 mu mol mol(-1)) and elevated (550 mu mol mol(-1), realised by a Free Air Carbon dioxide Enrichment system) [CO(2)], and measurements were performed during the third growing season of the second rotation. Elevated CO(2) did not affect Q(10) or specific stem CO(2) efflux (E(10)) of overall poplar shoots. The lack of any effect of N on stem CO(2) efflux indicated that nutrients were sufficient. Specific stem CO(2) efflux differed significantly between shoot sizes, emphasizing the importance of tree size when scaling-up respiration measurements to the stand level. Variation in stem CO(2) efflux could not be satisfactorily explained by temperature as the only driving variable. We hypothesize that transport of CO(2) with the sapflow might have confounded our results and could explain the high Q(10) values reported here. Predicting the respiratory carbon loss in a future elevated [CO(2)] world must therefore move beyond the single-factor temperature dependent respiration model and involve multiple factors affecting stem CO(2) efflux rate.