Substantial uptake of nitrous oxide (NO) by shoots of mature European beech.

Similar to soils, tree stems emit and consume nitrous oxide (NO) from the atmosphere. Although tree leaves dominate tree surface area, they have been completely excluded from field NO flux measurements and therefore their role in forest NO exchange remains unknown. We explored the contribution of leaf fluxes to forest NO exchange. We determined the NO exchange of mature European beech (Fagus sylvatica) stems and shoots (i.e., terminal branches) and of adjacent forest floor, in a typical temperate upland forest in Germany. The beech stems, and particularly the shoots, acted as net NO sinks (-0.254 ± 0.827 μg NO m stem area h and -4.54 ± 1.53 μg NO m leaf area h, respectively), while the forest floor was a net source (2.41 ± 1.08 μg NO m soil area h). The unstudied tree shoots were identified as a significant contributor to the net ecosystem NO exchange. Moreover, we revealed for the first time that tree leaves act as substantial NO sinks. Although this is the first study of its kind, it is of global importance for the proper design of future flux studies in forest ecosystems worldwide. Our results demonstrate that excluding tree leaves from forest NO flux measurements can lead to misinterpretation of tree and forest NO exchange, and thus global forest greenhouse gas flux inventories.