Similar to soils, tree stems emit and consume nitrous oxide (N2O) from the atmosphere. Although tree leaves dominate tree surface area, they have been completely excluded from field N2O flux measurements and therefore their role in forest N2O exchange remains unknown. We explored the contribution of leaf fluxes to forest N2O exchange. We determined the N2O 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 N2O sinks (-0.254 ± 0.827 μg N2O m-2 stem area h-1 and -4.54 ± 1.53 μg N2O m-2 leaf area h-1, respectively), while the forest floor was a net source (2.41 ± 1.08 μg N2O m-2 soil area h-1). The unstudied tree shoots were identified as a significant contributor to the net ecosystem N2O exchange. Moreover, we revealed for the first time that tree leaves act as substantial N2O 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 N2O flux measurements can lead to misinterpretation of tree and forest N2O exchange, and thus global forest greenhouse gas flux inventories.

Department of Ecosystem Trace Gas Exchange Global Change Research Institute of the Czech Academy of Sciences Belidla 4a CZ 60300 Brno Czech Republic

Department of Ecosystem Trace Gas Exchange Global Change Research Institute of the Czech Academy of Sciences Belidla 4a CZ 60300 Brno Czech Republic; Department of Geography Institute of Ecology and Earth Sciences University of Tartu 46 Vanemuise EST 51014 Tartu Estonia

Department of Life Science Systems School of Life Sciences Technical University of Munich Von Carlowitz Platz 2 DE 85354 Freising Germany

INRAE UMR EcoFoG CNRS Cirad AgroParisTech Université des Antilles Université de Guyane FR 97310 Kourou France

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