Boreal forests comprise 73% of the world's coniferous forests. Based on forest floor measurements, they have been considered a significant natural sink of methane (CH4) and a natural source of nitrous oxide (N2O), both of which are important greenhouse gases. However, the role of trees, especially conifers, in ecosystem N2O and CH4 exchange is only poorly understood. We show for the first time that mature Scots pine (Pinus sylvestris L.) trees consistently emit N2O and CH4 from both stems and shoots. The shoot fluxes of N2O and CH4 exceeded the stem flux rates by 16 and 41 times, respectively. Moreover, higher stem N2O and CH4 fluxes were observed from wet than from dry areas of the forest. The N2O release from boreal pine forests may thus be underestimated and the uptake of CH4 may be overestimated when ecosystem flux calculations are based solely on forest floor measurements. The contribution of pine trees to the N2O and CH4 exchange of the boreal pine forest seems to increase considerably under high soil water content, thus highlighting the urgent need to include tree-emissions in greenhouse gas emission inventories.

Department of Environmental and Biological Sciences University of Eastern Finland P O Box 1627 FI 70211 Kuopio Finland

Department of Food and Environmental Sciences University of Helsinki P O Box 56 FI 00014 Finland

Department of Forest Sciences University of Helsinki P O Box 27 FI 00014 Finland

Department of Physics University of Helsinki P O Box 48 FI 00014 Finland

Global Change Research Institute CAS Bělidla 4a CZ 603 00 Brno Czech Republic

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