Pinus sylvestris as a missing source of nitrous oxide and methane in boreal forest

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 (CH(4)) and a natural source of nitrous oxide (N(2)O), both of which are important greenhouse gases. However, the role of trees, especially conifers, in ecosystem N(2)O and CH(4) exchange is only poorly understood. We show for the first time that mature Scots pine (Pinus sylvestris L.) trees consistently emit N(2)O and CH(4) from both stems and shoots. The shoot fluxes of N(2)O and CH(4) exceeded the stem flux rates by 16 and 41 times, respectively. Moreover, higher stem N(2)O and CH(4) fluxes were observed from wet than from dry areas of the forest. The N(2)O release from boreal pine forests may thus be underestimated and the uptake of CH(4) may be overestimated when ecosystem flux calculations are based solely on forest floor measurements. The contribution of pine trees to the N(2)O and CH(4) 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.