Seasonal dynamics of stem NO exchange follow the physiological activity of boreal trees.

The role of trees in the nitrous oxide (NO) balance of boreal forests has been neglected despite evidence suggesting their substantial contribution. We measured seasonal changes in NO fluxes from soil and stems of boreal trees in Finland, showing clear seasonality in stem NO flux following tree physiological activity, particularly processes of CO uptake and release. Stem NO emissions peak during the vegetation season, decrease rapidly in October, and remain low but significant to the annual totals during winter dormancy. Trees growing on dry soils even turn to consumption of NO from the atmosphere during dormancy, thereby reducing their overall NO emissions. At an annual scale, pine, spruce and birch are net NO sources, with spruce being the strongest emitter. Boreal trees thus markedly contribute to the seasonal dynamics of ecosystem NO exchange, and their species-specific contribution should be included into forest emission inventories.