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.

Summary: Trees are known to emit methane (CH₄) and nitrous oxide (N₂O), with tropical wetland trees being considerable CH₄ sources. Little is known about CH₄ and especially N₂O exchange of trees growing in tropical rain forests under nonflooded conditions. We determined CH₄ and N₂O exchange of stems of six dominant tree species, cryptogamic stem covers, soils and volcanic surfaces at the start of the rainy season in a 400‐yr‐old tropical lowland rain forest situated on a basaltic lava flow (Réunion Island). We aimed to understand the unknown role in greenhouse gas fluxes of these atypical tropical rain forests on basaltic lava flows. The stems studied were net sinks for atmospheric CH₄ and N₂O, as were cryptogams, which seemed to be co‐responsible for the stem uptake. In contrast with more commonly studied rain forests, the soil and previously unexplored volcanic surfaces consumed CH₄. Their N₂O fluxes were negligible. Greenhouse gas uptake potential by trees and cryptogams constitutes a novel and unique finding, thus showing that plants can serve not only as emitters, but also as consumers of CH₄ and N₂O. The volcanic tropical lowland rain forest appears to be an important CH₄ sink, as well as a possible N₂O sink.

One of the characteristics of global climate change is the increase in extreme climate events, e.g., droughts and floods. Forest adaptation strategies to extreme climate events are the key to predict ecosystem responses to global change. Severe floods alter the hydrological regime of an ecosystem which influences biochemical processes that control greenhouse gas fluxes. We conducted a flooding experiment in a mature grey alder (Alnus incana (L.) Moench) forest to understand flux dynamics in the soil-tree-atmosphere continuum related to ecosystem N₂O and CH₄ turn-over. The gas exchange was determined at adjacent soil-tree-pairs: stem fluxes were measured in vertical profiles using manual static chambers and gas chromatography; soil fluxes were measured with automated chambers connected to a gas analyser. The tree stems and soil surface were net sources of N₂O and CH₄ during the flooding. Contrary to N₂O, the increase in CH₄ fluxes delayed in response to flooding. Stem N₂O fluxes were lower although stem CH₄ emissions were significantly higher than from soil after the flooding. Stem fluxes decreased with stem height. Our flooding experiment indicated soil water and nitrogen content as the main controlling factors of stem and soil N₂O fluxes. The stems contributed up to 88% of CH₄ emissions to the stem-soil continuum during the investigated period but soil N₂O fluxes dominated (up to 16 times the stem fluxes) during all periods. Conclusively, stem fluxes of CH₄ and N₂O are essential elements in forest carbon and nitrogen cycles and must be included in relevant models.

The role of trees in the nitrous oxide (N₂O) balance of boreal forests has been neglected despite evidence suggesting their substantial contribution. We measured seasonal changes in N₂O fluxes from soil and stems of boreal trees in Finland, showing clear seasonality in stem N₂O flux following tree physiological activity, particularly processes of CO₂ uptake and release. Stem N₂O 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 N₂O from the atmosphere during dormancy, thereby reducing their overall N₂O emissions. At an annual scale, pine, spruce and birch are net N₂O sources, with spruce being the strongest emitter. Boreal trees thus markedly contribute to the seasonal dynamics of ecosystem N₂O exchange, and their species-specific contribution should be included into forest emission inventories.