Short-term flooding increases CH₄ and N₂O emissions from trees in a riparian forest soil-stem continuum

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.