Mature riparian alder forest acts as a strong and consistent carbon sink

Abstract. Alder forests are widespread across the Northern Hemisphere, often occupying riparian zones and enhancing soil fertility through symbiosis with nitrogen-fixing bacteria. Despite their ecological importance, the ecosystem-level carbon and water exchange of alder forests remains poorly studied, particularly under contrasting hydroclimatic conditions. We studied ecosystem carbon and water fluxes over three contrasting years (“wet”, “drought”, “recovery”) in a mature riparian grey alder forest in Estonia. The forest was a strong and consistent net carbon sink with annual net ecosystem exchange (NEE) ranging from −496 to −663 g C m−2 yr−1, gross primary production (GPP) from −1258 to −1420 g C m−2 yr−1, ecosystem respiration (ER) from 595 to 923 g C m−2 yr−1 and evapotranspiration (ET) varied from 194 to 342 kg H2O m−2 yr−1. Moderate soil water saturation (40 %–50 %) enhanced all ecosystem fluxes. In contrast, progressive drought reduced ER, ET, and to a much lesser extent GPP, with elevated EWUE and suppressed canopy conductance indicating strong stomatal regulation to limit water loss while maintaining carbon sequestration. While soil saturation affected canopy conductance, its effect was outweighed by vapour pressure deficit during the drought year, even after soil water availability recovered. We observed a full recovery in the following year, which was supported by favourable temperature and precipitation, although partially suppressed canopy conductance suggested some vulnerability to possible consecutive droughts in the future. Overall, the forest demonstrated drought resilience and high net carbon uptake across contrasting years, underscoring the capacity of riparian alder stands to sustain carbon sequestration under variable hydroclimatic conditions.