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.

Riparian forests are known as hot spots of nitrogen cycling in landscapes. Climate warming speeds up the cycle. Here we present results from a multi-annual high temporal-frequency study of soil, stem, and ecosystem (eddy covariance) fluxes of N₂O from a typical riparian forest in Europe. Hot moments (extreme events of N₂O emission) lasted a quarter of the study period but contributed more than half of soil fluxes. We demonstrate that high soil emissions of N₂O do not escape the ecosystem but are processed in the canopy. Rapid water content change across intermediate soil moisture was a major determinant of elevated soil emissions in spring. The freeze-thaw period is another hot moment. However, according to the eddy covariance measurements, the riparian forest is a modest source of N₂O. We propose photochemical reactions and dissolution in canopy-space water as reduction mechanisms.

Tree stems play an important role in forest methane (CH₄) and nitrous oxide (N₂O) flux dynamics. Our paper aimed to determine the unknown diurnal variability of CH₄ and N₂O exchange in grey alder tree stems. The gas fluxes in tree stems and adjacent soil were measured using manual static and dynamic chamber systems with gas chromatographic and laser-spectroscopic analysis, respectively. The alder trees were predominant emitters of CH₄ and N₂O; however, N₂O emission from stems was negligible. The soil mainly emitted N₂O into the atmosphere and was both a source and sink of CH₄, depending on environmental conditions. Neither the tree stems nor the riparian forest soil showed significant differences in their CH₄ and N₂O fluxes between the daytime and nighttime, independently of the exchange rates. In contrast to several previous studies revealing a diurnal variability of greenhouse gas fluxes from tree stems, our investigation did not show any clear daytime–nighttime differences. On the other hand, we found quite clear seasonal dynamics initiated by changing environmental conditions, such as temperature and soil water conditions and tree physiological activity. Our results imply a transport role of tree stems for soil-produced CH₄ and N₂O rather than the production of these gases in tree tissues, even though this cannot be excluded.

Histosols cover about 8–10% of Lithuania’s territory and most of this area is covered with nutrient-rich organic soils (Terric Histosols). Greenhouse gas (GHG) emissions from drained Histosols contribute more than 25% of emissions from the Land Use, Land Use Change and Forestry (LULUCF) sector. In this study, as the first step of examining the carbon dioxide (CO₂) fluxes in these soils, total soil CO₂ efflux and several environmental parameters (temperature of air and topsoil, soil chemical composition, soil moisture, and water table level) were measured in drained Terric Histosols under three native forest stands and perennial grasslands in the growing seasons of 2020 and 2021. The drained nutrient-rich organic soils differed in terms of concentrations of soil organic carbon and total nitrogen, as well as soil organic carbon and total nitrogen ratio. The highest rate of total soil CO₂ efflux was found in the summer months. Overall, the rate was statistically significant and strongly correlated only with soil and air temperature. A trend emerged that total soil CO₂ efflux was 30% higher in perennial grassland than in forested land. Additional work is still needed to estimate the net CO₂ balance of these soils.

Nutrient-rich northern peatlands are often drained to enhance forest productivity, turning peatland soils into sinks of methane (CH₄) and sources of nitrous oxide (N₂O). However, further attention is needed on CH₄ and N₂O dynamics during the winter period to fully understand the spatio-temporal variability of fluxes. Besides soil, tree stems can also emit CH₄ and N₂O. However, stem contribution is not considered in most biogeochemical models. We determined the temporal dynamics of winter-time CH₄ and N₂O fluxes in a drained peatland forest by simultaneously measuring stem and soil fluxes and exploring the relationships between gas fluxes and soil environmental parameters. During sampling (October 2020–May 2021), gas samples from Downy Birch (Betula pubescens) and Norway Spruce (Picea abies) trees were collected from different tree heights using manual static chambers and analysed using gas chromatography. Soil CH₄ and N₂O concentrations were measured using an automated dynamic soil chamber system. Tree stems were a net source of CH₄ and N₂O during the winter period. The origin of stem CH₄ emissions was unclear, as stem and soil CH₄ fluxes had opposite flux directions, and the irregular vertical stem flux profile did not indicate a connection between stem and soil fluxes. Stem N₂O emissions may have originated from the soil, as emissions decreased with increasing stem height and were driven by soil N₂O emissions and environmental parameters. Soil was a net sink for CH₄, largely determined by changes in soil temperature. Soil N₂O dynamics were characterised by hot moments—short periods of high emissions related to changes in soil water content. Tree stem emissions offset the soil CH₄ sink by 14% and added 2% to forest floor N₂O emissions. Therefore, CH₄ and N₂O budgets that do not incorporate stem emissions can overestimate the sink strength or underestimate the total emissions of the ecosystem.

Drainage of organic soils is associated with increasing soil carbon dioxide (CO₂) efflux, which is typically linked to losses in soil carbon (C) stock. In previous studies, drained organic forest soils have been reported as both CO₂ sinks and CO₂ sources depending on, e.g., soil nutrient and moisture regime. However, most of the earlier research was done in the boreal zone, and both the magnitude of CO₂ efflux and the impact of soil moisture regime on soil C stock are likely to vary across different climatic conditions and ecosystems, depending further on vegetation. A 2-year study was conducted in hemiboreal forest stands with nutrient-rich organic soil (including current and former peatlands) and a range of dominant tree species (black alder, birch, Norway spruce, Scots pine) in the Baltic states (Estonia (EE), Latvia (LV), Lithuania (LT)). In this study, we analyzed the CO₂ balance of organic soil in drained (19) and undrained (7) sites. To assess the CO₂ balance, soil respiration was measured along with the evaluation of CO₂ influx into the soil through aboveground (aGV) and belowground (bGV) litter. To characterize the sites and factors influencing the CO₂ fluxes, we analyzed soil temperature, soil water table level (WTL), and physical and chemical parameters of soil and soil water. Irrespective of drainage status, the soils functioned as both CO₂ sinks and CO₂ sources. On average, a close-to-neutral soil CO₂ balance (+0.45 ± 0.50 $\mathrm{t}{\mathrm{CO}}_{\mathrm{2}}\text{-}\mathrm{C}{\mathrm{ha}}^{-\mathrm{1}}{\mathrm{yr}}^{-\mathrm{1}}$ ) was observed in drained sites dominated by black alder, birch, or Norway spruce, while drained Scots pine sites showed soil CO₂ removals with a mean rate of +2.77 ± 0.36 $\mathrm{t}{\mathrm{CO}}_{\mathrm{2}}\text{-}\mathrm{C}{\mathrm{ha}}^{-\mathrm{1}}{\mathrm{yr}}^{-\mathrm{1}}$ . In undrained birch- and spruce-dominated sites, soil functioned as a mean CO₂ sink at +1.33 ± 0.72 $\mathrm{t}{\mathrm{CO}}_{\mathrm{2}}\text{-}\mathrm{C}{\mathrm{ha}}^{-\mathrm{1}}{\mathrm{yr}}^{-\mathrm{1}}$ , while the undrained black alder stands showed an uncertain CO₂ balance of +1.12 ± 2.47 $\mathrm{t}{\mathrm{CO}}_{\mathrm{2}}\text{-}\mathrm{C}{\mathrm{ha}}^{-\mathrm{1}}{\mathrm{yr}}^{-\mathrm{1}}$ . Variation in the soil CO₂ balance was related to soil macronutrient concentrations and pH: forest types characterized by lower nutrient availability showed greater soil CO₂ sink. The reported soil CO₂ balance values may be used as regional emission factors (EFs).