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.

Summary: Trees are known to be atmospheric methane (CH₄) emitters. Little is known about seasonal dynamics of tree CH₄ fluxes and relationships to environmental conditions. That prevents the correct estimation of net annual tree and forest CH₄ exchange. We aimed to explore the contribution of stem emissions to forest CH₄ exchange. We determined seasonal CH₄ fluxes of mature European beech (Fagus sylvatica) stems and adjacent soil in a typical temperate beech forest of the White Carpathians with high spatial heterogeneity in soil moisture. The beech stems were net annual CH₄ sources, whereas the soil was a net CH₄ sink. High CH₄ emitters showed clear seasonality in their stem CH₄ emissions that followed stem CO₂ efflux. Elevated CH₄ fluxes were detected during the vegetation season. Observed high spatial variability in stem CH₄ emissions was neither explicably by soil CH₄ exchange nor by CH₄ concentrations, water content, or temperature studied in soil profiles near each measured tree. The stem CH₄ emissions offset the soil CH₄ uptake by up to 46.5% and on average by 13% on stand level. In Central Europe, widely grown beech contributes markedly to seasonal dynamics of ecosystem CH₄ exchange. Its contribution should be included into forest greenhouse gas flux inventories.

In order to understand the effect of phytotoxic tropospheric ozone (O₃) on terrestrial vegetation, we quantified the impact of current O₃ concentration ([O₃]) on net ecosystem production (NEP) when compared to the conditions of the pre-industrial era. We compared and tested linear mixed-effects models based on [O₃] and stomatal O₃ flux (F_sto). The managed ryegrass–clover (Lolium perenne and Trifolium pratense) mixture was grown on arable land in the Czech Republic, Central Europe. Values of [O₃] and F_sto were measured and calculated based on resistance analogy, respectively, while NEP was calculated from eddy covariance CO₂ fluxes. We found the F_sto-based model more precise when compared to measured NEP. High F_sto was found even at low [O₃], while broad summer maximum of [O₃] was not necessarily followed by significant NEP decline, due to low soil water content leading to a low stomatal conductivity and F_sto. Comparing to low pre-industrial O₃ conditions, current levels of O₃ resulted in the reduction of cumulative NEP over the entire growing season, up to 29.7 and 13.5% when the [O₃]-based and F_sto-based model was applied, respectively. During the growing season, an O₃-induced reduction of NEP ranged between 13.1% in May and 26.2% in July when compared to pre-industrial F_sto levels. Looking to the future, high [O₃] and F_sto may lead to the reduction of current NEP by approximately 13.3% on average during the growing season, but may increase by up to 61–86.6% in autumn, indicating further O₃-induced acceleration of the senescence. These findings indicate the importance of F_sto and its inclusion into the models estimating O₃ effects on terrestrial vegetation. The interaction between environmental factors and stomatal conductance is therefore discussed in detail.

We investigated how reduced summer precipitation modifies photosynthetic responses of two model tree species—coniferous Norway spruce and broadleaved sessile oak—to changes in atmospheric CO₂ concentration. Saplings were grown under mountainous conditions for two growing seasons at ambient (400 μmol CO₂ mol^–1) and elevated (700 μmol CO₂ mol^–1) CO₂ concentration. Half were not exposed to precipitation during the summer (June–August). After two seasons of cultivation under modified conditions, basic photosynthetic characteristics including light-saturated rate of CO₂ assimilation (A_max), stomatal conductance (G_Smax), and water use efficiency (WUE) were measured under their growth CO₂ concentrations together with in vivo carboxylation rate (V_C) and electron transport rate (J) derived from CO₂-response curves at saturating light. An increase in A_max under elevated CO₂ was observed in oak saplings, whereas it remained unchanged or slightly declined in Norway spruce, indicating a down-regulation of photosynthesis. Such acclimation was associated with an acclimation of both J and V_C. Both species had increased WUE under elevated CO₂ although, in well-watered oaks, WUE remained unchanged. Significant interactive effects of tree species, CO₂ concentration, and water availability on gas-exchange parameters (A_max, G_Smax, WUE) were observed, while there was no effect on biochemical (V_C, J) and chlorophyll fluorescence parameters. The assimilation capacity (A_sat; CO₂ assimilation rate at saturating light intensity and CO₂ concentration) was substantially reduced in spruce under the combined conditions of water deficiency and elevated CO₂, but not in oak. In addition, the stimulatory effect of elevated CO₂ on A_max persisted in oak, but completely diminished in water-limited spruce saplings. Our results suggest a strong species-specific response of trees to reduced summer precipitation under future conditions of elevated CO₂ and a limited compensatory effect of elevated CO₂ on CO₂ uptake under water-limited conditions in coniferous spruce.

Elevated atmospheric CO2 (EC) can enhance photosynthesis, biomass accumulation and water-use efficiency (WUE), but responses depend on water and nutrient availability and vary among plant species. Studies testing these interactions in multifactorial designs remain scarce. We examined the interactive effects of EC (~700 μmol mol–1), nitrogen supply (nitrogen addition, N+; no nitrogen addition, N–) and water availability (well-watered, WW; drought-stressed, DS) on physiological and metabolic responses of anisohydric European beech (Fagus sylvatica) and isohydric Norway spruce (Picea abies) saplings.Across both species, EC produced the largest overall shifts in gas exchange and metabolite profiles, while drought and low N strongly modified EC responses. Under EC, net CO2 assimilation increased despite reduced stomatal conductance, resulting in higher WUE, with the strongest increase under EC×DS×N+. In beech, EC was associated with photosynthetic downregulation, reflected in lower Rubisco carboxylation capacity (VCmax) and electron transport rate (Jmax), most evident under DS×N– conditions. This response was accompanied by stronger stomatal closure, lower succinate and tissue N, and reduced protective metabolites such as glycine betaine and γ-aminobutyric acid, whereas stress-related amino acids (proline, asparagine and methionine) and phenolics (syringic and vanillic acids) increased. In contrast, spruce maintained VCmax and Jmax under EC even under DS×N–, alongside lower abscisic acid and higher phenolic-based defence metabolites (kaempferol, quercetin and caffeic acid).Overall, our findings indicate contrasting acclimation strategies under EC: beech relies on flexible but stress-sensitive metabolic adjustments, whereas spruce maintains a more conservative physiology, supporting resilience under CO2 enrichment and resource limitation.