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.

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.

The impact of drought on total ozone flux in a mountain Norway spruce forest | Thomas Agyei, Stanislav Juráň, Kojo Kwakye Ofori-Amanfo, Ladislav Šigut, Otmar Urban, Michal V. Marek | Agricultural Journals

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.