With their dominant share in global plant biomass carbon (C), forests and their responses to atmospheric CO 2 enrichment are key to the global C balance. In this free air CO 2 enrichment (FACE) study, we assessed respiratory losses from stems and soil, and fine root growth of ca. 110-year-old Picea abies growing in a near-natural forest in NW Switzerland. We anticipated a stimulation of all three variables in response to a ca. 150 ppm higher CO 2 concentration in the tree canopies. During the first 2.5 years of the experiment, stem CO 2 efflux (R stem ) remained unresponsive to CO 2 enrichment. This indicates that there is no enhancement of metabolic activity in phloem and xylem of these mature trees. Soil CO 2 efflux (R soil ) beneath trees experiencing elevated CO 2 (eCO 2 ) showed a slight but significant reduction compared to R soil under control trees. High CO 2 trees did not increase their fine root biomass in ingrowth cores after 20 months under FACE relative to the fine root fractions collected in undisturbed soil. Tree growth (stem radial increment, not shown here) remained completely unchanged although earlier experiments showed largest responses (if any) during the early years after a step increase in atmospheric CO 2 concentration. The data presented here suggest C saturation of the study trees at the current close to 400 ppm CO 2 ambient concentrations. Together with the high local atmospheric N-deposition rates (ca. 20 kg N ha -1 a -1 ), our findings imply that factors other that C and N supply appear to constrain growth and metabolism of these mature P. abies trees under eCO 2 .
Understanding the effects of elevated atmospheric CO2 on carbon (C) relations of mature forest trees is central to understanding ecosystem C fluxes and pools in a future high-CO2 world. Here, we investigated the CO2-induced photosynthetic enhancement and the diurnal variation in shoot carbon assimilation, stem CO2 efflux and soil respiration associated with ca. 110-year-old and 37 m tall Norway spruce trees (Picea abies (L.) H.Karst.) growing under free air CO2 enrichment (FACE; eCO2) in a mixed, near-natural forest in Northern Switzerland. Diurnal measurements of these major C fluxes were conducted simultaneously on three occasions: one week before and after the start of CO2 enrichment, and one year later. Under controlled leaf chamber conditions, an increase in the atmospheric CO2 concentration of ca. 150 ppm above ambient stimulated light-saturated rates of photosynthesis in previous-and current-year upper-canopy shoots equally by 73 2 %. In the course of the day such large differences in C assimilation between eCO2 and aCO2 trees only occurred around midday under non-limiting light conditions. The CO2 efflux rates from spruce stems (CEstem) and surrounding soil (Rsoil) shared a similar range during night-and
The photosynthetic light saturation curve in duckweed was lowered by 20–25% after ozone exposure (300 nmol mol−1, 1 h). The light flux and oxygen concentration during ozone‐exposure had no effect on reduction of net photosynthesis. Net photosynthesis and photorespiration were both depressed by about 40% after exposure for 1 h to 360 nmol mol−1 ozone. We could not find any change in dark respiration after ozone exposure below 300 nmol mol−1. When the concentration of ozone was doubled from 150 nmol mol−1 to 300 nmol mol−1, the uptake of ozone in duckweed changed from 100 nmol m−2 s−1 to 170 nmol m−2 s−1. We found no differences in fluorescence (pattern) between ozone treated plants and the control plants during a period of 150 min after ozone treatment, but there was an increase in synthesis of the Dl‐protein and a significant reduction in degradation after ozone treatment (300 nmol mol−1, 1 h). These results, together with fluorescence measurements, indicate that photochemical electron transport was not responsible for the ozone‐induced reduction in net photosynthesis.
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