Interactive Effect of Elevated CO2 and Reduced Summer Precipitation on Photosynthesis is Species-Specific: The Case Study with Soil-Planted Norway Spruce and Sessile Oak in a Mountainous Forest Plot (original) (raw)

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 CO 2 concentration. Saplings were grown under mountainous conditions for two growing seasons at ambient (400 µmol CO 2 mol -1 ) and elevated (700 µmol CO 2 mol -1 ) CO 2 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 CO 2 assimilation (A max ), stomatal conductance (G Smax ), and water use efficiency (WUE) were measured under their growth CO 2 concentrations together with in vivo carboxylation rate (V C ) and electron transport rate (J) derived from CO 2 -response curves at saturating light. An increase in A max under elevated CO 2 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 CO 2 although, in well-watered oaks, WUE remained unchanged. Significant interactive effects of tree species, CO 2 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 ; CO 2 assimilation rate at saturating light intensity and CO 2 concentration) was substantially reduced in spruce under the combined conditions of water deficiency and elevated CO 2 , but not in oak. In addition, the stimulatory effect of elevated CO 2 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 CO 2 and a limited compensatory effect of elevated CO 2 on CO 2 uptake under water-limited conditions in coniferous spruce.