Downward adjustment of carbon fluxes at the biochemical, leaf, and ecosystem scale in beech-spruce model communities exposed to long-term atmospheric CO2 enrichment

Young beech (Fagus sylvatica) and spruce (Picea abies) trees from different provenances or genotypes were grown in competition in large model ecosystems and were exposed to two concentrations of atmospheric CO2 (370 vs 570 mu mol mol(-1)), two levers of wet nitrogen deposition (7 vs 70 kg N ha(-1) yr(-1)); and two native forest soils (acidic vs calcareous) for four years in open-top chambers. The 2 x 2 x 2 factorial experimental design was fully replicated (n = 4) with each CO2 x N combination applied to each soil type. Exposure to atmospheric CO2 enrichment stimulated daytime net ecosystem CO2 flux (NEC) as measured during sunny days in the middle of the third growing season. Nevertheless, we observed substantial down-regulation of NEC, with larger adjustments on acidic than on calcareous soil. NEC adjustment was associated with slightly reduced leaf area index (LAI) on the acidic soil (no response on calcareous soil), enhanced soil CO2 efflux from both substrate types, and, most importantly, with down-regulation of CO2 uptake at the leaf scale. Downward adjustment of light-saturated single-leaf photosynthesis (A) and of Rubisco was more pronounced in beech than in spruce and these species-specific differences increased over time. By year four, A adjustment (except in one specific treatment combination in each species) had become complete in beech but had disappeared in spruce. At no time did we observe a genotype or provenance effect on the downward adjustment of carbon fluxes, and nitrogen deposition rate generally had little effect as well. Overall, our results suggest that tree species and soil quality will have profound effects on ecosystem CO2 fluxes under continued atmospheric CO2 enrichment.