Effects of elevated CO₂ and soil quality on leaf gas exchange and above-ground growth in beech–spruce model ecosystems

Abstract

Responses of leaf gas exchange and above-ground growth of beech (Fagus sylvatica L.) and Norway spruce (Picea abies Karst.) to atmospheric CO₂ enrichment (374 μl l⁻¹ vs. 590 μl l⁻¹) and increased wet deposition of N (5 vs. 50 kg N ha⁻¹ a⁻¹) in combination with two natural forest soil types (‘acidic’ and ‘calcareous’) were studied in large open-top chambers. Eight juvenile beech and spruce trees from different provenances, together with a ground cover composed of five understorey species, were established in each of 32 model ecosystems. Both beech and spruce showed sustained enhancement of photosynthesis in response to atmospheric CO₂ enrichment during the first 2 yr of treatment. Nevertheless, switching measurement CO₂ concentrations revealed partial downward adjustment of photosynthesis in trees grown in elevated CO₂, beech generally showing more pronounced downward adjustment than spruce. The responsiveness of photosynthesis to CO₂ enrichment did not vary significantly among trees from different provenances. Stomatal conductance was reduced under elevated CO₂ in both tree species. In spruce, the radial growth of the main stem and the annual production of wood (shoot-wood dry mass of current-year lateral shoots), needle dry mass, and assimilation area per tree were stimulated both by CO₂ enrichment and increased N deposition, but were not significantly affected by soil type by year 2. In contrast, in beech, the radial growth of the stem and the total leaf number, foliage dry mass, and assimilation area per tree were all not significantly affected by elevated CO₂ and increased N deposition when responses of the two soil types were pooled, but were greater on calcareous than on acidic soil by year 2. However, CO₂ interacted with soil type in beech: irrespective of the N deposition rate, saplings showed growth stimulation on the calcareous soil but responded negatively to CO₂ enrichment on the acidic soil (where growth was slower). Our results suggest that complex interactions between CO₂, species and soil quality need to be accounted for when attempting to predict forest development in a future CO₂-rich world.