EFFECTS OF ELEVATED CO2 AND SOIL QUALITY ON LEAF GAS-EXCHANGE AND ABOVEGROUND GROWTH IN BEECH-SPRUCE MODEL-ECOSYSTEMS

Responses of leaf gas exchange and above-ground growth of beech (Fagus sylvatica L.) and Norway spruce (Picea abies Karst.) to atmospheric C O2 enrichment (374 mu l l(-1) VS. 590 mu l l(-1)) and increased wet de position of N (5 vs. 50 kg N ha(-1) a(-1)) in combination with two nat ural forest soil types ('acidic' and 'calcareous') were studied in lar ge open-top chambers. Eight juvenile beech and spruce trees from diffe rent provenances, together with a ground cover composed of five unders torey species, were established in each of 32 model ecosystems. Both b eech and spruce showed sustained enhancement of photosynthesis in resp onse to atmospheric CO2 enrichment during the first 2 yr of treatment. Nevertheless, switching measurement CO2 concentrations revealed parti al downward adjustment of photosynthesis in trees grown in elevated CO 2, beech generally showing more pronounced downward adjustment than sp ruce. The responsiveness of photosynthesis to CO2 enrichment did not v ary significantly among trees from different provenances. Stomatal con ductance was reduced under elevated CO2 in both tree species. In spruc e, the radial growth of the main stem and the annual production of woo d (shoot-wood dry mass of current-year lateral shoots), needle dry mas s, and assimilation area per tree were stimulated both by CO2 enrichme nt and increased N deposition, but were not significantly affected by soil type by year 2. In contrast, in beech, the radial growth of the s tem and the total leaf number, foliage dry mass, and assimilation area per tree were all not significantly affected by elevated CO2 and incr eased N deposition when responses of the two soil types were pooled, b ut were greater on calcareous than on acidic soil by year 2. However, CO2 interacted with soil type in beech: irrespective of the N depositi on rate, saplings showed growth stimulation on the calcareous soil but responded negatively to CO2 enrichment on the acidic soil (where grow th 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.