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.