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Beech (Fagus sylvatica L.) seedlings were cultivated from seeds sown in pots or directly in the ground in outdoor chambers that were transparent to solar radiation, and provided either ambient air or CO(2)-enriched air (ambient + 350mgr;mol mol(-1)). The rooting volume was high in all experiments. In the short-term experiment, potted plants were assigned to a factorial CO(2) x nutrient treatment (optimal nutrient supply and severe nutrient shortage) for 1 year. In the long-term experiment, plants were grown directly in the ground and received an optimal supply of water and nutrients in both CO(2) treatments for 3 years. Nutrient stress caused carboxylation capacity (V(m)) to decrease in the potted seedlings exposed to CO(2)-enriched air during their first growing season. In the long-term experiment with optimal nutrient supply, CO(2)-enriched air did not affect V(m), but caused an upward acclimation of maximum electron transport rate (J(m)). Consequently, there was a 14% increase in the J(m)/V(m) ratio, indicating nitrogen reallocation to maintain an equilibrium between RuBP consumption and RuBP regeneration. Both V(m) and J(m) decreased during the growing season in both CO(2) treatments. Although upward acclimation of J(m) was no longer apparent at the end of the third growing season, plants in CO(2)-enriched air maintained a higher J(m)/V(m) ratio than plants in ambient air, indicating that photosynthetic acclimation always occurred. Second flush leaves appeared during each growing season. When expressed on the basis of foliar nitrogen concentration, their photosynthetic characteristics (V(m) and J(m)) were enhanced compared with other leaves. Because the number of second flush leaves was also increased in the elevated CO(2) treatment, this response should be taken into account when modeling the effects of elevated CO(2) concentration on canopy photosynthesis. Stomatal conductance decreased in response to atmospheric CO(2) enrichment; however, the stomatal response to irradiance followed a single relationship based on two stomatal conductance models.

CO2 diffusion into the leaf to the chloroplast not only depends on stomatal conductance but also on the conductance along the path CO2 follows into the leaf mesophyll from the substomatal cavities to the catalytic sites of Rubisco. Recent methodological advances showed that a significant drop in the CO2 partial pressure may occur along this path [1–5]. Such a drop reveals a low conductance to CO2 diffusion in the mesophyll. A lower conductance in the mesophyll has been shown in leaves of woody plants than in herbaceous plants [4–7].

We review the results of several experiments on growth of potted and free rooted Castanea sativa and Fagus sylvatica in soils of differing nutrient contents under CO2 enrichment with respect to the CO2 responses of gas exchange, biomass increments, tissue chemistry, allocation and canopy development. The stimulation of leaf photosynthesis and plant growth by elevated CO2 is shown to be modulated by the balance of resource availability. High soil resources allow for high increases in photosynthesis and growth. Fagus shows a higher growth response to elevated CO2 than Castanea. The feedbacks between stimulation of leaf photosynthesis by elevated CO2 and LAI expansion are important factors to be taken into account when it comes to extrapolation of experimental results on young trees to mature forests.

Two-year-old beech (Fagus sylvatica L.) saplings were planted directly in the ground at high density (100 per m 2 ), in an experimental design that realistically mimicked field conditions, and grown for two years in air containing CO 2 at either ambient or an elevated (ambient + 350 ppm) concentration. Plant dry mass and leaf area were increased by a two-year exposure to elevated CO 2 . The saplings produced physiologically distinct types of sun leaves associated with the first and second growth flushes. Leaves of the second flush had a higher leaf mass per unit area and less chlorophyll per unit area, per unit dry mass and per unit nitrogen than leaves of the first flush. Chlorophyll content expressed per unit nitrogen decreased over time in plants grown in elevated CO 2 , which suggests that, in elevated CO 2 , less nitrogen was invested in machinery of the photosynthetic light reactions. In early summer, the photosynthetic capacity measured at saturating irradiance and CO 2 was slightly but not significantly higher in saplings grown in elevated CO 2 than in saplings grown in ambient CO 2 . However, a decrease in photosynthetic capacity was observed after July in leaves of saplings grown in CO 2 -enriched air. The results demonstrate that photosynthetic acclimation to elevated CO 2 can occur in field-grown saplings in late summer, at the time of growth cessation.