The effects of elevated atmospheric CO 2 concentration on growth of forest tree species are difficult to predict because practical limitations restrict experiments to much shorter than the average life-span of a tree. Long-term, processbased computer models must be used to extrapolate from shorter-term experiments. A key problem is to ensure a strong flow of information between experiments and models. In this study, meta-analysis techniques were used to summarize a suite of photosynthetic model parameters obtained from 15 field-based elevated [CO 2 ] experiments on European forest tree species. The parameters studied are commonly used in modelling photosynthesis, and include observed light-saturated photosynthetic rates (A max ), the potential electron transport rate (J max ), the maximum Rubisco activity (V cmax ) and leaf nitrogen concentration on mass (N m ) and area (N a ) bases. Across all experiments, light-saturated photosynthesis was strongly stimulated by growth in elevated [CO 2 ]. However, significant down-regulation of photosynthesis was also observed; when measured at the same CO 2 concentration, photosynthesis was reduced by 10-20%. Wullschleger (1993) and Ryan et al. (1994), are therefore invaluable for improving model predictions.These challenges were faced by the ECOCRAFT network, a group of laboratories conducting field-based experiments on the effects of elevated [CO 2 ] on European forest tree species. The network has existed since 1991, and the experimental results up to 1995 have been compiled by Jarvis (1998). Besford, Mousseau & Matteucci (1998) reviewed observations of photosynthetic rates in the ECOCRAFT experiments, and concluded that 'both up and down-regulation of photosynthesis has been found. Downregulation appears to be associated with either poor nutrient status or accumulation of starch, occurs more often late in the growing season and in the older needles of conifers.'The group then faced the problem of quantifying these results in such a way that they could be included in models. This problem was addressed by establishing a central relational database of model parameters (Medlyn & Jarvis 1999). The parameters required, and the methods of deriving them from experimental data, were agreed upon by project working groups comprising both experimentalists and modellers. This paper reports on the photosynthesis parameters from the database, presenting them in formats useful for modelling. First, lists of photosynthesis parameters for different species, extracted from the database, are presented. This catalogue should provide a useful resource for modellers. Second, quantitative methods (i.e. meta-analysis) are used to estimate the effects of elevated [CO 2 ] on the parameters across experiments. Different hypotheses for the effects of long-term elevated [CO 2 ] on photosynthesis are examined. This analysis aids our understanding of photosynthetic responses to elevated [CO 2 ] and suggests ways in which we can improve model formulation.Photosynthesis is a key process when stu...

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• Morphological and anatomical characteristics of needles from different whorl levels were examined on Scots pines ( Pinus sylvestris ), grown for 4 yr under either ambient or elevated (ambient + 400 µ mol mol -1 ) CO 2 concentrations in open-top chambers.• Needle characteristics were studied using light microscopy, scanning electron microscopy and laser scanning confocal microscopy.• Under the elevated CO 2 treatment stomatal density was reduced on both adaxial and abaxial needle surfaces although the number of rows of stomata did not change significantly. Needle cross-sectional area increased by 10%; this was largely the result of an increase in needle thickness and, to a lesser extent, needle width. The increase in needle thickness was due to a large increase in mesophyll tissue. The relative area (i.e. proportion of the total area) of epidermis plus hypodermis, of resin canal, of xylem and of central cylinder decreased, whereas the relative area of needle phloem significantly increased.• The results suggest that a prolonged exposure to elevated CO 2 has an effect on needle structure, anatomical and stomatal characteristics of Scots pine needles.

Three-year-old Scots pine (Pinus sylvestris L.) seedlings were grown for two years in the ground in open-top chambers supplied with either an ambient or elevated (ambient + 400 &mgr;mol mol(-1)) CO(2) concentration. Phenological observations and measurements of height and stem diameter growth, absolute and relative growth rates, starch and soluble carbohydrate concentrations of the needles, and crown structure and needle properties were made at frequent intervals throughout the two growing seasons. Elevated CO(2) significantly advanced the date of bud burst in both years. The increase in total needle area in response to elevated CO(2) was accounted for by longer shoots and an increase in individual needle area in the first year, and by an increase in the number and length of shoots in the second year. Stem diameter and tree height were enhanced more by the elevated CO(2) treatment in the first year than in the second, indicating a decreased effect of CO(2) on growth over time. This was confirmed by a study of absolute and relative growth rates of leader shoots. During the first growing season of CO(2) enrichment, mean weekly relative growth rates over the growing season (RGR(m)) were significantly enhanced. During the second year, RGR(m) in ambient CO(2) closely matched that in elevated CO(2).

Five-year-old Scots pine (Pinus sylvestris L.) seedlings were grown in open-top chambers at ambient and elevated (ambient + 400 &mgr;mol mol(-1)) CO(2) concentrations. Net photosynthesis (A), specific leaf area (SLA) and concentrations of nitrogen (N), carbon (C), soluble sugars, starch and chlorophyll were measured in current-year and 1-year-old needles during the second year of CO(2) enrichment. The elevated CO(2) treatment stimulated photosynthetic rates when measured at the growth CO(2) concentration, but decreased photosynthetic capacity compared with the ambient CO(2) treatment. Acclimation to elevated CO(2) involved decreases in carboxylation efficiency and RuBP regeneration capacity. Compared with the ambient CO(2) treatment, elevated CO(2) reduced light-saturated photosynthesis (when measured at 350 &mgr;mol mol(-1) in both treatments) by 18 and 23% (averaged over the growing season) in current-year and 1-year-old needles, respectively. We observed significant interactive effects of CO(2) treatment, needle age and time during the growing season on photosynthesis. Large seasonal variations in photosynthetic parameters were attributed to changes in needle chemistry, needle structure and feedbacks governed by whole-plant growth dynamics. Down-regulation of photosynthesis was probably a result of reduced N concentration on an area basis, although a downward shift in the relationship between photosynthetic parameters and N was also observed.