The temperature dependence of C 3 photosynthesis is known to vary with growth environment and with species. In an attempt to quantify this variability, a commonly used biochemically based photosynthesis model was parameterized from 19 gas exchange studies on tree and crop species. The parameter values obtained described the shape and amplitude of the temperature responses of the maximum rate of Rubisco activity ( V cmax ) and the potential rate of electron transport ( J max ). Original data sets were used for this review, as it is shown that derived values of V cmax and its temperature response depend strongly on assumptions made in derivation. Values of J max and V cmax at 25 °°°° C varied considerably among species but were strongly correlated, with an average J max : V cmax ratio of 1·67. Two species grown in cold climates, however, had lower ratios. In all studies, the J max : V cmax ratio declined strongly with measurement temperature. The relative temperature responses of J max and V cmax were relatively constant among tree species. Activation energies averaged 50 kJ mol − − − − 1 for J max and 65 kJ mol − − − − 1 for V cmax , and for most species temperature optima averaged 33 °°°° C for J max and 40 °°°° C for V cmax . However, the cold climate tree species had low temperature optima for both J max ( 19 °°°° C) and V cmax (29 °°°° C), suggesting acclimation of both processes to growth temperature. Crop species had somewhat different temperature responses, with higher activation energies for both J max and V cmax , implying narrower peaks in the temperature response for these species. The results thus suggest that both growth environment and plant type can influence the photosynthetic response to temperature. Based on these results, several suggestions are made to improve modelling of temperature responses.

Summary• Data from 13 long-term (> 1 yr), field-based studies of the effects of elevated CO 2 concentration ([CO 2 ]) on European forest tree species were analysed using meta-analysis and modelling. Meta-analysis was used to determine mean responses across the data sets, and data were fitted to two commonly used models of stomatal conductance in order to explore response to environmental conditions and the relationship with assimilation.• Meta-analysis indicated a significant decrease (21%) in stomatal conductance in response to growth in elevated [CO 2 ] across all studies. The response to [CO 2 ] was significantly stronger in young trees than old trees, in deciduous compared to coniferous trees, and in water stressed compared to nutrient stressed trees. No evidence of acclimation of stomatal conductance to elevated [CO 2 ] was found.• Fits of data to the first model showed that growth in elevated [CO 2 ] did not alter the response of stomatal conductance to vapour pressure deficit, soil water content or atmospheric [CO 2 ]. Fits of data to the second model indicated that conductance and assimilation responded in parallel to elevated [CO 2 ] except when water was limiting.• Data were compared to a previous meta-analysis and it was found that the response of g s to elevated [CO 2 ] was much more consistent in long-term (> 1 yr) studies, emphasising the need for long-term elevated [CO 2 ] studies. By interpreting data in terms of models, the synthesis will aid future modelling studies of responses of forest trees to elevated [CO 2 ].

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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