Responses of two provenances of Fagus sylvatica seedlings to a combination of four temperature and two CO2 treatments during their first growing season: gas exchange of leaves and roots.

Physiological responses of two provenances of European beech (Fagus sylvatica) were studied in seedlings grown at two [CO2 ] in combination with four temperature treatments. For the local Danish provenance, the average effect of elevated [CO2 ] during growth was to increase light-saturated net photosynthesis (An ) and instantaneous water-use efficiency or transpiration efficiency (ITE). These increases were strongly related to the temperature treatment. Stomatal conductance (gs ) was reduced in seedlings in high [CO2 ], but there was no statistically significant effect of temperature treatment. Stomatal conductance was 13-26% lower at elevated [CO2 ] and ITE was 89-156% higher, depending on growth temperature. The effects of [CO2 ] on An were considerably larger than those shown for many other woody species, but similar to those in other studies on European beech. The absolute value of An for a Romanian provenance of beech was 5-18% lower than in the Danish provenance at low [CO2 ] and 14-26% lower at high [CO2 ]. There was no statistically significant interaction between the provenances and [CO2 ], or between provenance and temperature. A model of the response of An to [CO2 ] at different temperatures gave predictions close to the measured results, except at the lowest temperature treatment where the model over-predicted the effect of elevated [CO2 ]. This and measurements of An made at a common, low [CO2 ] indicated a down-regulation of photosynthesis in the lowest temperature treatment at high [CO2 ]. Root plus soil respiration on a whole-tree basis (Rtr ) was increased by elevated [CO2 ] at all but the lowest temperature, but no effect was seen of [CO2 ] on root respiration per unit root d. wt. Mean Rtr on any given date was significantly correlated with An , except at the lowest temperature treatment. It is hypothesized that low temperature limited the ability of the roots to use photosynthates resulting in a feedback inhibition of An when elevated [CO2 ] was combined with low temperature.