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 (A(n)) and instantaneous water-use efficiency or transpiration efficiency (ITE). These increases were strongly related to the temperature treatment. Stomatal conductance (g(s)) 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 A(n) 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 A(n) 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 A(n) to [CO2] at different temperatures gave predictions close to the measured results, except at the lowest temperature treatment where the model overpredicted the effect of elevated [CO2]. This and measurements of A(n) 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 (R-tr) 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 R-tr on any given date was significantly correlated with A(n), except at the lon est temperature treatment. It is hypothesized that low temperature limited the ability of the roots to use photosynthates resulting in a feedback inhibition of A(n) when elevated [CO2] was combined with low temperature.