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

Abstract

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 over-predicted 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 lowest 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.