Effects of Atmospheric CO2 Enrichment on CO2 Exchange Rates of Beech Stands in Small Model Ecosystems

Abstract

CO2 enrichment experiments were performed during two vegetation periods on young beech stands in four closed mini-greenhouses. The houses were climatized according to the outside microclimate (± 0,5 °C, ± 15% rel. air humidity, wind speed approximately to outside in the range of 0.5-2.5 m s-1, max. 17% PAR reduction). The model ecosystems — consisting of 36 young beech (2.5 yr-old) in a soil block of 0.38 m3 and an air volume of 0.64 m3 — were exposed to CO2 concentrations of the unchanged ambient air (350 ± 34 ppmv, control) and of 700 ppmv (698 ± 10 ppmv). Plant growth parameters were measured non distructively and at the end of the 1st season samples were taken for weighing the phytomass. CO2 gas exchange of the stands taken as a whole were continuously measured with two entire mini-greenhouses and, in addition, a compact mini-cuvette system (CMS 400, Walz) was used for measuring dark respiration and CO2 net assimilation rates of single leaves in both stands. Under the influence of the additional CO2 supply stem diameter (2 cm above the first lateral roots) was increased by 13.5%, stem height by 27.4%, and the number of leaves/tree by 33% at the end of the 2nd season. The number of buds was not significantly different and the effect on mean area per leaf was insignificant. Leaf area index was by 1.4 units greater. All dry weights of the main organs were increased after the 1st season: leaf 60%, stem 34%, bud 54%. Roots 2 mm Ф 1.7-fold more under elevated CO2. CO2 gas exchange of two systems was measured. Whole system CO2 losses during night as well as photosynthetic CO2 gains during days were greater at 700 ppmv than in the control system. However, if one balances CO2 gains with CO2 losses over a period of five days in August both model-ecosystems taken as a whole were sinks for CO2. During this selected time period of 5 days at the peak of the season the beech stand at 350 ppmv was the greater sink. At 350 ppmv CO2 (control) the average leaf respiration for 20 °C amounted to 0.31 ± 0.18 and at 700 ppmv to 0.57 ± 0.42 µmol CO2 m-2 s-1 (n = 35/40, t = 3.48, a < 0.05), and correlated positively with leaf temperature. At light saturation the mean net assimilation rate was 4.48 µcool m-2 (leaf area) s-1 in the control and 6.21.tmol m-2 s-1 at the high CO2 concentration corresponding with an enhancement factor of 1.39 for the selected time period.