Annual CO 2 budget of spruce model ecosystems in the third year of exposure to elevated CO 2

Abstract

Clones of 4-year-old spruce trees ( Picea abies) were grown in competition in model ecosystems with nutrient-poor natural forest soil and natural understory vegetation and were exposed to three CO 2 concentrations (280, 420 and 560 μmol mol −1) for three years. Diurnal net ecosystem CO 2 uptake (NEC d), nocturnal net ecosystem CO 2 loss (NEC n) and soil CO 2 efflux were measured repeatedly in the third year of CO 2 exposure and were used to estimate an annual ecosystem CO 2 budget. The CO 2-induced stimulation of NEC d varied over the year with no measurable stimulation in spring and fall but a high mid-season CO 2 stimulation. Respiratory losses of whole ecosystems and soil CO 2 efflux alone were both progressively increased with increasing CO 2, thus counteracting the CO 2 stimulation of photosynthesis per unit ground area. Consequently, the annual net ecosystem CO 2 uptake was only moderately and non-linearly stimulated by CO 2 (+8%=84 g C m −2 a −1 at 420 and +9%=90 g C m −2 a −1 at 560 compared to 280 μmol CO 2 mol −1). We conclude that the rising atmospheric CO 2 concentration may lead to an increase in annual net ecosystem carbon gain of rather nutrient-poor spruce communities. Our results further suggest that CO 2 fertilization effects may be greatest under current CO 2 concentration and that relative increases of net ecosystem CO 2 uptake will become relatively smaller as atmospheric CO 2 will continue to rise.