Photosynthesis and biomass under elevated CO2 in a semi-natural grassland - a six-years' field study

Abstract

In a six years' field study (1995-2000) we have studied the effects of elevated CO2 on photosynthesis, above- and below-ground production and root dynamics in a species-rich semi-natural grassland with a long history of grazing in Central Sweden (59° 48'N, 17° 38'E, 10 m a.s.l.). We used open-top chambers with ambient (A, ca 350 µmol/mol) and elevated (E, ca 700 µmol/mol) CO2 treatments, and a control treatment consisting of rings without mounted chambers. Elevated CO2 was supplied to treatment E continuously during the field season (April-October/November). During each field season, elevated CO2 stimulated light-saturated leaf-level photosynthesis of Taraxacum vulgare and Plantago media. Although photosynthesis showed a significant decrease during each growing season, the CO2 effect (treatment E/A) was rather constant. We found no trend in photosynthesis or the effect of CO2 hereon over the years, but during the dry and extremely dry summers of 1996 and 1999, respectively, the CO2 effect on leaf-level photosynthesis was significantly larger than during rainy summers. Elevated CO2 stimulated above-ground production (standing crop in early August) in 1995-1997, but not in 1998-2000. Root production (measured with minirhizotrons and ingrowth-bags) was stimulated during each growing season. The lack of above-ground production response during the latter years may be explained by unusual weather circumstances (1998 and 2000 being extremely wet and overcast with only few sunny days, and 1999 being extremely dry). However, increasing nutrient limitation might also be involved.