Abstract:
The rising concentration of carbon dioxide in the atmosphere ([CO2]) has a direct effect on terrestrial vegetation
through shifts in the rates of photosynthetic carbon uptake and transpirational water-loss. Free Air CO2 Enrichment (FACE)
experiments aim to predict the likely responses of plants to increased [CO2] under normal climatic conditions. The Giessen
FACE system operates a lower [CO2] enrichment regime (480 mmol mol–1) than standard FACE (550–600 mmol mol–1),
permitting the analysis of a mixed species temperate meadow under a [CO2] level equivalent to that predicted in 25–30 years.
We analysed the physiological and morphological responses of six species to investigate the effect of moderate [CO2] on
spring biomass production. Carbon dioxide enrichment stimulated leaf photosynthetic rates and supressed respiration,
contributing to enhanced net assimilation and a 23% increase in biomass. The capacity for photosynthetic assimilation was
unaffected by [CO2] enrichment, with no downregulation of rates of carboxylation of Rubisco or regeneration of ribulose-
1,5-bisphosphate. Foliar N content was also not influenced by increased [CO2]. Enhanced [CO2] reduced stomatal size, but
stomatal density and leaf area index remained constant, suggesting that the effect on gas exchange was minimal.
