SUMMARY

Gas vent areas, where CO2 of deep origin is naturally released into the atmosphere, provide a valuable opportunity to study long-term effects of CO2 enrichment on natural vegetation. A study of adaptive traits in relation to elevated CO2 was carried out on genetically isolated populations of Scirpus lacustris, a rhizomatous emergent wetland sedge growing at several CO2 springs in Italy. Plants were grown in the laboratory from rhizomes collected in CO2 springs and control sites and their photosynthetic capacities were compared using gas-exchange techniques. Carbon isotope discrimination, nitrogen content and stomatal density were then measured in the field on plants of the same species growing along a transect traced from gas vents outwards. Photosynthetic rates of plants grown in the laboratory from CO2 springs were not significantly different from those of control plants. The absence of decline in photosynthetic capacity with increasing external CO2 concentration was supported by results from plants grown along the CO2 gradient in the field. They showed no change in nitrogen content and in Ci/Ca ratio, and exhibited a downward regulation of stomatal density. The possible role of wetland ecosystems as sinks for atmospheric CO2 under greenhouse climate conditions is discussed.

INTRODUCTION

Short-term exposure to elevated CO2 concentration is known to cause an increase in the photosynthetic rate in most C3 species, whereas long-term growth under elevated CO2 concentration often results in a downward regulation of photosynthetic capacity (Sage, Sharkey & Seemann, 1989; Arp, 1991).