EFFECTS OF ELEVATED CO2 ON GAS-EXCHANGE CHARACTERISTICS OF ALPINE GRASSLAND

The ecosystem-level gas exchange characteristics of an alpine grasslan d treated with a combination of elevated CO2 and moderate additions of NPK fertilizer during the third season of experimental treatments are described. Mid-season maximum daytime net ecosystem CO2 flux (NEC)inc reased significantly under elevated CO2 (+45%), whereas nighttime NEC was unaffected by the CO2 treatment. Since daytime NEC under elevated CO2 underwent a seasonal decline, only moderate carbon surpluses accum ulated under elevated CO2. The observed seasonal decline in daytime NE C may be due to reduced sink strength once maximum aboveground biomass is attained, and appears to be a reg ulatory mechanism of ecosystem c arbon accumulation. Moderate additions of NPK fertilizer stimulated bo th day- (+39%) and nighttime NEC (+29%) due to increased plant biomass , independent of CO2 treatment. Yet there is no indication that enhanc ed mineral nutrient status will increase ecosystem responsiveness to e levated CO2.