Response of tussock tundra to elevated carbon dioxide regimes: analysis of ecosystem CO2 flux through nonlinear modeling.

The response of tussock tundra to elevated atmospheric concentrations of CO2 was measured at Toolik Lake, Alaska in the summer of 1983. Computer-controlled greenhouses were used to determine diurnal ecosystem flux of CO2 under four treatments: 340 ppm, 510 ppm, and 680 ppm CO2, as well as 680 ppm CO2 with a four degree centrigrade increase in temperature. For the seven days of data analyzed, net daily CO2 flux was significantly different between treatments. Net uptake was positively correlated with CO2 concentration in the chamber and negatively correlated with temperature. A nonlinear model was used to analyze this data set and to determine some of the reasons for different net CO2 flux. This model allowed an estimation of light utilization efficiency, total conductance of CO2, and a comparable measure of total respiration. From this analysis we conclude that nutrient limitations in the arctic decrease the capacity of tundra plants to make use of elevated CO2 concentrations. The plants respond by decreasing conductance in the presence of elevated CO2, which results in approximately equal gross uptake rates for the three CO2 treatments. Apparent changes in system respiration result in higher net uptake under elevated CO2 but this may be due to biases in the data. The treatment with increased temperature exhibited higher conductances and, consequently, higher gross uptake of CO2 than the other treatments. Higher temperatures, however, also increase respiration with the result being lower net uptake than would be expected in the absence of temperature inscreases.