Net carbon dioxide exchange in canopies of burned and unburned tallgrass prairie

Summary Net carbon dioxide exchange (NCE) rates were measured in a tallgrass prairie, a grassland with high productivity, to determine photosynthetic rates of the canopy. Canopy measurements were made in large, plexiglass chambers (1.21 m long; 0.91 m wide; 1.40 m tall) placed on burned and unburned areas of the prairie. The NCE rates of the canopy were compared with those of individual leaves ofAndropogon gerardii Vitman (big bluestem). In addition, CO₂ flux from the soil was quantified and compared with net photosynthetic flux. The canopy NCE rates were generally lower than those made on individual leaves. In mid-summer (11 July 1987), the maximum canopy NCE rates were 55% and 64% of those measured on individual leaves in burned and unburned treatments, respectively. Canopy NCE rates were lower than individual-leaf NCE rates for two reasons. First, the individualleaf measurements were made on young, unshaded, healthy leaves, while the canopy measurements were made on all types of leaves including senescing, shaded, and damaged leaves. Second, soil CO₂ flux into the chambers lowered NCE values. The CO₂ flux from the soil ranged from 7.2% to 28.4% of the total NCE. One needs to add soil CO₂ flux rates to the measured canopy NCE rates to obtain canopy NCE rates closer to individual-leaf NCE rates. Soil CO₂ flux decreased when conditions became dry, reaching a low of 0.06 mg CO₂m^–2s^–1, but increased after rain to 0.16 mg CO₂m^–2s^–1. Also, after rain, when plants were well watered, they were not light saturated at 1 900 µEm^–2s^–1. The NCE rates on the burned treatment were either higher or similar to those on the unburned treatment. For example, on 11 July 1987, NCE rates were higher on the burned treatment (0.66 mg CO₂m^–2s^–1) compared to the unburned treatment (0.47 mg CO₂m^–2s^–1). During the rest of July and August, the rates of the two treatments were not significantly different. But in September and October, the NCE rates were again higher on the burned treatment compared to the unburned treatment. The results indicated that canopy NCE rates may be more indicative of the productivity of the prairie than individual-leaf measurements made only on young, highly productive leaves.Contribution No. 89-82-J from the Kansas Agricultural Experiment Station. This research was supported, in part, by Grant No. DE-FG02-84ER60253.A000.With 4 Figures