Effect of Fire and Drought on the Ecophysiology of Andropogon gerardii and Panicum virgatum in a Tallgrass Prairie

The effect of fire and drought on net photosynthesis, stomatal conductance to water vapor diffusion, leaf water potential, leaf osmotic potential, leaf nitrogen content, and growth were evaluated for the dominant tallgrass prairie species, Andropogon gerardii, and a less common (more mesic) grass, Panicum virgatum, at the Konza Prairie Research Natural Area in northeast Kansas. Early in the growing season in a burned plot, leaves of A. gerardii exhibited greater photosynthesis (maximum CO₂ uptake = 41.6 @mmol°m— ² @?s— ¹), leaf conductance, leaf nitrogen content, nitrogen use efficiency, leaf thickness, and shoot biomass compared to shoots from an unburned plot, Postburn differences in P. virgatum were less pronounced, and CO₂ uptake rates (maximum = 46.0 @mmol°m— ² @?s— ¹) were not significantly different between burned and unburned sites for this grass. Both grasses had broad temperature optima for photosynthesis, and neither was light—saturated at 2.2 mmol°m— ² @?s— ¹ quantum flux. The response of A. gerardii to burning reflects the documented positive vigor of this grass, and is indicative of a substantial shift in the water, energy, and nutrient dynamics of tallgrass prairie following fire. Due to a midseason drought in 1983, leaf water potentials at midday decreased to < —6.6 and —5.74 MPa in A. gerardii, and —4.53 and —3.51 MPa in P. virgatum in burned and unburned plots, respectively. Lower leaf water potential in the burned site was a result of greater leaf area and transpiration per unit ground area in burned than in unburned prairie. As water stress increased, the osmotic potential at full turgor and at zero turgor decreased more rapidly in the burned plot for both grasses, but minimum values of each parameter (—2.04 and —2.68 MPa in A. gerardii; —2.04 and —2.56 MPa in P. virgatum) were recorded at the end of the season in the unburned plot. When water stress was most severe, photosynthesis decreased to near zero for both grasses at both sites, yet following substantial late season precipitation, photosynthesis increased from 28 to 48% of the early season rates. Attributes of A. gerardii that may contribute to its dominance in tallgrass prairie were the ability of this grass to maintain high rates of carbon gain over a greater range of leaf temperatures and at lower leaf water potential than P. virgatum.