Growth and gas exchange of Andropogon gerardii as influenced by burning

Abstract

Late spring burning response of the dominant big bluestem (Andropogon gerardii) was studied on a tallgrass site in central Oklahoma (USA) during a dry (1984) and a wet (1985) year. During active growth (May and June) when temperatures were not limiting, photosynthesis (PS) was higher for burned (25-27 micromoles-2-1) relative to unburned plants (20-25 micromoles m-2 s-1); but during summer drought, PS declined to <10 micromoles m-2 s-1 and treatment rank reversed. However, the 2 treatments had similar transpiration per unit leaf area, and burned plots had much higher peak big bluestem leaf area indices (6.4 in 1984 and 4.5 in 1985) than unburned plots (2.0 both years). Apparently higher transpirational demand in burned plots lowered soil moisture, thereby increasing late season moisture stress and lowering PS relative to unburned plots. Burning resulted in a doubling of big bluestem tiller numbers (997-1,034 and 498-600 tillers m-2 for burned and unburned plots, respectively). Peak aboveground biomass of big bluestem was about 3 times higher on burned relative to unburned prairie during both years. During both years burned vs. unburned big bluestem had higher peak values of % leaf nitrogen (N) and more total leaf N (%N* leaf mass). Thus, burning big bluestem increased leaf area during the active growth period and stimulated PS, resulting in higher carbon uptake of burned relative to unburned plants.