Effect of Fire on Big Bluestem Production

Abstract

Microclimatic conditions associated with fire-induced changes in a tall-grass prairie environment were investigated, and estimates of the net photosynthetic response of Andropogon gerardi Vitm. to these changes were made by laboratory determination of the temperature dependence of net photosynthesis of A. gerardi plants collected from burned and unburned sites. Plants from both sites had temperature optima for net photosynthesis between 25 C and 30 C, and similar maximum rates. Increased production of A. gerardi in the field following burning was attributed primarily to the more favorable environmental conditions for net photosynthesis on the burned site from the time of leaf emergence through late June. Warmer soil temperatures and more light on the burned site appear to be important in initiating early season growth. Total carbon gain per unit area was greater on the burned site than on the unburned, primarily because more photosynthetic leaf and shoot area was available on the burned site earlier in the growing season. Beginning in late June and continuing for the rest of the growing season, measured environmental parameters were similar on both sites. INTRODUCTION An increase in dry matter production and flowering when tallgrass prairie is burned has been amply documented (cf., Brown, 1967; Curtis and Partch, 1948; Daubenmire, 1968; Ehrenreich and Aikman, 1963; Hadley and Kieckhefer, 1963; Kucera and Dahlman, 1968). Several factors including release from allelopathic effects and decreased competition from cool season grasses have been suggested (Robocker and Miller, 1955; Curtis and Partch, 1948; Ehrenreich, 1959; Old, 1969) as reasons for increased production after fire, but litter removal has often been cited as the primary factor (Curtis and Partch, 1950; Old, 1969). On unbumed tall-grass prairie the light-colored litter layer is often at least 10 cm deep, and quite dense. Its removal affects many microclimatic parameters and it is not clear which of these changes are responsible for increasing production. The most obvious ways in which litter removal affects the microclimate are by allowing increased light penetration and warmer temperatures near and below the soil surfaces (Hurlbert, 1969). These factors have been cited by Ehrenreich and Aikman (1963), Ehrenreich (1959) and Weaver and Rowland (1952) to account for earlier growth on burned compared to over 114 gm2 (1000 lbs acre-1) unburned areas. An additional effect of litter on microclimate was proposed by Brown (1967), who observed less net radiation in litter areas, sug1 Present address: Department of Vegetable Crops, Cornell University, Ithaca, New York 14850. 2 Department of Biology, Central State University, _Edmond, Oklahoma 73034,