BIOMASS AND DENSITY RESPONSES IN TALLGRASS PRAIRIE LEGUMES TO ANNUAL FIRE AND TOPOGRAPHIC POSITION

Annually burned tallgrass prairie is purported to be a nitrogen-limite d system, especially when compared to unburned prairie. To test the hy pothesis that legumes, potential nitrogen-fixers, would increase in re lative abundance in annually burned sites, we assessed their density a nd biomass for two seasons on upland and lowland soils in annually bur ned and unburned watersheds. Total legume density was significantly hi gher in burned (8.0 +/- 1.0 [SE] stems/m(2)) than in unburned watershe ds (3.0 +/- 0.3 stems/m(2)). Species with higher (P < 0.05) densities in burned than in unburned prairie included Amorpha canescens, Dalea c andida, Dalea purpurea, Lespedeza vioiacea, Psoralea tenuiflora, and S chrankia nuttallii. Desmodium illinoense was the only legume that resp onded negatively to annual fire. Total legume biomass did not differ b etween burned (11.3 +/- 1.3 g/m(2)) and unburned prairie (10.5 +/- 0.9 g/m(2)). Biomass productions of Dalea candida and Psoralea tenuiflora were higher (P < 0.05) in burned than in unburned sites, but biomasse s of other legumes were similar between burn treatments. Average indiv idual stem masses of Amorpha canescens and Baptisia bracteata were sig nificantly greater in unburned than in burned prairie. Legumes were af fected differentially by topographic location. Total legume density wa s higher (P < 0.05) on lowland soils (6.6 +/- 1.0 stems/m(2)) than on upland soils (4.3 +/- 0.5 stems/m(2)). However, total legume biomass w as not different between lowland soils (12.0 +/- 1.2 g/m(2)) and uplan d soils (9.9 +/- 1.0 g/m(2)). Densities and biomasses of Amorpha canes cens, Desmodium illinoense, and Lespedeza capitata were higher on lowl and sites than on upland sites, whereas densities and biomasses of Bap tisia bracteata and Dalea purpurea were higher on upland than on lowla nd soils. Most legume species are either fire tolerant or exhibit a po sitive response to fire and their persistence in annually burned prair ie suggests that they may play an important role in the nitrogen budge t of this ecosystem.