Succession and Fire Season In Experimental Prairie Plantings

Fire season influenced the cover of species and flowering guilds of plants in replicated grass and forb plantings of tallgrass prairie species in Wisconsin. Over two burn cycles at 3—yr intervals, cover increased in the rhizomatous perennials Andropogon gerardii, Aster simplex and Solidago altissima in plots burned in spring, or summer, or left unburned, but cover showed dramatic positive or negative responses to spring or summer burns in Agropyron repens, Erigeron annuus, Panicum virgatum, Phalaris arundinacea, and Rudbeckia hirta. Aggregate response of flowering guilds was stronger. By the last (1993) census, the phenological guild flowering before mid—July (summed cover values for nine species) accounted for 4% cover in unburned plots and <2% cover in plots burned in early spring, but 32% cover in plots burned in midsummer. The late—flowering guild (summed values for 21 species) accounted for 93% cover in unburned plots and 97% cover in plots burned in early spring, but 66% cover in plots burned in midsummer. Remaining space was taken by a mid—season guild (11 species), only one of which (E. annuus) was common enough for individual statistical analysis. These results have implications for succession of prairie vegetation under different seasonal fire regimes. Over 8 yr, summer fires that simulated the timing of lightning fires retarded progression to dominance of large, late—flowering C4 grasses, allowing early—flowering species eliminated in other treatments to persist or even prosper. Primaeval lightning fires in midsummer may have produced quite different communities than anthropogenic fires, which are rarely set during the middle of the growing season.