Effects of fire season on composition of wet-mesic prairie plantings were studied for 20 years near Viola, WI. Burns started in 1989, and continued every third season thereafter. Seven 12x15 m plots were burned in April or May, 7 plots in July, and 7 were unburned. Cover and species counts tested two non-exclusive hypotheses against the null. The first is that burn season favors species not in vigorous growth at the time (i.e. early fires favor late-emerging species, late fires early-emerging species), the second is that growing season fire suppresses dominant vegetation, releasing subdominant vegetation of whatever phenology. Fire season changed relative cover, but not components of species richness. Plots averaged 23 ± 1 species in 1988, three seasons after planting, but 15 ± 1 species by 2006 (P < 0.001). Treatments did not influence the number of planted species, nor the incidence of native or exotic volunteers. Early-season burns favored late-season Andropogon gerardii (P < 0.05). Mid-summer burns suppressed Andropogon and released early-successional species, as evidenced by changes in cover (P < 0.03). Other species-specific responses include: Solidago altissima and S. gigantea were favored in unburned plots, and Phalaris arundinacea and native biennials and pioneer species, as well as subdominant late-season species, in summer-burn plots. Given the bias in favor of late-season species in the 1986 planting, a surprise is that summer burns preserved higher cover of planted, and mostly late-season, species than other treatments (P < 0.05), apparently through suppression of Andropogon.
