Temporal changes in vegetation composition and structure in a fire-managed prairie fen.

There have been few quantitative studies of fire effects on graminoid fens in the tallgrass prairie region of North America. These "prairie fens" (Moran 1981) have strong floristic relations to wet prairie and sedge meadow, and fire is hypothesized to play an integral role in maintaining their structure and diversity (Curtis 1959, Moran 1981, Kohring 1982, Zimmerman 1983). In general, fire is said to structure prairie vegetation by increasing cover and annual biomass of graminoid plants, reducing total species diversity and cover of woody plants, and causing variable responses among different forb groups; but lack of fire reduces prairie species richness over time by allowing litter accumulation and woody plant invasion (e.g., Collins and Glenn 1988, Collins and Gib-son 1990, Johnson and Knapp 1995). How well this model applies to prairie fens, however, is unknown. These peatland plant communities present challenges for restoration management and monitoring when they are degraded by past land use practices such as grazing and fire protection. This paper analyzes vegetation change between 1986—87 and 1992, after an 11-year period of fire management to restore graminoid vegetation structure at the Bluff Spring Fen Nature Preserve (BSF), a 40-ha prairie fen in Cook County, Illinois. ABSTRACT: Prairie fens, rare peatland features of the landscape of the midwestern United States, are often fire-managed to restore or maintain species diversity and prevent shrub and tree invasion. We quantified the effects of such management at Bluff Spring Fen Nature Preserve (Cook County, Illinois, USA), where eight dormant-season prescription burns supplemented by shrub cutting took place from 1982 to 1992. Baseline transects established across the fen in 1986—87 were resampled in 1992 in relatively undisturbed spring run/marl flat, calcareous seep, graminoid fen, and sedge meadow communities, and in disturbed graminoid fen vegetation where woody plant invasion and species change had occurred after previous grazing and fire suppression. Analyses of plot richness or diversity found no significant temporal changes. Correlation coefficients and indices of floristic similarity and coefficients of conservatism were sensitive to significant vegetation changes in sedge meadow and in disturbed fen habitats, but not in less-disturbed vegetation. More rigorous analyses found significant temporal changes throughout the fen, which paralleled known fire effects on grassland. In most relatively undisturbed communities, combined frequencies of graminoid species increased, forb frequencies decreased, and woody vegetation remained unchanged. At the species level, this pattern was supported by significant increases in cover of most dominant grasses and sedges. The decline in overall forb frequency was cumulative, as few individual forb species declined in cover or frequency. One exception was the common fen goldenrod Solidago ohioensis, which decreased in cover but not frequency in two communities. Disturbed fen underwent structural and successional changes that varied with habitat. Gray dogwood (C'ornus racemosa), the most aggressive invader of disturbed fen, was more resistant to fire and cutting in unshaded fen and less resilient and more controllable in partial shade of bur oak (Quercus macrocarpa). The prairie grass Andropogon gerardii increased only in unshaded areas of disturbed fen. Successional changes in herbaceous species included loss of alien species and gain of prevalent fen species, although some species that appear to represent late-successional graminoid fen have not yet appeared in disturbed graminoid fen. More precise and frequent monitoring and experimental management treatments are needed to determine the relevance of these findings. Because Cornus racemosa is also present in higher quality graminoid fen and in calcareous seep, continued control of woody vegetation may be required; it is doubtful that dormant-season prescribed burning or cutting without application of appropriate herbicides can eliminate this resilient species.