Response of a prairie remnant to a historically rare growing season flooding event

Background/Question/Methods

The Faville Prairie is a remnant wet prairie in the glaciated region of southern Wisconsin. Aldo Leopold’s pessimistic essay, Exit Orchis, was written about this site. Studies by UW student Max Partch in the 1940’s and in 1978-79 provide a rich source of information about the past state of the prairie. In 2008 southern Wisconsin experienced unprecedented rainfall that caused flooding that was extreme in maximum stage and  timing. Peak stage was achieved in June, well into the growing season. When the waters receded, most of the above ground portions of plants were dead. This study was conducted in 2010-2011 to assess the degree to which the composition of this valuable remnant suffered from the prolonged flooding. The expectation was that the more flooding tolerant species would expand, as would the aggressive invaders, especially Phalaris arundinacea. The permanently marked 4 m² Partch plots (n = 121) were resampled for species presence. Cover was estimated by the point method. The earlier Partch samples and the 2010 samples were compared to assess the changes.

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Results/Conclusions

By 2010 the cover had substantially recovered but the quadrat level richness and the composition had shifted significantly. The mean species richness of the quadrats decreased from 24.2 (4.7 SD) to 20.5 (6.6 SD). Species turnover as measured by the Jaccard index was 78.4 (7.4 SD). Formerly abundant prairie species, Liatris pycnostachya and Dodecatheon meadii are apparently extinct on the site. Other prairie species were substantially reduced. Silphium terebinthenceum dropped from 81% to 9% occurrence. As expected grasses and forbs classed as obligate or facultative wetland species expanded while most of the more upland species contracted. The P. arundinacea dominated area located in the frequently flooded lower portion of the prairie expanded into the formerly species-rich prairie by as much as 0.4 ha in two growing seasons after the flooding. A suite of five invasive exotics, including P. arundinacea, increased from 5% to 22% occurrence. Ordinations confirm that the relationships among species has shifted significantly. We conclude that if, as predicted, the frequency of such early summer flooding increases with climate change this prairie area will most likely convert to a P. arundinacea wetland with a sparse residuum of prairie species. Leopold’s despair over a lack of resources to preserve biodiversity must be expanded to include alarm at the lack of concern for the effects of human induced climate change.