Trends of Change in Composition and Structure of Chicago Region Wetland Vegetation

To understand long-term changes in Chicago region wetlands, in 2002 we investigated the status of 31 high quality grassor sedge-dominated wetland plant communities that were originally sampled in 1976. Our objectives were to quantify vegetation changes over time by resampling transects and then correlating changes with site fire histories, as well as other factors linked with compositional and structural change. Three of the sites sampled in 1976 were destroyed. The 28 remaining sites included ten graminoid fens, ten sedge meadows, four calcareous floating mats, two marshes, a single calcareous seep and one graminoid bog. Analysis showed that 20 % or more fire frequency maintained species richness, but that most sites had less frequent burning, and the majority of these sites declined in species richness. Almost all communities underwent significant increases in alien and woody vegetation as well as invasion by narrow-leaved cattail, which also had a negative relationship with native species richness. Marshes and fens underwent the most significant declines. There was a 50% loss of species richness in marshes that was largely replaced by an increase in narrow-leaved cattail. In fens, woody vegetation increased in abundance while grasses and sedges declined, and compositional changes included replacement of characteristic species by more generalist wetland and alien species. Infrequent fire and altered environmental factors appear to be driving these changes. Applied research is needed to determine how combinations of fire management and control of other environmental factors can maintain high quality wetland vegetation.