Using GIS technology to compare aerial photos of several Green Bay wetlands from the past 40 years, Frieswyk found that the sedge meadows had shrunk in recent years with the drop in Green Bay water levels. During the same time, hybrid cattail populations had expanded rather than receding like those of native cattails. Because they are much hardier and more aggressive, she explained, the hybrid cattails can withstand a broader range of water levels.
The cattails are gaining ground in areas with stable water levels as well. Aaron Boers, another graduate student in Zedler’s lab, found that hybrid cattails thrive in the presence of abundant phosphorus, and they grow bigger and take up more of the nutrient when they are constantly flooded. The Nature Conservancy is putting Boers’ research findings to immediate use as it works with a lake management association in southeastern Wisconsin.
Once an area is taken over with hybrid cattails, it may be difficult to restore, the researchers say. Frieswyk found that soils under hybrid cattails had few seeds to regenerate sedge meadows. In fact, the most common seed found was purple loosestrife, another notorious invasive plant. According to Zedler, this is a red flag for wetland managers seeking to restore sedge meadows by simply removing hybrid cattails with fire or herbicides. “You’d be trading one problem for another,” she said. —Kathleen Schmitt