UW Sea Grant Research
Sedges under Siege 4/12/2006
Hybrid Cattails Run Amok in Lake Michigan Wetlands
By Kathleen Schmitt
Since the 1800s, half of Wisconsin’s primeval wetlands have been drained and filled in for agriculture, roads, houses, and industry. Now the remaining wetlands face another threat—a pernicious, pushy invasive plant. UW–Madison restoration ecologist Joy Zedler is studying how this invader lays siege to Lake Michigan wetlands and drives out diversity.
Cattails blowing in the wind may be a familiar sight, but a closer look reveals they’re not all alike. The native species typically grow straight, sparse leaves that leave room for other types of plants to thrive. But the hybrid Typha x glauca, a cross between native and invasive cattails, is less hospitable.
“The hybrid cattails form very dense stands that use all the light above ground, and below ground the rhizomes fill up all the space. Hardly anything can coexist with it,” said Zedler, who was awarded Sea Grant funding to study the problem.
At risk, said Zedler, are sedge meadows, one of the region’s most diverse types of wetland. The roots of native sedges, which are grass-like plants, form mounds that create nooks and crannies for other types of plants to live. They also provide excellent cover for birds and small animals.
The fluctuating water levels of the Great Lakes historically have sustained sedge meadows and kept cattails in check. Native cattails hug the waterline, while sedges grow just inland. Both plants prefer wet soil, and together they move upslope when water is high and recede when water levels drop.
It’s a different story with hybrid cattails, according to Christin Frieswyk, who received her doctorate working with Zedler. 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 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.
A likely reason, according to Boers, is that flooding alters soil chemistry so that phosphorus previously locked up in the soil becomes available for uptake by plants. It’s a mechanism that has been researched in lake sediments and some wetland soils, but this is the first study to document it in invasive wetland plants. Scientists call it “internal eutrophication.” It explains why cattails dominate the wetlands of Lake Ontario, which has had regulated water levels since the St. Lawrence Seaway opened in 1959.
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.
Zedler will continue to look for ways to restore sedge meadows, using segments of the UW-Madison Arboretum as testing grounds. To aid the process, Frieswyk developed a formula that quantifies which species dominate a certain wetland and in what manner. Her formula—called a species dominance index—provides an objective way to track changes in wetlands and measure progress during restoration efforts.
The constant threat from invasive species can be daunting, but Zedler claims two rewards.
“Two marvelous students received excellent training through this Sea Grant study,” she said. “They’re becoming well equipped to solve the critical problems facing our wetlands.”