Plant wars kick in when dams come out
By Leslie Mertz
About 12 to15 dams are removed every year from the Great Lakes region. And when they are, plant warfare begins.
As upstream reservoirs revert to river channels, newly exposed bottomland becomes wide-open habitat primed for colonization.
“These are places where invasive plant species can come in and set up shop if left to their own devices,” said Emily Stanley, a professor in the Department of Zoology and Center for Limnology at the University of Wisconsin. “It’s a blank slate, and the reason these plants are invaders is because they have traits that make them really good at getting to places and growing rapidly. Certainly in southern Wisconsin you can absolutely bank on reed canarygrass showing up if nothing is done to stop it, and then whatever other garden-variety invaders that are around in that particular area at that particular time.”
Besides reed canarygrass, a perennial grass that can reach 6 feet tall, other troublesome invasive plants include purple loosestrife, the common reed Phragmites, and Canada thistle. All can quickly blanket the newly exposed bottomlands of former dam impoundments, leaving little if any room for native plants.
Typical strategies to stave off invaders involve spreading native plant seeds and pulling or poisoning invasive plants.
Seeding is necessary because plants are unlikely to sprout on their own in areas that have been underwater for several decades, Stanley said. One of her graduate students researched whether viable, naturally occurring native seeds – a seed bank – remained in the soil from before the dam was installed.
“My student couldn’t find any evidence of a viable seed bank in these old reservoir sediments,” she said.
Restoration projects often include perennials, which may require more than one season to establish. Additional annuals not only compete with sprouting invasive plants, they quickly stabilize the newly exposed bottomland which might otherwise wash into the river with a rain or high-water event.
“A lot of folks use annual ryes or other very quickly germinating species that won’t persist through the site over the long term, but will definitely get it vegetated as soon as possible,” said Sara Strassman, American Rivers director of river restoration in the Upper Midwest.
Seeding is part of the approach where three dams along the Boardman River in northwest Lower Michigan are to be removed to reconnect nearly 3.5 miles of waterway to the Great Lakes. The first dam was removed in late 2012, leaving 50- to 100-foot-wide exposed bottomland on either side of the new channel that runs through heavily wooded parklands known as the Brown Bridge Quiet Area.
The plan included reseeding the entire corridor within the three-dam project, but that was scaled back, said Steve Largent, chair of the Bottomlands Committee of the Boardman River Dams Project.
“Once we determined the location of the (original) channel we dredged something like 200,000 cubic yards of material,” said Largent, who is also Boardman River program coordinator for the Grand Traverse Conservation District that manages parklands along the river.. “It had such a great organic content to it that we’re going to see what actually comes up on its own. We’ll be reseeding the floodplain and about 13 acres of upland area, but everything else we’re going to let come up naturally and control invasives.”
The seed mix will include a variety of native plants, but probably no annual rye for a quick cover crop.
“I don’t think we’re going to need it, being that the river is back to where it’s flowed for thousands of years prior to when we dammed it up in 1922,” Largent said. “We’re confident that the river is going to stay where it’s at, and we’re not going to be getting any slumpage erosion or anything like that.”
Robin Christensen, program coordinator for the regional Invasive Species Network, is similarly hopeful about the Brown Bridge bottomlands. As the reservoir has been drawn down over the past three years in preparation for the dam removal and bottomlands have become exposed, surveys show a considerable native plant population returning, she said. “Brown Bridge Quiet Area is remote enough that there’s not a ton on invasive plants around there. Sure, we have Phragmites because there are a lot of inland lakes nearby, but everything else is pretty much in early infestations.”
By hand-pulling and using aquatically approved herbicides, they are ahead of the game. She anticipates staying that way: “I’m pretty optimistic that what’s going to be there is native, and we’re going to be able to keep it native with not a ton of effort by just keeping on top of it.”
She is less positive about the reservoirs around the additional two dams slated for removal. “The other ponds are in more of an urban location closer to the city, where there is a ton of invasive-species pressure,” she said. “We’ve already been preparing for that by trying to remove some of the seed source from the surrounding areas, and doing surveys on what’s been exposed. That’s definitely on our radar and will be an active part of our job for the next couple of years.”
Elsewhere, other projects are putting time and effort into the revegetation of bottomlands, said Strassman.
“A lot of restoration practitioners are trying to get in front of the potential for invasives at their sites. It’s definitely an issue, especially in the Midwest, where you’re liable to have sediments there that are really nutrient rich, and are just a perfect setup for invasives getting established.”