Iowa's river islands need a little pruning

Cutting trees to save forests may seem paradoxical, but biologists are doing just that on islands in the Upper Mississippi River.

“By cutting trees on the edges of islands, we hope to save the islands themselves and all the other trees growing on them,” said Rich King, manager of the McGregor District of the Upper Mississippi River National Fish and Wildlife Refuge.

The trees targeted for cutting, King explained, are “leaners” that were eventually going to topple into the river anyway.”

“We're spending millions (through the U.S. Army Corps of Engineers Environmental Management Program) to build new islands, and it only makes sense to do what we can to protect the ones we already have,” said Mike Griffin, the Iowa Department of Natural Resources' biologist who introduced the concept to refuge personnel.

“Mike has been talking about it for years. It is a practical and inexpensive way to protect a valuable resource,” said Clyde Male, assistant manager of the refuge's McGregor District, which covers Mississippi River pools 9, 10 and 11, stretching from Genoa, Wis., to Dubuque.

Griffin said wave action has been eroding refuge islands for decades. The standard response, armoring shorelines with big rocks, is extremely expensive and in many cases cost-prohibitive, he said.

The trees on the edges of islands, mostly silver maple and cottonwood, have shallow root systems limited by the high water table in the river's backwaters.

With their disproportionately small root systems, they are top-heavy and especially vulnerable to strong winds when they are fully leafed out, said Griffin.

“It's just a matter of time before the leaners are uprooted,” he said.

Rather than allowing nature to take its course, King and his colleagues are cutting the ‘leaners' before they fall, taking tons of island soil with them into the river and exposing their banks to further erosion..

Doing so, they say, leaves the stump and its root system in place to protect the island from erosion.

In most cases, the stump will sprout suckers and remain a living organism on the edge of the island.

Griffin said siltation reduces river bottom diversity and fills in holes and depressions that some fish species depend on to survive the winter.

Island living

Besides keeping harmful sediment out of the river, maintaining the islands preserves nesting and roosting trees for bald eagles, the signature bird of the Upper Mississippi. The islands also provide critical habitat for migrating songbirds, and the islands' higher elevations support hardwoods like the swamp white oak, whose acorns nourish wood ducks and other wildlife.

The logs and branches of the felled trees are allowed to settle into the backwaters, where they will increase habitat for fish, turtles and other aquatic animals.

Crews cut many leaners this winter on the edges of long, narrow islands in Harpers Slough at the lower end of pool 9. Islands in pools 10 and 11 have been targeted for additional work after ice out this spring, King said.

Biologists will be monitoring the outcome of the experimental program.

If it works, it will be much less expensive than using some of the more traditional mechanical methods to slow down island erosion, King said.