How has flooding impacted Belle Isle’s ‘imperiled’ Wet-Mesic Flatwoods forest?

Kevin Bingham estimates the age of a dead oak tree, a piece of which is being used to block off a flooded portion of Belle Isle’s Central Avenue, at 195 years. The oak was likely killed years before the flooding that has hit much of the woods on the eastern end of the island. But it represents the types of specimens that are being lost here in large numbers.

Drone photographs show masses of dead canopy trees on Belle Isle, especially where Lake Okonoka has inundated the island’s Wet-Mesic Flatwoods ecosystem, a swampy type of forest that the Michigan Natural Features Inventory lists as “imperiled” in Michigan. Belle Isle’s woodland represents one of the largest remaining examples of this ecosystem type. It’s also home to several species of concern listed by the MNFI, including the threatened Pumpkin Ash (Fraxinus profunda) and special-concern Shumard Oak (Quercus shumardii). 

Bingham, a Detroit-based arborist, and co-owner of tree service company Singing Tree, estimates that roughly a third of the forest’s overstory has died since the floodwaters began rising in the spring of 2019.

“When they opened up Lake Okonoka, that water just whooshed in,” Bingham says, referring to the Lake Okonoka Habitat Restoration project. The project was designed to reconnect “Belle Isle’s internal waterways to the river and restore the Wet-Mesic Flatwoods forest to enhance habitat for a great diversity of animal and plant species,” according to the project website. But so far, the woods have been inundated.

“There was no outlet. Just looking at the drone photos, you can tell how [the water] is going into a bowl there and then spilling over into the forest,” said Bingham. 

Indeed, looking south from the bridge that passes over the channel between the lake and the lagoon, a wall of dead trees rises from the lake and appears to extend to the partially submerged Central Avenue that runs through the forest. The damage sweeps southwest to the Nashua Canal Trail, where the canal has overflowed its banks and washed out much of the footpath. Dead and dying trees are everywhere, some of them possibly victims of the oak-wilt and emerald ash borer that have been here for years. Yet others appear to have just died or are undergoing early autumn senescence.

According to the MNFI abstract describing Wet-Mesic Flatwoods, species composition in the ecosystem is “regulated by winter and spring inundation followed by soil desiccation in late summer and fall, when the water level drops well below the soil surface.” But when subjected to year-round inundation, conditions begin to exclude trees as wetland-type species are favored. This appears to be happening to the Wet-Mesic Flatwoods on Belle Isle.

Ecosystem restoration backfire?

The Lake Okonoka Habitat Restoration project was designed to restore the island’s connection to the Detroit River, improving spawning grounds for fish and offering habitats like mudflats for shorebirds that will come and go as water levels in the Detroit River rise and fall. Projects like these “soften shorelines”, allowing them to infiltrate stormwater and change with variations in water levels —  an important adaptation for climate change, which is expected to create more frequent and severe cycles of drought and flooding. 

However, when the waters of the Detroit River were connected to Lake Okonoka in April of 2019, it coincided with the beginning of nearly two years of record high water levels that have washed out roads and flooded portions of the Flatwoods across the eastern end of Belle Isle.

The forest is a historically significant part of the island that park designer Fredrick Law Olmsted considered its essential element. “The key to all improvements of Belle Isle must be found in the character of the existing wood,” he writes in his essay “The Park for Detroit”. 

Much of the Flatwoods are very old. Bingham estimates that some of the oak trees have been here for more than 300 years. But on the eastern end of the island, the woods historically transitioned between marsh, prairie, and forest as the waters from the river rose and fell — at least before the park’s footprint expanded to create the area around the Blue Heron Lagoon, with the ring road and added land mass creating protection from floodwaters for the woods.

Some park users have expressed frustration with what they see as reluctance to take responsibility for damage that flooding has caused on the island on the part of Michigan’s Department of Natural Resources (DNR), which oversees the island as a state park. In a statement to Planet Detroit, the DNR lays the blame for the die-off on pre-existing disruptions to natural drainage.  

“Significant loss of trees in the Wet-Mesic Flatwood is due to the cumulative impacts park development has had on forest hydrology over many decades, rather than the recent high-water levels in the Flatwoods,” the statement reads.

Issues with hydrology, pests, and disease have doubtless had an impact on trees here. But it’s unclear if even healthy trees could have withstood the roughly 15 months of flooding that parts of this forest have experienced since April of 2019. When scientists discuss the effect of high waters on tree mortality, they reference trees withstanding weeks or months of flooding before dying, even in the cases of floodplain species that have developed a tolerance to high waters, such as those found in the Wet-Mesic Flatwoods. That flooding seems to hit old or very young trees hardest, and could account for the dead forest canopy with scrubby growth underneath.

“I think this year was the final blow,” Heidi Frei, a natural resources steward with the DNR, told Planet Detroit. “But driving through and being around in that area on foot over the last several years, we’ve seen a considerable decline in a lot of those canopy trees.”

Bingham doesn’t believe the DNR’s explanation that the die-off was caused by pre-existing conditions. “For some reason, all the surviving trees didn’t have their feet wet,” he says.

HIgh water is not a new problem

Park managers have always had issues managing water on the island in one way or another. 

“This is not something that just started yesterday,” Ron Olson, chief of the DNR’s Parks and Recreation Division says. Olmsted backs him up on this, devoting several passages of his 1882 volume to solving drainage problems on the island. “The good news is we have a game plan to deal with it. The bad news is that over the years, the woods have been transformed into a de-facto wetland.”

The roads that crisscross the eastern end of Belle Isle have also disrupted the natural hydrology of the Flatwoods, and piling of fill dirt on the disused Tanglewood Street behind the old Belle Isle Zoo has caused water to pool in that area. Suzan Campbell, a former senior naturalist at the Belle Isle Nature Center, says this last issue was an especially harmful development and slowly undermined a high-quality portion of the woodland over many years. Likewise, the construction of the Belle Isle Nature Center created a bowl for standing water that killed several acres of forest.

The DNR was supposed to install a removable cofferdam at the entrance to Lake Okonoka in the spring but found they lacked the appropriate equipment and expertise to do so. After hiring a contractor, they finally put the structure in place on August 14 and have pumped out 16 inches of water, putting an end to the major flooding. 

This drawdown is expected to give standing water in higher areas of the island a chance to drain into the canals and thus dry out the woods.

It’s worth noting that even without the Lake Okonoka project, Belle Isle might have experienced flooding in the past two years on account of storm drains on the island’s ring road that may have backed up during the high water. And the stop log structure that previously helped regulate the flow of water at the junction of the Blue Heron Lagoon and Lake Okonoka could have very well spilled over. 

Going forward, a bigger danger might be from even higher waters washing over the road itself. 

“Belle Isle is a flat island in the Detroit River,” Sam Lovall, project manager for Friends of the Detroit River, told Planet Detroit. “If it goes up another 18 inches —  if climate change does that to us — it’s lights out.”

Campbell points out that as the health of many areas of the forest has declined, there are fewer large trees–which can transpire hundreds of gallons of water a day–creating an increasingly wet situation that leads to further die-offs.

What the future holds for the Belle Isle’s Wet-Mesic Flatwoods

The DNR is carrying out a multi-year, $3.7 million project, funded by the Environmental Protection Agency’s Great Lakes Restoration Initiative, to restore the natural flow of water on the island, re-engineering roads and removing the fill dirt from Tanglewood, among other things. This work could help the remaining woodlands on the island drain more quickly and perhaps reverse the decline created by the many factors stressing the forest.  

But, to some extent, the damage is done. A large portion of the area will be filled with standing or fallen dead timber and new ecosystems will emerge.

“It is a very dramatic look, especially right now,” the DNR’s Frei says. “But I think it also tells the story of how forests change, and how forests are a very dynamic system.” 

The openings created in the canopy could allow certain species like oaks–which often benefit from disturbance–to emerge. Campbell says that we could also see Great Lakes Marsh and Lakeplain Wet Prairie  turn up in some areas. Like the Flatwoods, these are ecologically significant and imperiled habitats that are native to the area.

Limiting disturbance will be crucial for ecological restoration going forward. This would include leaving the dead trees, which can help provide homes for wildlife and act as nurse logs for plants trying to establish themselves by providing a nutrient-rich growing medium and offering small lifts in elevation that can help plants stay above water in wetland habitats.

Campbell says that the soil seed bank on the island, along with seeds moved from intact portions of the woodland by squirrels and other animals, should be adequate to begin re-establishing forest and other habitats. 

“We think we can go in and recreate something like that,” she says. “The reality is, we can’t. But if we don’t screw up the soil, if we don’t screw up the drainage, if we don’t remove the seed sources, it will restore itself.” Managing invasive species that might colonize these disturbed areas is likely to be an important task.

In the meantime, Detroiters will likely witness the kind of ecological succession — from forest to wetland and perhaps back again — that was once common along the Detroit River before shorelines were hardened to protect infrastructure against the fluctuating Great Lakes. 

Bingham says that he’s already beginning to see these changes, and he admits that the fishing is better on the island since its waterways were reconnected to the river. 

“But it’s a huge tree loss,” he says. “To me, it’s a big event.”