Rising from the Ashes: What happens when the trees disappear from the forest?

Chippewa National Forest—Scan the horizon of this vast, open swamp and forest land and you’ll see black ash trees. Millions and millions of black ash, and not much else.

When—not if—the Emerald Ash Borer (EAB) arrives here in north central Minnesota, it will kill all these trees in just a few years, through a process in which the ash borer larvae basically prevent nutrients from moving between the leaves and roots and cause trees to die. Because there are so many black ash trees in this forest, and so little else, the insects will feast, and in their wake, leave a dramatically changed forest landscape.

A collaborative project between CFANS scientists, the U.S. Forest Service Northern Research Station, the Chippewa National Forest and the Minnesota Forest Resources Council is preparing for that scenario, whenever it comes. With help from a grant from the Minnesota Environment and Natural Resources Trust Fund, scientists are simulating the future landscape at 32, 4-acre sites in the forest using three different situations: in one, all the ash trees have been removed. In another, the ashes are slowly dying. In a third, small patch openings show what might happen if smaller groups of ashes die. The rest of the sites are monitored as controls but unaltered by the scientists.

Few trees will remain after the ash borer comes through. “In lowland areas in this part of the state, black ash makes up 90 percent of the native forest,” says Tony D’Amato, associate professor in the Department of Forest Resources and a co-principal investigator for the research project. “Unfortunately, the options for replacing them are pretty limited.”

Why black ash?

Black ash trees have adapted over time to the wet, marshy conditions in the National Forest. The trees can live many years—one tree found as part of the team’s research efforts was estimated to be more than 300 years old—and can withstand wet roots for extended periods. But like their cousins the green and white ash, they’re highly susceptible to the EAB, and D’Amato estimates that when the pests almost inevitably expand their range to the Chippewa National Forest and other portions of northern Minnesota, the trees’ mortality rate will be about 100 percent.

What will replace them?

Ash has thrived here but few other trees currently do because of the marshy conditions. Ash trees absorb water from the swamps, and early experiments have shown that when the ashes are gone, the water table goes up, D’Amato says, which could have a significant effect on streams and lakes if large parts of the forest are affected and no other trees take root. “If 1,000 acres of black ash is gone, where would that water go?” he asks. “At the very least, we want to hold this as forested wetland” because of the potential impact on waterways as well as wildlife. As part of the project, CFANS scientists also are tracking how surface and groundwater are affected by the simulated disappearance of black ash.

While foresters and entomologists are closely studying the effects of EAB in urban forests, few are considering its impact in the large tracts of undeveloped forest lands like those in northern Minnesota, D’Amato says. 

What next?

Walk into the research sites deep in the forest and it’s clear how devastating the impact could be. To address this, scientists have planted new seedlings of possible replacement trees, including local wetland species such as tamarack, black spruce and white cedar, but also species from more southerly locations in Minnesota, including hackberry and swamp white oak. All of these species could potentially grow in these forests. At one time, elm trees also grew here, but the invasion of Dutch Elm Disease three decades ago took out nearly all of the only other trees that occurred in high abundance in these wetlands.

The research team also is testing a new, non-native species, the Manchurian ash, which is resistant to EAB. Black ash is used for traditional basket weaving by Native Americans in the region, including the local Leech Lake Band of Chippewa, and so losing all of it would affect tribal culture, D’Amato says. The Manchurian ash has similar characteristics and could eventually be a replacement for the lost black ash trees. Because it’s a non-native species, researchers are only planting the seedlings to see how they survive the forest’s tough weather and growing conditions. But before the trees are able to reproduce they have to be destroyed.

The clear-cut site shows most dramatically what may happen to the landscape, but more realistic scenarios are found in the research sites where trees are dying because their bark has been shaved off using a technique called girdling, or where trees have been removed in smaller patches, says Brian Palik, a Forest Service research ecologist who’s D’Amato’s co-principal investigator on the project. “This is more like what would happen in nature,” he says while standing in a research plot where the girdled trees are slowly dying.

When the trees start to die from EAB, it happens fast, he adds. At a Forest Service site in Michigan, 40 to 50 acres of trees were gone in a year; the EAB is so devastating that it even attacks the sprouts of regrowth after a tree is cut down.

Some early indications are that the EAB can’t tolerate the extremely cold temperatures of northern Minnesota, but a warming climate may remove that barrier to the pest's expansion, D’Amato says.

No one knows when the ash borer will reach this forest, although it has just this summer been found in Superior, Wis. But researchers are in it for the long haul, Palik adds. The project is funded for five years and is being duplicated in another forest in Upper Michigan. “We’ve built all this, and we’re going to follow what happens as long as we can,” he says.

D’Amato says projects like this help scientists plan ahead for what seems inevitable. “Can we do things ahead of time that will make this forest more resilient? We don’t know yet. If not, it’s pretty depressing and overwhelming to think what could happen. We won’t have time to play catch-up.”

By Becky Beyers.  For full article, visit: http://www.cfans.umn.edu/Solutions/Fall2013/RisingAshes/index.htm

University of Minnesota College of Food, Agricultural and Natural Resources Sciences: Solutions