Preserving hardwood diversity through new management

MSU researchers are working to preserve hardwood diversity, which protects the forest ecosystem from threats such as pathogens and insect predation.

In the early days of Michigan statehood, it was a common belief that a squirrel could easily cross the entire state without ever setting foot on the ground because of the plentiful forests. Though much of that landscape has since changed, forests remain tremendously important to Michigan’s economy and culture.

With the state boasting approximately 20 million acres of woodland, Michigan’s forest products industry includes more than 800 logging and trucking companies, and over 1,000 manufacturers that depend on Michigan lumber, according to the Michigan Department of Natural Resources (MDNR). The density and diversity of those forests are dwindling, however, and a Michigan State University (MSU) AgBioResearch team is working to remedy that.

For the past half-century, the majority of Michigan’s northern hardwood forests were managed using a method called single tree selection silviculture. In this practice, emulating the dynamics of old-growth forests, loggers enter a stand of woodland every 10 to 20 years, cutting down single trees and leaving small gaps in which the ecosystem would naturally replenish itself. This should have led to diverse ages among the trees, but forest managers returning to stands over decades discovered that they remained fairly even aged. Instead of rejuvenating the stands, they were effectively thinning them out, threatening the long-term sustainability and diversity of the forests.

Species diversity protects the forest ecosystem from threats such as pathogens and insect predation because what targets one species has less impact on others. A healthy, resilient forest, therefore, features a wide assortment of trees, from the commercially and ecologically significant sugar maple to yellow birch, red maple and others. Single tree selection combined with high deer population, has been found to have a homogenizing effect, limiting the species that are able to regenerate. Worse still, those species that do regenerate tend to be less preferable economically and environmentally, such as American beech and ironwood.

Taken together, these issues represent a threat to one of Michigan’s most important natural resources. That’s why the MDNR approached MSU AgBioResearch forest ecologist Michael Walters for help. Walters has dedicated his academic career to developing better, more sustainable ways of managing forests.

“There’s been a growing awareness of these problems for about 20 years,” said Walters, associate professor in the MSU Department of Forestry. “Many of our forests are not being managed sustainably, and that’s left them vulnerable to not only disease and pests but also to larger forces, such as climate change. It’s nearing the end of the road, and we have to do something.”

The findings of numerous studies have suggested that the root cause of these problems is a combination of the small gaps in the forest left by single tree selection and overgrazing by large white-tailed deer populations. Through an approximately $1 million grant from the MDNR, Walters and his colleague, MSU AgBioResearch wildlife ecologist Gary Roloff, are leading a six-year project to develop a new, more sustainable model for northern hardwood forest management.

The team designed an experiment that compares single tree selection with three alternative silvicultural systems:

  • Seed-tree silviculture, in which all trees in a stand except a select few of diverse species are harvested. The remaining trees reseed the next generation.
  • Shelterwood silviculture, in which enough trees are cut to thin the canopy, allowing enough light to reach the ground for tree seedlings to grow but not so much that weeds and less desirable species flourish.
  • Gap-based silviculture, in which trees are harvested to leave larger multi-tree gaps of varying sizes in the forest, allowing more space for young trees to grow.

In addition to testing the effects of various harvesting methods on tree seedlings, the team is also looking at other ways to limit the impact of deer on regenerating trees.

“When you get concentrated numbers of deer in an area, they can significantly impact tree survival,” explained Roloff, associate professor in the MSU Department of Fisheries and Wildlife. “They love to browse a lot of important species, such as yellow birch and red oak, and when you have all these other factors already limiting those species, adding deer on top of that makes the perfect storm for failure.”

Using 250 motion-activated cameras, Roloff and his team are monitoring how deer interact with tree stands and finding ways to shift their attention away from the species the project is trying to protect. One experiment involves having loggers leave the top limbs and branches of harvested trees on the ground, creating a barrier that would shift deer to other areas of the stand.

The project encompasses 142 30-acre stands ranging from the western Upper Peninsula to the mid-Lower Peninsula and is being carried out in cooperation with both state forest managers and private industry. The team will enter each stand every two years, taking measurements and gauging both the regeneration and the diversity of the stands.

“The scope we’re operating at is unprecedented for this type of project,” Walters said. “We need it to be operationally accurate, however, so that the practices we test are realistic in terms of what companies and the state can apply.”

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