Looking at forests in a new light
New light shed on importance of tree diversity.
Michigan spans 36.3 million acres, and more than half of that is forestland. Michigan’s state-owned forest system, managed by the Michigan Department of Natural Resources (DNR), is nearly 4 million acres — larger than any other state-owned system in the United States. In addition to the recreational, tourism and indirect economic benefits derived from this natural resource, Michigan’s forests support more than 13,000 timber/wood product jobs and more than 2,200 companies.*
Michigan forests, however, have unique problems. Michigan State University (MSU) AgBioResearch forest ecologist Mike Walters studies factors that can have negative impacts on forests, especially the nearly 7 million acres of northern hardwood forests that thrive in Michigan, primarily in the Upper Peninsula (U.P.) and northern Lower Peninsula.
“The legacies of past forest harvesting practices and high deer populations have combined to contribute to low tree diversity and the perpetuation of just a few tree species that northern hardwood forests support, and some of these are undesirable for management,” explained Walters, an associate professor in the MSU Department of Forestry.
Hardwood forests have been predominantly managed by a single tree selection system. Every 10 to 20 years, there are partial harvests of individual trees dispersed through the stand. This process creates small gaps in the overstory, the uppermost layer of the forest foliage forming the canopy. The assumption is that natural regeneration of species desirable for management will fi l the tree voids.
Walters has found, however, that natural regeneration often does not occur. And when it does, he found that only a couple of species — oftentimes ones undesirable for management — regenerated.
“Northern hardwood forest can support 20- plus species, but now trees regenerating in these forests are dominated by sugar maples and some beech and ironwood,” explained Walters, who has an appointment with Partnerships in Ecosystem Research and Management between MSU and the DNR. “Some species that were formerly abundant, such as hemlock, white pine, yellow birch and paper birch, have declined markedly.”
Walters and his colleagues are identifying factors that reduce tree diversity and searching for practicable ways to restore it. By integrating information from a 1-million- acre natural experiment in the U.P. and a multistand, manipulative experiment in the northern Lower Peninsula, Walters has found that, for seedlings to become full- grown, they need to escape being eaten by deer and emerge through competing vegetation, such as raspberries and elderberries.
“Deer love tree seedlings, and it’s no coincidence that tree regeneration is being dominated by ironwood and beech, because deer browse these species only when nothing else is available,” he said.
A blend of tree species is ideal.
“A greater diversity ought to make the forests more resilient to disturbance. For example, currently 70 percent of the trees in the northern hardwood forests where I work
are sugar maples. If the Asian long-horned beetle, which is already in parts of the U.S., comes to Michigan and attacks sugar maples, it could take out large portions of our forests,” Walters said.
A more diverse mix of trees would also provide an economic buffer against changes in market demand. Increasing forest diversity is challenging, however, for several reasons:
- There have to be nearby adult trees of the species you want to increase to supply seeds, and some of the species needed to increase diversity are rare in the forests.
- The forest floor lacks a good substrate for seedlings to grow on. Because of forest management practices, few trees die, fall over and rot, and gentle logging practices expose little mineral soil surface, both of which make good substrates.
- The low light created by small forest harvest gaps allows only the seedlings of shade-tolerant species to thrive.
- Managing the deer populations in the forests is difficult — there are no readily available solutions to the problem of deer browsing seedlings.
In spite of these challenges, Walters and DNR and industry foresters with whom he works have some ideas, including changing the single tree selection harvesting method on sites where it is not working. They also recommend methods that provide for larger openings in the forest overstory, allowing for more light to reach seedlings.
“Scarifying” or exposing the mineral soil on the forest floor to allow the seedlings of smaller seeded species (e.g., birches) to grow is another recommendation. Walters said this could be done by promoting tree harvests in the summer, especially in areas where seeds of desirable species are available.
“The other thing is that seed is cheap,” Walters said. “You could probably reforest all of Michigan with one big bag of paper birch seeds, so perhaps broadcasting seeds or planting seedlings will work on some sites. All of these are viable options.”
*Source: Michigan Department of Natural Resources