What Makes a Healthy Forest?
Guest Feature by Denny Eilers
Most Midwest forest owners and managers turn that question around and ask it like this: “What makes a forest unhealthy?”
Jacque Holloway, a private landowner in Linn County, Iowa, says the number one problem in maintaining a healthy forest is invasive species. Jacque and her husband Denny manage 40 acres of hardwoods, and are the major sponsor of Trees Forever’s Our Woodland Legacy program.
“We have a horrible infestation of garlic mustard,” she states. “When spring flowers come up, garlic mustard just crowds them out.”
The Holloways started with 20 acres of timberland 35 years ago and have added another 20 acres since. Their stand is a hardwood mix of oak, hickory and maple they manage for natural beauty.
“Oriental bittersweet is also a problem, as it can climb into the canopy and get so heavy it pulls trees over,” she adds. “The vines get as large as your wrist.” While invasive species like garlic mustard, and insect pests like Emerald Ash Borer (EAB), threaten the health of Midwest forests, there are management steps that can fight these attacks and help move a forest back to the healthy side.
Dr. Michael Jeffords, entomologist at the University of Illinois, says a healthy forest has four distinct layers:
• Ground layer
• Herbaceous layer (where the wildflowers are)
• Shrub layer (understory for replacement trees)
• Trees in canopy stage
“If a forest has this structure, it’s less likely to be invaded by exotic species,” he states. As a landowner, you also need to look at your forest in terms of tree size and diversity, Jeffords advises.
“With a forest all the same age you’ll be more likely to end up with invasive species. Also, make sure dominant trees in the canopy are reflected in the understory. For example, if it’s an oak-hickory forest, and the entire understory is maple, you’ll have issues in getting oak regeneration.”
Dr. Jeffords adds that bush honeysuckle, kudzu, Russian olive and Bradford pear are becoming invasive problems in parts of the Midwest, along with garlic mustard and Oriental bittersweet.
“An invasive species will overtake and crowd native species,” he reports. “An invasive normally doesn’t have natural enemies and food chain organisms don’t recognize it.”
The Holloways credit Trees Forever and its volunteer force for help in fighting invasive species. “They organize volunteer groups to help rid public lands of unwanted species,” says Jacque Holloway. Trees Forever strives to be an advocate and educator on the benefits of trees in both urban and rural forests.
When looking at forest health, Paul Tauke, Forestry Bureau Chief for the Iowa DNR, divides tree stands into urban and rural.
For urban forests a diversity of tree species is essential for long-term health, he notes. “Typically, you don’t want more than 10 percent of any one species and no more than 30 percent of any one genus,” he explains.
“For example, for the oaks you could have 10 percent each of red oak, white oak and pin oak, but you’ll never want more than 30 percent in the oak family.” Tauke says the growing problem with Emerald Ash Borer supports this plan. “There will be a big impact on communities where 10 to 50 percent of tree resources were planted to ash,” he says. “When you get a pest like this you’re potentially wiping out that tree species from your urban forest. The more diverse the urban forest is, the healthier it will be and the easier it will be to combat pest problems that come up periodically.”
In a rural area, Tauke notes the forest should reflect the landowner’s goals. “The landowner, or manager, can contact his or her district forester [or a consulting ecologist or forester] and walk the forest together,” he suggests. “The forester can point out what’s going on in the stand and present options for the future; for example, what the forest can look like 5, 10 or 20 years down the road.” The key, Tauke says, is to develop a management plan for improving your forest and use it as a guide to keeping it healthy for the long term.
A healthy forest is a complex organism. No one thing can tell you if a forest is healthy. There are many factors that when added together showcase a healthy forest. These include:
• Four distinct layers (see article)
• Absence of exotic or invasive species
• Natural regeneration of hardwood trees
• Diverse plant species like those above
• Lack of gully erosion in ravines