Monitoring the health of Michigan’s forests by air and on land

Oak wilt – an invasive, exotic disease – can be spread in infected firewood.
(DNR photos)
Forestry is a relatively young science in the Lake States. Native insects and diseases influence the forest
differently as the forest grows older, as the forest succeeds to different mixtures of trees, and as climate extremes – droughts and warming trends – change the playing field.

The Michigan Department of Natural Resources' Forest Health program monitors the occurrence and impacts of both native and exotic forest insects and diseases. Understanding the impacts of pests and other stressors helps resource managers make short-term and long-range plans to keep Michigan’s forest ecosystems functioning sustainably and productively.

“We spend time flying over the forests looking for problems like defoliation, discolored leaves and tree mortality,” said DNR forest health specialist Dr. Robert Heyd. “We spend time evaluating the seriousness of problems detected from the air or reported by forest resource managers and the public.

“Mostly what we see as we fly Michigan’s 20 million acres of forest land are healthy forests. Occasionally we have widespread outbreaks like the forest tent caterpillar and gypsy moth defoliation of hardwoods in the northern Lower Peninsula, which lasted for several years and ended in 2011.”

Different tree species have different site requirements (e.g., soil, moisture and climate). Trees growing on suitable sites are healthier and less likely damaged by native pests.

“When our forests are stressed by events like extended droughts, trees occurring on less-than-optimum sites, and trees that are toward the end of their natural life, they are most heavily impacted,” Heyd said. “Impacts include declining health as evidenced by branches in the upper crown dying and discoloring. These weakened trees are more susceptible to ‘secondary pests’.”

Large beech trees killed by beech bark disease were
removed along a trail in Tahquamenon State Park
to ensure the safety of park visitors.
Heyd explained that secondary pests are those that only impact weakened trees. Examples include bark beetles of pine and root rots of hardwoods. Healthy pines produce pitch such that when bark beetles try to chew through the bark, they are “pitched-out”. When pines are weakened, they produce less pitch, allowing bark beetles to successfully attack the tree. In hardwoods, healthy trees produce chemicals and stored energy such that root rots cannot invade roots. But stressed trees have less energy for such chemicals and are successfully invaded.

It takes a few years of normal rainfall for trees to completely recover energy reserves and corresponding defenses to pest attacks.

“Adding an inch of water to the rooting zone of yard trees during periods of drought is the single most important way to keep the trees healthy and resistant to native pests,” said Roger Mech, DNR forest health specialist. “Of course, exotic pests like oak wilt and the emerald ash borer attack both healthy and weakened trees.”

DNR forest health specialist Roger Mech and DNR pilot
Bill Green take aerial photographs to record and assess
the severity of impacts of forest health events.
The DNR also recommends keeping the following in mind:

If planting a tree or thinning a woodlot, it is important to match selected tree species to the planting site. Be sure that the tree selected actually grows in that area and that is is matched to the site in terms of soil and the availability of sunlight.

In a woodlot, promote trees that are best adapted to the site, are growing vigorously and have no serious pest problems.

It is always best to seek the assistance of a consulting forester when managing a woodlot.

In addition to native pests, people continue to import exotic insects, diseases and plants, some of which greatly influence the function and appearance of forest ecosystems. Although there are quarantines preventing the introduction of pests from other countries, inspecting the approximately 300 million shipping containers received annually is a daunting task.

Monitoring insect populations using traps, such as this
one being emptied by DNR forest health specialist Roger Mech,
allows entomologists and managers to decide when control
may be needed, and what controls are most appropriate.
Well-known examples of exotic pests include Dutch elm disease, chestnut blight, beech bark disease, the
emerald ash borer, oak wilt and the gypsy moth. Once introduced, imported insect and disease problems continue to move around the country in firewood and nursery stock. Exotic plant species such as purple loosestrife, spotted knapweed, garlic mustard and buckthorn are examples of exotic plants that can disrupt native ecosystems.

During the growing or bug season, forest health specialists said they are busy answering daily inquires like: "What's killing (or feeding on) my tree?" or "What bug is this?"

“Answering these questions serves many purposes,” Mech said. “Understanding that a condition or pest is not damaging is as important as knowing what to do about serious pests. Knowing that an insect isn't really harmful creates peace of mind, improves environmental awareness, saves the caller the expense of treatment, and eliminates or reduces the unnecessary use of pesticides.”

When not surveying Michigan's forests or evaluating the impact of a current pest situation, DNR forest health specialists work with state, federal, private and industrial resource managers. Helping natural resource managers recognize and manage current and long-term forest health problems is an important part of the job. News releases, reports and a Forest Health program website ( are used to inform and educate professionals and the public.

The DNR’s Forest Health program monitors the long-term health of Michigan's forest resource by using a network of permanently established survey plots that are monitored over time. This helps to detect more subtle changes in forest condition, growth and productivity.

Entire hillsides are defoliated by the exotic, invasive gypsy
moth, such as shown here in Barry County. DNR forest health
specialists monitor these events and take management actions,
as appropriate, to conserve the forest resources.
Monitoring the health of Michigan's forest has been and continues to be greatly enhanced by advances in
computer-aided data analysis, navigation, mapping and image-processing technologies. So, as in many professions these days, there is a vital place for the application and advancement of technology.

“All of these tools help us meet the Forest Health program goal of keeping forest ecosystems functioning well over long periods of time to provide resilience to short-term stress and adaptation to long-term change,” Heyd said. “The health and sustainability of Michigan’s forests are vital to ensuring this natural resource can be enjoyed and used by current and future generations.”

For more information about the DNR’s Forest Health program, or to read the 2012 Forest Health Highlights Report, visit