Even the healthiest of woodlots can harbor dangerous zombie trees. Credit: Julia Bayly / BDN

Like the human versions, zombie trees appear living when they are actually dead. Unlike human zombies, zombie trees are real and potentially dangerous.

According to tree experts, a tree that otherwise appears perfectly healthy can be hiding severe structural damage. And that damage can make a tree a danger to any people or property around it.

One of the major factors that can create a zombie tree is stress. Stress management is as important for tree health as it is for human health, according to Allison Kanoti, state entomologist with the Maine Department of Agriculture, Conservation and Forestry.

“Sometimes these stressors are obvious and sometimes they are harder to determine,” Kanoti said. “We all know if we can help manage our own stress we can manage our health and the same is true for trees.”

In Maine one of the recent major stressors on trees has been the ongoing drought conditions that have severely limited the moisture trees need.

“Trees are not fleeting residents on the landscape,” Kanoti said. “They can’t get up and move so if they experience drought conditions they can expect root damage.”

At the same time, too much rain can also damage tree roots.

Defoliation from insect infestations, along with human activities including construction, chemicals, road salt and soil compaction can also stress trees. All that stress leaves a tree vulnerable to damaging parasitic activity that further weakens its structure. When a tree or its branches become too weak to support itself, a strong wind or snow load can break or topple it.

“If it’s out in the forest there is really no risk, but you don’t want them to be in places where they will hurt someone,” Kanoti said.

The trick, according to William Livingston, associate professor of forest resources at the University of Maine, is recognizing a zombie tree before it can do any harm. That can take a trained professional.

“When the foliage is on the trees they will look for discoloration of leaves or needles at the top of trees that can be an indication of root damage,” Livingston said. “Zombie trees will also attract insects so they will look for any signs of insect activities like holes in the bark and evidence of sawdust at the base of a tree.”

Once a tree shows those signs, there is no way it can be brought back to life. But if caught early enough, some trees can be saved.

“I had a neighbor with a large white spruce that was losing needles,” Livingston said. “I told him his tree needed water and that helped it.”

A tree can actually be getting along just fine but have significant internal decay, according to Livingston.

“The tree is alive, but it has so much decay and has lost so much internal wood that the ability to support itself has become compromised,” he said. “Those are the trees that will blow over in a windstorm or lose branches.”

All a tree needs are leaves to get to the sun and the roots to pull nutrients from the soil, according to Kanoti. As long as it has those, the wood in the middle of the tree contributing to its overall structure can become quite thin or nonexistent.

Zombie trees can fool people into believing they are just fine especially when there are no obvious injuries or damaging events.

“One indicator of decay are injuries to the tree that do not heal,” Livingston said. “But that can take years to develop.”

Sometimes, Kanoti said, a tree will start to show stress due to events that took place dozens of years in the past.

“It can be hard to know if a tree is going to be a problem if you don’t know its history,” she said.

The growth of certain fungal species on or below the tree is a sure sign of a zombie, Kanoti said.

“One of the more famous mushrooms is the armillaria fungus. The fruiting bodies for this fungus are the honey mushrooms and if they are growing at the base of a tree, it’s almost a sure thing the fungus is on the tree and weakening its structure.”

Conchs or brackens — those hard, shelf-like fungal growths on the trunks of trees — are also indicators of tree decay.

The United States Department of Agriculture has a publication that can help identify the signs of a zombie tree. It’s available online or as a hard copy through the Maine Department of Agriculture, Conservation and Forestry.

But with so many factors contributing to a tree becoming a dangerous zombie, Livingston said it’s a good idea to call in the experts.

“Find a good arborist,” he said. “They know these trees very well and can take a look and make recommendations.”

Some licensed arborists are certified for tree risk assessment, Kanoti said, and have undergone special training to detect problem trees.

At the same time, these dead and dying trees in the forest are a crucial part of the landscape. They provide food and shelter for wildlife and contribute to the building up of soil on the forest floor when they fall.

“From the standpoint of wildlife, these trees are functioning the way they should,” she said. “Those trees are important to wildlife and are of high value to wildlife, but not so much if they are over your kid’s swing set.”

Julia Bayly

Julia Bayly is a reporter at the Bangor Daily News with a regular bi-weekly column. Julia has been a freelance travel writer/photographer since 2000.