Nature Notes
Norway Maples May Look Healthy, But Be Rotten at the Core
By Patricia Martin

For a number of years, those of you who have faithfully read my column (and I hope there are at least a few) have heard about some of the environmental problems created by Norway maples (Acer platanoides). Norway maples are exotics that were introduced to Mackinac Island 70 or more years ago. Part of the problem with these trees is that they grow very rapidly and seed in profusely. In some areas of the Island, they've moved aggressively into the woods, out-competing our native trees. They also reduce the amount of light reaching the forest floor because of their dense canopy, affecting the flowers that grow on the forest floor, and remove necessary nutrients and water from the top layer of the soil. According to one dendrologist I spoke with, they also produce a chemical that inhibits the growth of other plants in the immediate vicinity.

Recently, I was rereading the pamphlet on Norway maples that was produced by those of us on the Environmental Advisory Committee of the Mackinac Island Community Foundation, and I realized that we had failed to mention another major problem with these trees: They can look healthy and be rotten to the core. This was aptly demonstrated a couple of weeks ago, when a large Norway fell on La Chance Cottage during a storm.

Norway maples have weak wood and are prone to rot. This Norway maple fell on the former La Chance Cottage during a recent thunderstorm. The trunk shows that the tree was rotting.

Early Friday morning, June 8, a towering Norway maple about 60 feet high with a diameter of at least three feet crashed into La Chance (now part of Harbour View Inn), primarily damaging the porches. This particular Norway was thought to be 60 or 70 years old. Tons of wood fell when that tree came down, and it's amazing that the damage wasn't worse. Upon inspection of the fallen tree, it was evident that the heart of the tree had rotted. Norway maples, because of their fast-growing nature, don't produce very dense or strong wood to start with, so when the middle of the tree goes, there isn't much to hold it up. I've had personal experience with the problem of weak wood in Norways. About 10 years ago, a branch from a 60-year-old Norway on the edge of my property fell on the gazebo next door (I since have removed all of the trees on my property), and a few years later a branch of one of my neighbor's Norways fell on my roof. Over the years, Norway maple trees, or at least their branches, have come crashing down on city streets, including Church Street or Cadotte Avenue for, example, and crews have had to remove the debris and wood.

Even with rot in the middle of the tree, it can "look" all right, producing a full complement of healthy leaves. This is because the center of the tree is "dead" or heart wood, which means that it doesn't have living cells that conduct water, nutrients, and food. The part of the tree that does the transport is near the outer surface of the tree and is known as "sap wood," as it contains the sap of the tree. In a sugar maple (Acer saccharum), this is the area in which you put taps to collect sap for making maple syrup. The main function of the heart wood is structural support for the rest of the tree, but if the heart wood is rotted, only the outer shell keeps the tree upright. If the sap wood isn't dense or strong, the tree can easily be broken in a wind.

Since the fall of the La Chance tree, people may have become concerned about the Norway maples near their homes or property, and may be wondering if they have potential problems. There are some clues that might indicate that there could be problems. I'll give you an example: There is a large Norway in the yard behind the Beaumont Memorial just down the hill from Trinity Church's parish hall. The first thing that you note is that the tree has been cabled together. This is an indication that some time ago, someone noticed that the tree was perhaps splitting at the crotch, and bolted or cabled it so that the weight of the branches would not further split the tree. There are also vertical cracks from pressure in the bark and wood of the tree, and on one particular limb of the tree, Pileated woodpeckers have been having a great time. The presence of the woodpecker holes indicates that there are probably carpenter ants living in the tree (the favorite food of the Pileated), which is not a good sign. This tree is a good candidate for removal, before it falls on one of the adjacent buildings. There are a couple of other indicators of trouble in these Norway maples. If other trees, shrubs, or plants are growing in the crotch of the tree, they first should be removed before they begin to split the tree, and they may indicate that there is some rot in the tree. If branches are falling, that may indicate problems. If the tree is near a road or construction site, where the roots may have been cut or compacted by heavy equipment, it's worth taking a look at the tree.

Remember that most trees have an underground root system that corresponds in horizontal size to the spread of the branches. Recently, the State Park removed a sugar maple in the middle of a sidewalk. It had been dying, branch by branch. On one side was a street, and on the other side was a building, so that its root system was blocked or cut, with the exception of about one-quarter of its natural growing area. They thought that they might have difficulty removing the stump, but because of its limited root system, it popped right out with one tug. There was little holding this tree up.

Another way to detect problems is to use a machine that was recently developed, which can help determine the amount of rot or hallow in a tree. It's called a shigometer, and I'll discuss it in more detail in a later column.

It's expensive to remove and replace the Norway maples that many people have growing near houses or other buildings. I know that the removal and disposal of the two large ones on my lot, because of the location and the difficulty getting at them, cost me several thousand dollars. It is sometimes difficult to get other trees to grow after a Norway has been in the area, and the influence of these maples on the soil may be felt by other plants for up to 10 years. It's a hard decision to remove these large trees, because of the cost and the aesthetic change of removing them, but when considering the cost of damage done to buildings and property, not to mention the environmental impact of the distribution of these trees into our forests, it might behoove people to consider it.

Trish Martin is a yeararound resident of Mackinac Island, has earned a master's degree in botany from Central Michigan University, and owns Bogan Lane Inn.