By Troy Holcomb, DNR Cooperative Forest Management Forester for Aitkin County
Minnesota’s forests truly are an amazing and dynamic organism. It is so easy to forget that the tree in our front yard or the forest zooming past our windshield are part of a complex and connected ecosystem.
A constant part of that ecosystem is change. Forests are always changing - they always have and they always will. One of the biggest changes people notice about their trees or woods is when they seem unhealthy. With the current summer heat and drought on everyone’s minds, it is a good time to consider forest health.
Even with recent rains, most of Minnesota is in the middle of a severe drought. Couple this with above average temperatures during most of the summer, and trees can become stressed. The signs of drought-stressed trees are often delayed and may not show up until late summer or early fall. Trees will also remain stressed for several years after a drought. If you start to notice wilting, yellowing, or loss of leaves in the upper branches of a tree, you may be seeing the signs of drought stress.
Image of drought-stressed pagoda dogwood. Photo Credit: Jennifer Teegarden.
Drought stress alone is usually not enough to kill a tree. What it does, however, is predispose a tree or woods to a variety of insects or diseases that could lead to tree death. For example, many people are aware of forest tent caterpillars, or “army worms.” These native caterpillars have population peaks about every 10 years. On their own, the defoliation forest tent caterpillars cause won’t kill a tree. However, as their populations increase in the next few years, and coupled with drought stress, we may see mortality on the aspen, basswood, and other hardwood trees they target. Two-lined chestnut borer is another pest we may see more of after this drought. This native wood boring beetle will kill upper branches of bur and red oak trees. Two-lined chestnut borer is always present in our woods, but when trees have reduced vigor, we will see increased damage to our oaks. Again, these native pests have evolved with our trees for millions of years. Native tree pests are an important part of our forest ecosystem as they create gaps in the tree canopy for new trees to grow, provide dead standing trees for wildlife habitat, and decomposing vegetation for nutrient recycling.
Some non-native pests can also cause harm to our forests. Non-native pests are more disturbing because they have not evolved with our ecosystem. Because they are from a different continent, in some cases, few, or sometimes no natural enemies control their populations. Our local trees have also not evolved natural defense mechanisms to prevent or minimize damaged caused by a non-native pest attack. White pine blister rust, oak wilt, emerald ash borer, and Dutch elm disease are well-known examples.
So with all these potential negative impacts on our forests, what can people do to lessen the blow? In forested areas, reduce competition between trees by thinning areas that have become too dense. This allows the remaining trees to be healthier and less susceptible to insect, disease, and environmental stress. When planting trees, choose a tree that will grow well in future growing conditions. To help take care of trees in your yard, water the soil under the entire canopy of mature trees with a slow trickle of water using a soaker hose for several hours once a week when we are in a drought. For trees planted with the last five years, provide 25 gallons of water when it has rained less than an inch in the past week (using a water bag is the easiest way to water a young tree). The best time to water is in the early morning or late at night to conserve water. Make sure you check with your local municipality for any watering restrictions. Another way to keep trees healthy is to avoid creating additional stress on the tree. For example, don’t compact, add or remove soil around trees, and avoid pruning trees. Also, lawn herbicides and fertilizers can cause stress on a tree. For your forested property, consider developing a management plan with a forester to make sure you are keeping your woods healthy.
Trees and forests are so easy to appreciate that it can seem like a bad thing when they get sick or die. As you follow these tips to keep your trees and woods healthy, consider that forest health issues have always been part of the forest ecosystem.