Exotic forest plants

Exotic pests are one of the more serious threats to forest health. While few exotics pose serious challenges, there are some insects, diseases and plants that do. Eradication is possible if small infestations can be detected early.

Glossy buckthorn prefers moister sites near wetlands while common buckthorn is a more upland invader. Photo credit: Bill Cook l MSU Extension

Glossy buckthorn prefers moister sites near wetlands while common buckthorn is a more upland invader. Photo credit: Bill Cook l MSU Extension

Over 40,000 exotic species have been introduced into North America, and most have proven to be useful to humans and benign to the natural environment. Most exotic introductions fail to establish, but some have grown invasive and damaging.

According to Michigan State University Extension, a large number of exotic plants cause problems in our forests, lakes, wetlands and other habitats. The forest has a fewer number, due to the inherent characteristics of most exotic plants, but their presence is important and widespread across much of Michigan.

Successful exotic plant invasions are often facilitated by other plant and animal species that cause disruptions of forest ecology. This pattern is sometimes called a “cascade effect.” Interestingly, eradication of the exotic plant will not necessarily restore earlier conditions. Garlic mustard is one such exotic plant.

Garlic mustard does best when white-tailed deer, slugs (exotic), earthworms (another exotic) and other forest components produce this cascade effect. When garlic mustard is successfully eradicated, restoration of the forest floor still fails.

Garlic mustard tends to dominate the forest floor over time, helping to preclude regeneration of other plant species. It prefers richer, mesic soils that also support northern hardwood forest types. Viable seeds can persist in the soil for many years.

Glossy buckthorn and European or common buckthorn are shrubs that occupy more open woodland sites and can quickly colonize after a timber harvest. Glossy buckthorn prefers moister sites near wetlands while common buckthorn is a more upland invader. Both can overtake habitat by displacing, sometimes almost completely, native plants. The attractive fruit is spread by birds but provides little nourishment.

European Buckthorn

European Buckthorn. Photo credit: Bill Cook

Buckthorn is an alternative host for certain agricultural pests and diseases. The shrubs, along with other exotic species (especially earthworms), can alter soil conditions that favor buckthorn and work against many native species. Contrary to what the name suggests, the species has no benefit to white-tailed deer.

There is a native species of buckthorn; therefore, identifying the species correctly is important.

Autumn olive, once recommended as a wildlife planting, is an aggressive invader of old fields. Dense stands with sharp false thorns are daunting. Flowers are fragrant and distinctive, easily caught in the spring air. The red fruits are tart and tasty. The foliage often has a green-gray shade. With practice, one can easily identify the structural form.

Autumn Olive

Autumn olive. Photo credit: Bill Cook

Various species of exotic honeysuckle are yet another invasive shrub group, a common sight in many residential areas and urban parks. Like other exotic shrubs, honeysuckles can be part of the “cascade effect” that leads to semi-permanent habitat degradation. Watery berry-like fruits are orange to red, depending upon the species. There are also native honeysuckles but they do not reach shrub size.

Exotic honeysuckle

Exotic honeysuckle. Photo credit: Bill Cook

Lastly, Japanese barberry is a common problem in much of southern Michigan and certain areas in the rest of the state. The ornamental shrub is a landscape favorite but an enemy of the forest. A small-leaved plant, with red berries and sharp spines make it reasonably easy to identify.

Japanese barberry

Japanese barberry. Photo credit: Bill Cook

None of these exotic species are particularly palatable to deer, so they have a competitive advantage over many native species in areas where browsing pressure is high. Each of them has reproductive strategies and environmental tolerances that have contributed to their success.

More information about these, and other, exotics can be found from either the Michigan Natural Features Inventory (MNFI) or the Midwest Invasive Species Information Network (MISIN). The MNFI has a good field guide. None of these species are likely to be eliminated from Michigan’s landscape; however, the populations might be managed to minimize their impact, if research-based strategies are used with ecosystem restoration in mind. Sometimes, simple removal will not be enough.

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