Nature Notes
Island Continues Battle With Invasive Norway Maples, Periwinkle
By Patricia Martin

So far this year, I've not mentioned any of the invasive species at Mackinac. This is not because the problems with some of our alien species have lessened, but because I didn't want to keep "preaching to the choir," but then again, perhaps I should. Just to refresh your memory, alien or exotic species are ones that have been introduced to an area and that have an unfair advantage over native species in that habitat. Native species have their own set of predators and diseases that keep their populations in check, but exotic species often are unaffected by those controls and so are free to dominate the habitat, crowding out the native species. Two plants in particular that I would like to mention in this regard are the Norway maple (Acer platanoides) and periwinkle/myrtle (Vinca minor).

Myrtle is a plant, that though originally native to northeastern United States, is not a native to Mackinac Island. It is a lowgrowing plant, usually less than four inches in height, that likes to hug the ground. It is a broadleaved evergreen with dark, green, glossy, oval leaves, which form a low dense, decorative ground cover, and is especially popular for shaded areas. The flower is a decorative, starshaped, five-petaled blue blossom that appears in early spring and continues for several months. According to one of my garden books, it prefers moist, well-drained acid soil in shade or full sun. This surprises me, as most of the soils on Mackinac are pretty basic owing to the limestone rock, and yet the myrtle grows well here. I do admit, however, that some of the places that it grows best are under northern white-cedar, whose needles will acidify the soil to some extent. This plant is further described as a "good weedsmothering ground cover, particularly in deep shade where other ground covers will not grow." This weed-smothering capability makes this non-native a rather dangerous invasive when it escapes into the woods, where it chokes out the native wild flowers. It spreads by vines that creep out from a central plant, gradually, until it forms a dense mat. On the Island, it's found in abundance near Anne's Tablet, near the cemeteries in the adjacent woods, and in isolated patches along Morning Snack, Beechwood, and Soldier's Garden Trail, among others, where it quietly expands inch by inch, year by year.

In May, a group of boys who are part of the Christian Service Brigade, lead by Dave Card, worked with Jeff Dykehouse, the naturalist at the state park, to remove "hot spots" of myrtle near Great Turtle Park.

Contrast this with the Norway maple, a large tree from Europe, which has been widely planted in North America, particularly in the 20th century to replace many of the elm trees that were lost to Dutch Elm disease. These trees are fast growing, heavy seed producers that grow easily in many soil conditions and habitats and are quite disease-resistant. They have milky sap and have leaves with five to seven lobes, which are large and flat. The buds are rounded with only a few bud scales, and their seeds have wings (samaras), which diverge widely. (As kids, we used to call them helicopters).

In a paper written by Steven Wangen and Christopher Webster, published in the Journal of Applied Ecology, the growth and expansion of the Norway maple on Mackinac was discussed. These trees were brought to Mackinac in the first half of the 20th century, and after a lag time of about 34 years (which is the time it takes for these trees to mature enough to begin to produce seeds), they began to increase in the area they occupied at a very fast rate of about 5.6 hectares per year for about 35 years, when the rate began to slow in the 1990s. Numerous satellite populations were established during the expansion period. The initial expansion of the Norways in the forest of Mackinac began on the south end, where they were planted, and radiated northward from there. Trails and roads provided important corridors for seed dispersal from the developed areas around the harbor. The hauling of yard waste and manure to the solid waste disposal site in the center of the Island probably helped in the establishment of these isolated satellite populations. It is thought that the expansion rate has slowed, perhaps because the best areas near town have already been used, but there is a concern that because of the lag time for the trees to develop, the satellite populations at places like British Landing, the old Battle Field, and others will begin their own expansion as the trees in these areas mature and begin to produce seed, so the expansion does not appear to have halted.

The problem with the invasion of the Norway maples is that they could out-compete our native trees, and could eventually produce a monoculture of

Norways, instead of the mix of different deciduous and coniferous tree species in some areas of the Island. In addition, because of the shallow root system and the dense canopy, these trees could eliminate or reduce the number and kinds of plants growing beneath them.

Efforts are being made to deal with these invasive species. Private landowners are removing Norways and trying to prevent the spread of garden plants like the myrtle into Mackinac's forest. Greater understanding of the problems these invaders pose is being gained through the studies of Mackinac that are being done by the School of Forest Resources and Environmental Science of Michigan Technological University. In May, a group of boys who are part of the Christian Service Brigade, lead by longtime Island friend, Dave Card, worked with Jeff Dykehouse, the naturalist at the state park, to remove some "hot spots" of myrtle near Turtle Park, and removed small Norway maples in an area near Cass Cliff along Huron and Arch Rock roads. Recently, the state park was granted money from the environmental funds (the Erin Shufelt and the Natural Resource Preservation Funds) of the Community Foundation to help pay for the removal of some of the Norway maples in some of the satellite areas on the north end of the Island. If anyone wants to help, you can do so by removing Norway maples from your property, and make sure seeds and clippings from exotic species grown in your garden do not spread. Try not to grow plants which are known to become invasive, particularly if you live near the woods. Donate money to one of the environmental funds, which help support the efforts to control invasive species, and educate yourself and others about the problems caused by exotic species.

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