Lake lusters

Natural History, Feb, 1997 by Robert H. Mohlenbrock

Thousands of lakes of all sizes dot the Upper Peninsula of Michigan. Some are quite muddy; others are fairly clear, with an organic bottom built up from the remains of plants and animals. The clearest lakes, however, have a sandy bottom. Among these is Wolf Lake, which lies in the heart of Hiawatha National Forest. An important feature of this Wolf Lake--there are other Wolf Lakes in Michigan--is the presence of various plant species typical of the Atlantic Coastal Plain of the United States.

I visited Wolf Lake in early August, when I knew most of the plants would be flowering or fruiting (grasses, sedges, and rushes are otherwise hard to identify). Forest Road 2696, the closest access to the lake, was a narrow cut through a forest of jack pines, scattered red pines, and such broad-leaved trees as red maple, quaking aspen, and white birch. Through the trees, I could see water gleaming in the middle of a thirty-acre opening.

The forest was bordered with shrubs, including two species of Spiraea--one with pink clusters of flowers, called steeplebush or hardhack, and a white one known as meadowsweet. Beyond this transitional zone, the terrain sloped imperceptibly toward the lake. In the early spring, as a result of snowmelt and heavy precipitation, water nearly covers the open area. As the weeks pass, the water gradually recedes, and the lake is ringed with vegetation. At the outer margins, where the soil is first exposed, tall grasses, sedges, and wildflowers predominate. Smaller plants that flower and fruit more rapidly lay claim to the inner circles.

I first passed through a zone of twig rush, two-foot-tall plants with reddish brown clusters of flowers. I then entered a green band, perhaps twenty feet wide, containing a mixture of plants six to twenty-four inches high. Ringing the lake next was a bright orange-red swath about fifteen feet wide, with plants no more than six inches tall. The soil was very soggy, covered here and there with a film of water. The orange-red color was due to an abundance of fruit-bearing bulblet rush. At the edge of the lake, where perhaps an inch of water covered the sand, grew a carpet of hatpins, two-inch-tall plants aptly named for their tiny spherical heads of white flowers. Farther in, the lake was still shallow enough for me to wade across. Peering through the clear water, I could see submerged plants growing in the sand.

Some biologists term Wolf Lake a coastal plain lake because the plants growing in and around it include species mainly found in the Atlantic Coastal Plain east of the Appalachians. Examples are twig rush, water hyssop, Tuckerman's panic grass, and hatpins. Naturalist-writer Donald Culross Peattie called attention to such coastal plants in the Great Lakes region in 1922. Subsequent studies found coastal plants most prominent in the southeastern Georgian Bay region of Ontario, in the sandy plains near Lake Michigan in southwestern Michigan and northern Indiana, and in sand deposits in Wisconsin. Lesser concentrations appear near Lake Erie, in north-central Illinois, and in the Upper Peninsula of Michigan, where Wolf Lake is located.

Botanist Anton Reznicek has found that the coastal plain plants live primarily in sandy, soft-water ponds and small lakes with fluctuating water levels. The plants are common during years when the water level is low. When the level remains high, they survive primarily as seeds in the soil under water.

Peattie suggested that the coastal plain plants gradually spread to the Great Lakes along the shores of lakes and streams that formed when the last ice-age glaciers melted, about 10,000 years ago. These would not have provided an uninterrupted path for migration, however. Another suggestion has been that the plants spread by random, long-distance dispersal, probably by birds. All the plants have succeeded in migrating to the same far-inland areas, however, a coincidence that is hard to explain if they were dispersed randomly. Reznicek takes an intermediate view, suggesting that the coastal plants migrated into the Great Lakes regions along postglacial drainages, taking short to medium jumps to suitable habitats along the way, probably with the aid of birds.

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The outer zone around the lake consists mostly of a green swath dominated by Euthamia remota, a grass-leaved goldenrod. Named for their very narrow leaves, most grass-leaved goldenrods grow in moister areas than other goldenrods, and their clusters of flowers are usually flat topped rather than elongated. Other plants include a pinkish flowered mint known as water hyssop; a tiny white-flowered mint called water horehound; and a small, round-fruited grass called Tuckerman's panic grass. The twig rush that rings this green swath is actually not a rush but a sedge, Cladium mariscoides.

The inner zone receives its orange-red color from the fruit crowding the stems of bulblet rush. This is one way the rush propagates. But many of this plant's flowers, instead of forming pollen grains and ovules (immature seeds), develop into tufts of tightly rolled plant tissue, called bulblets, which fall to the ground and sprout directly into new plants. Among the small plants growing inconspicuously alongside the bulblet rush is a delicate buttercup, called creeping buttercup, and slender water milfoil, a plant that bears little resemblance to the water milfoils popularly used in aquariums. Most water milfoils have leaves divided into many threadlike segments, and rather obscure flowers and fruits. Slender water milfoil consists of erect stems nearly devoid of leaves; near the top of each stem a few four-petaled, purple flowers appear, which eventually give rise to small nutlets. Slender water milfoil may grow submerged or stranded on land.