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On a weathered sandstone cliff sculpted by wind and the waters of the Kickapoo River, a half-dozen tenacious plants cling to a damp, narrow cleft in the sheer rock face. Their tough, woody stems bear waxy, evergreen leaves and, in May, sprays of vibrant magenta flowers. The plant's scientific name is Rhododendron lapponicum, but it goes by the more common title Lapland azalea. It's Wisconsin's only native member of the plant group containing the rosebays, azaleas and rhododendrons.
What's remarkable about these small, scraggly shrubs is not their beautiful flowers or their ability to grow in such a precarious place, but that they're found in Wisconsin at all. The Kickapoo's Lapland azaleas are far from home – nearly 900 miles away, in fact! They're what plant geographers call a "disjunct" species – a plant growing in a natural setting far distant from its home range. The Lapland azalea's primary range circles the globe in arctic and alpine regions of the northern hemisphere (including Lapland) and in the higher elevations of the Adirondack Mountains. Only two other small populations of this plant are known to exist in North America outside of its main range – one in the Dells of the Wisconsin River (where it is protected in a DNR-owned State Natural Area) and the other in a northern Minnesota cedar swamp.
Wisconsin is home to a number of other interesting disjunct species, two of them ferns. Growing under a canopy of red maples and towering white pines in the moist, peaty soil of Jackson and Monroe counties is the Massachusetts fern, Thelypteris simulata.
As its common name suggests, it is a plant of the northeastern U.S., ranging from Nova Scotia south to Virginia.
The nearest population is in central Pennsylvania. In our state, it typically grows with a variety of other ferns, clubmosses, sphagnum and sedges. Look for Massachusetts fern in the Jay Creek Pine Forest State Natural Area and in the Black River State Forest.
The other disjunct fern is lobed spleenwort (Asplenium pinnatifidum), a tiny species with fronds only three or four inches long. It is "epipetric" meaning it grows only on rock, in this case sandstone. In Wisconsin, lobed spleenwort has been found in the cracks and crevices of sandstone cliffs and pillars in the driftless area of Iowa County. Lobed spleenwort grows only on very hard, well-cemented sandstone and seems to prefer the sunniest, driest, and harshest conditions on a cliff; crustose lichens often are the only other species found growing alongside this hardscrabble fern. Lobed spleenwort's home range is far to our south and east, with the nearest population nearly 400 miles away in southern Illinois.
The low, flat bed of glacial Lake Wisconsin dominates the central sands landscapes of Adams, Juneau, Sauk, Monroe, Jackson, Portage, and Wood counties. In this region, small shallow softwater ponds and lakes with widely fluctuating water levels mimic the conditions of the marshes and bogs along the Atlantic coastal plain, the area of the eastern U.S. between the coast and the piedmont.
Growing among the sedges, rushes and other common wetland species is an intriguing array of disjunct plants commonly found on the coastal plain, but rarely inland. They're adapted to damp, sandy shorelines that are exposed during low water levels and then periodically flooded. These species survive by reestablishing themselves from seeds buried in the soil when it is exposed during dry periods.
Among Wisconsin's Atlantic coastal plain disjuncts are the bald rush (Psilocarya scirpoides), netted net-rush (Scleria reticularis), cross milkwort (Polygala cruciata), and meadow beauty (Rhexia virginica). Meadow beauty is perhaps the showiest of these, sporting bright rose-pink flowers with large yellow anthers. An excellent place to see these denizens of the east coast is Sohlberg Silver Lake State Natural Area in Adams County. Look for these plants from late July through August.
Although botanists are still somewhat puzzled over exactly how the disjuncts found their way to Wisconsin, they have proposed several hypotheses to explain the plants' presence here.
Some think species such as Lapland azalea normally found in, and adapted to, frigid areas to our north may have migrated south in front of advancing glaciers. It's possible they were more widespread here in the past, especially in the unglaciated Driftless Area of southwest Wisconsin. As the post-glacial climate warmed, these species shifted back north, but hung on in a few places that retained cool micro-environments, like the gorge of the Kickapoo River. Both of Wisconsin's azalea cliffs have cool water seeping from crevices and cracks in the sandstone. The water maintains a consistently cool, damp substrate for the plant's roots – similar to that in its arctic homeland.
The presence of Massachusetts fern and lobed spleenwort is believed to be a result of the long-distance dispersal of its spores – exceptionally tiny, dust-like "seeds" borne by the wind. The spores could have been swept into Wisconsin on strong breezes, eventually landing in hospitable habitat. How the spores moved from east to west, given the prevailing westerly winds, and why Massachusetts fern and lobed spleenwort aren't found in intervening areas with suitable habitat, remains a mystery.
Long-distance seed dispersal probably played a role in the occurrence of Atlantic coastal plain disjuncts in central Wisconsin. Instead of being blown by the wind, the heavy seeds may have hitched a ride on birds, primarily waterfowl and shorebirds migrating from eastern wetlands through Wisconsin. A second theory involves the westward migration of coastal plain plants along the extensive shoreline habitat of connected lakes and drainages that were present immediately following the retreat of the glaciers. Some botanists think a combination of these two schemes is the most likely explanation.
Not surprisingly, most disjunct species are considered rare in Wisconsin. Lapland azalea, of which there are only two populations here, and lobed spleenwort, of which there are only five populations, are listed as state-endangered species. Most of the others are listed as species of special concern – plants believed to be rare, but for which not enough information is available to rank them as endangered or threatened.
The DNR's Bureau of Endangered Resources tracks the status and location of these far-flung plants. If you happen upon a new population, bureau staff would appreciate knowing more about it. Please call (608)266-7012 to share your discovery.
Thomas A. Meyer is a natural areas specialist in the State Natural Areas Program of the DNR's Bureau of Endangered Resources in Madison.