| Summary: | The results of a rare plant, floristic, and plant community survey of the Grand Traverse Islands archipelago are presented. Stretching from Wisconsin’s Door Peninsula to Michigan’s Garden Peninsula, these Lake Michigan and Green Bay islands are largely underlain by Silurian dolomite that outcrops along shorelines as high, white cedar-dominated cliffs (on west coasts), low wave-washed shelves (east coasts), and, occasionally, interior escarpments. Most islands experienced intensive human use in the 19th century (fishing, concomitant logging and farming) but have now recovered to second- and third-growth forests. An exception is Washington Island, the largest in the archipelago, which has a permanent human population and much cultivated or formerly cultivated land. Islands larger than about 100 ha have interiors dominated by beech (Fagus grandifolia)—sugar maple (Acer saccharum) forests that may have rich spring ephemeral displays, including, on Washington and Rock Islands, the rare disjunct broad-leaved wood sedge (Carex platyphylla). These mesic forests are best developed on the western and northern sides of islands, where glacial till overlies dolomite. The eastern shores often have boreal conifer-dominated beach ridge and swale complexes (best developed at Jackson Harbor on Washington Island) with regional endemic calciphiles such as dwarf lake iris (Iris lacustris) and boreal disjuncts like northern comandra (Geocaulon lividum). Small dune complexes may have regional endemics such as Pitcher’s thistle (Cirsium pitcheri) and Lake Huron tansy (Tanacetum huronense), but these appear to be declining due to intensive human use of Great Lakes beaches. Dolomitic shoreline bedrock communities—now known as Great Lakes alkaline rockshores—are present on eastern and southeastern coasts of islands and are best developed on Summer, Poverty, Washington, Detroit, and possibly St. Martin Islands. On Poverty and Summer Islands, such communities grade inland into alvar, a globally rare community. Chambers Island is low and sandy and has extensive second-growth hemlock-hardwood forests and small remnant Great Lakes barrens communities with southern sand prairie or pine barrens species that are rare or absent elsewhere along the shores of northern Lake Michigan. Washington Island has a number of interesting interior wetlands including boreal fens at Little Lake, Coffee Swamp, and Big Marsh. The plants and communities of all of the islands are experiencing a “crunch” of negative factors including: Deer herbivory on large islands, which is affecting tree regeneration and the survival of understory herbs such as those in the lily and orchid families; and, invading colonial waterbirds on small (less than about 10 ha) islands—the birds have nearly killed all arboreal vegetation. Some 797 vascular plant species are recorded from the islands, including two federally listed species, 59 species listed as endangered, threatened, or special concern by the state of Wisconsin, and nine listed by endangered, threatened, or special concern by the state of Michigan.
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