2009 THE MICHIGAN BOTANIST 97 RARE PLANT SPECIES AND PLANT COMMUNITY TYPES OF MANITOU ISLAND AND GULL ROCK, KEWEENAW COUNTY, MICHIGAN Janet Keeney Marr Michael R. Penskar 23180 Highway Location Road Michigan Natural Features Inventory Calumet, MI 49913 Michigan State University Extension Stevens T. Mason Building, PO Box 30444 Lansing, MI 48909-7944 Dennis A. Albert 4073 Agriculture and Life Sciences Building Horticulture Department Oregon State University Corvallis, OR 97331-4501 ABSTRACT Botanical and ecological surveys of Manitou Island and Gull Rock were conducted in 1996 and 2005 to relocate and update the status of rare species associated with volcanic bedrock shoreline, as well as search for additional rare vascular plant species not previously known for Manitou Is - land. An additional objective of 2005 surveys was to search the interior of Manitou Island for rare plants, and to more comprehensively classify and describe the vegetation types of Manitou Island. Manitou Island is located in Lake Superior about 5 km east of the tip of the Keweenaw Penin - sula in Keweenaw County, Michigan. Manitou Island’s shoreline is comprised primarily of cobble beaches, steep sloping bedrock, and cliffs. The island’s bedrock shoreline has one of the highest concentrations of rare plant species for this natural community type in Michigan. One state en - dangered, nine state threatened, and four special concern species, several of which have boreal and arctic affinities, are now documented for Manitou Island, including two newly documented species, northern reedgrass ( Calamagrostis lacustris ) and fir club-moss ( Huperzia selago ), dis - covered in the island’s interior during the recent 2005 surveys. Low summer temperatures and fogs along the Lake Superior shore create arctic-alpine-like climatic conditions, and in combination with volcanic bedrock along the shoreline, provide habitat similar to that found farther north, where these plants are more common. No rare plant species were documented during 2005 surveys of Gull Rock, a small bedrock island of ~0.2 hectare located about 1 km west of Manitou Island. Keywords: Keweenaw County, Manitou Island, Gull Rock, rare plants, Lake Superior, bedrock shoreline, Great Lakes islands INTRODUCTION “From its flora, that island [Manitou Island] looks to me like a place as interesting as Isle Royale .” E. G. Voss (pers. comm.). The 30,000 islands of the Great Lakes are globally unique and significant in terms of their high level of biological diversity (Crispin 1998) including the botanically-rich Isle Royale, Apostle, and Slate archipelagos of Lake Superior. These island groups have been the subject of numerous collecting expeditions, research projects, and status surveys (e.g. Foster et al. 1998; Judziewicz 1993, 98 THE MICHIGAN BOTANIST Vol. 48 1997; Slavick and Janke 1993; McGregor 1974). Lake Superior’s Manitou Is - land, located 5 km east of the tip of Michigan’s Keweenaw Peninsula, exem - plifies the biological richness and importance of a smaller Lake Superior is - land, which despite its proximity to the mainland has been studied much less intensively. In addition to providing critical habitat for a diversity of breeding and migra - tory bird species (Wood 1933; Youngman 2004), Manitou Island is known for supporting a relatively large number of rare plant species. Seven Michigan en - dangered, threatened, and special concern species were collectively documented for this Keweenaw County island over a period of botanical exploration from 1950 –1982 (Richards 1952; Thompson and Wells 1974; Voice et al. 1982). One rare taxon, the state endangered northern marsh violet (Viola epipsila ) has not been documented elsewhere throughout the eastern United States (Ballard 1985). Most of the rare plant taxa discovered on Manitou Island in studies from 1950 –1982 were documented in natural communities on or near the island’s bedrock shoreline (Figure 1). The less accessible upland, wetland, and aquatic communities present in the island’s interior, all with the potential to harbor addi - tional rare plant species, do not appear to have received comparable examination. The botanical and ecological surveys conducted in 1996 (Albert et al. 1997) and in 2005 (Marr 2006) were dedicated in part to relocating and providing up - dated status information for historical and otherwise very dated occurrences of rare species. These surveys were also focused on searching for additional rare FIGURE 1. Copper Harbor Conglomerate bedrock shoreline of Manitou Island. Photo taken near the island’s west end looking toward tiny Gull Rock and its light station, 1 km to the west. Keweenaw Peninsula mainland on the horizon. (Photo by Michael R. Penskar.) 2009 THE MICHIGAN BOTANIST 99 vascular plant species not previously known for Manitou Island, based on its abundance of bedrock shoreline formations known to be rich in such rare taxa, and as demonstrated in the floral diversity of adjacent mainland shores along the Keweenaw Peninsula. Additional objectives of 2005 surveys were to con - duct surveys on nearby Gull Rock, to more intensively search the interior of Manitou Island for rare plants, and to more comprehensively classify and de - scribe the vegetation types of Manitou Island 1. Location and access Manitou Island 2 is located in Lake Superior about 5 km east of the northern tip of Michigan’s Keweenaw Peninsula (Figure 2). This 409 hectare island is about 5 km long and 1.6 km wide. According to Nichols and Nyholm (1995), Manitou is an Ojibwe word meaning “god” or “spirit .” Manitou Island has a single dock that is located in a north shore cove about 1 km west of the Mani - tou Island Light Station at the island’s eastern tip. The island’s sole trail leads inland from the dock and then branches, one branch heading to the light station and the other west to the head of Fishermans Bay (T58N R26W Sec 21 NE ¼). Due to Lake Superior’s low water levels in recent years, much of Manitou Is - land’s rocky shoreline was easily accessible for hiking in 2005. Gull Rock is an approximately 0.2 hectare island located about 1 km west of Manitou Island and ca. 4 km east of the northern tip of the Keweenaw Peninsula. Due to shallow reefs offshore and the lack of a dock, Gull Rock is most safely accessed by boat from nearby Manitou Island. Ownership About 230 hectares (56%) of Manitou Island is owned by the federal gov - ernment, 130 hectares (32%) owned by the State of Michigan, and 37.4 hectares (9%) owned by the Keweenaw Land Trust (Manitou Island Light Sta - tion Preserve) , who are currently restoring the light station. The remaining 12 hectares (3%) of the island is privately owned . Gull Rock is owned by the Gull Rock Lightkeepers , who are in the process of restoring the light station and outbuildings (Peter Annin, pers. comm.). Geology, topography, and soils The bedrock of Manitou Island, Gull Rock, and the Keweenaw Peninsula formed about a billion years ago during the Keweenawan Period of the Pre - cambrian, a period of extensive surface volcanic activity (Reed and Daniels 1987; Dorr and Eschman 1984). During this period, approximately 400 basaltic 1 Selected areas of Manitou Island’s bedrock shoreline were revisited as part of a state lands re - view (Cohen et al. 2008), during which some rare plant occurrences previously documented were ob - served and then subsequently updated; however, no known collections were obtained nor were any new taxa reported during this survey. 2 There are several other Manitou Islands in the Great Lakes region, including a second Lake Su - perior Manitou Island located in Wisconsin’s Apostle Islands National Lakeshore. Leelanau County’s North and South Manitou Islands are located in northern Lake Michigan within the Sleep - ing Bear National Lakeshore. 100 THE MICHIGAN BOTANIST Vol. 48 FIGURE 2. Location of Manitou Island and Gull Rock in Lake Superior off the tip of Michigan’s Keweenaw Peninsula. lava flows or “traps” were deposited, with 20 to 30 interbedded conglomerate and sandstone layers. The weight of these 25 km-thick Keweenawan rock layers caused the for - mation of a structural sag known as the Lake Superior Syncline, a portion of which is now occupied by Lake Superior. As the earth’s crust sagged, the thick horizontal layers of basaltic lava on the Keweenaw Peninsula dipped (bowed downward) as much as 40 degrees to the northwest into Lake Superior, while forming cliffs along the Keweenaw Peninsula’s southeastern shore. 2009 THE MICHIGAN BOTANIST 101 These thick Keweenawan basaltic lavas include about 10 km of Portage Lake Volcanics, upon which were deposited several kilometers of Copper Har - bor Conglomerate (Reed and Daniels 1987). See Elmore (1984) for technical treatment of the Copper Harbor Conglomerate and Cornwall and White (1955) for a detailed geologic map of the Manitou Island quadrangle. The Copper Harbor Conglomerate along the northern shoreline of the Keweenaw Penin - sula, and all of Manitou Island and Gull Rock, consists largely of conglomer - ate (strongly cemented pebbles and cobbles of volcanic origin), but also in - cludes several lava flows collectively known as the Lake Shore Traps (Bornhorst and Rose 1994). Conglomerate makes up about 80% of Manitou Is - land’s shoreline including much of the island’s south shore (Bill Rose, pers. comm.). Although the Lake Shore Traps mostly occur on Manitou Island’s north shore such as at the point directly north of the island’s lone dock, they also are visible at the light station on the island’s east end and inland as cliffs NW of Perch Lake (Bill Rose, pers. comm.). Perch Lake may have been an embayment on Glacial Lake Nipissing, with the west side of Perch Lake perhaps representing Nipissing’s shoreline of about 4000 years ago (Bill Rose, pers. comm.). Perch Lake would then have become isolated from Lake Superior as water levels receded to present-day Su - perior levels, exposing about a 150 m stretch of old cobble beach ridges that run from Perch Lake east to the head of Fishermans Bay.