A REVIEW OF THE ALVARS OF THE GREAT-LAKES REGION - DISTRIBUTION, FLORISTIC COMPOSITION, BIOGEOGRAPHY AND PROTECTION

Alvars are naturally open areas of thin soil over essentially flat lim estone or marble rock with a more or less sparse vegetation cover of s hrubs and herbs with trees absent or at least not forming a continuous canopy. Drought and extremes of soil moisture are major factors limit ing tree cover. Alvars are important as sites for (1) the protection o f biodiversity including threatened plant communities, rare and threat ened species of flora and fauna, and germplasm of crop relatives; (2) biological research and environmental monitoring; and (3) ecotourism. Approximately 85% of alvar sires and more than 90% of alvar landscape area in the Great Lakes region is in southern Ontario. With the except ion of small isolated areas, most notably western Lake Erie and Lake C hamplain area, the alvars of the Great Lakes region occur near the con tact line of the granitic Canadian Shield upland with the Ordovician a nd Silurian limestones and dolomites. The main area of occurrence exte nds from the north shore of Lake Michigan east to the islands of north ern Lake Huron, such as Drummond Island, east across Manitoulin Island , southeast to the south end of Georgian Bay, east to the Lake Simcoe area and the Garden limestone plain, discontinuously east to the Napan ee limestone plain between Trenton and Kingston, then, following a gap due to the Frontenac axis of granitic rocks, and/or Lake Ontario, app earing again in New York State at the east end of Lake Ontario and on the Smiths Falls limestone plain in the Ottawa Valley. A list of 347 v ascular plant taxa found on alvars in seven alvar regions is included based on examination of 59 sites in Ontario, one in Ohio and three in New York State, as well as on the literature and on personal communica tions. On the basis of floristic composition and environmental factors , alvars may be divided into two types: shoreline alvars along rivers and lakes and plateau alvars. Four major plant communities related to soil depth and moisture availability are identified on plateau alvars: alvar grassland, alvar pavement, alvar savanna and pavement ridge. Bi odiversity is highest on the alvars of western Lake Erie, Manitoulin I sland and the Napanee Plain. The alvar floras of seven regions are app arent as three major groups in both a phenogram and in the principal c oordinate plot derived from a matrix of Jaccard's coefficients: (1) th e western Lake Erie alvars with a proportionally high component of pla nt species occurring to the south, but a relatively small proportion o ccuring to the north; (2) the alvars of the Bruce Peninsula and Manito ulin Island with a high proportion of northern and endemic plant speci es at the expense of southern species; and (3) the alvars of central O ntario, eastern Ontario and northern New York with a moderate represen tation of plant species occurring also to the north, but the major pro portion being southern species. The endemics and boreal elements of th e northern Lake Huron alvars are probable relicts of the Picea Parklan d and tundra-like environments that existed in front of the Wisconsin glacier more than 9000 years ago, and the alvars of this region are of great interest in their apparent similarity to the periglacial enviro nment. The primarily western species on alvars may have also originate d from this environment or may have migrated along pathways of open ha bitat that extended from western North America eastward into the Great Lakes region during glacial or postglacial times. Although some of th e best alvars in the Great Lakes region are protected, not enough site s are protected to achieve adequate representation.