Introduction

“Alvars are naturally open areas of thin soil over essentially flat limestone or marble rock with trees absent or at least not forming a continuous canopy” (Catling et al. 1975; Catling and Brownell 1995). Workers familiar with similar habitats in Sweden, Denmark, and Estonia first applied the term alvar in North America to areas near Kingston, Ontario. The term is still widely used in Europe (e.g., Krahulec, Rosen and van der Maarel 1986). Although alvars of the Great Lakes region may be structurally similar to sites in northern Europe and subject to similar ecological processes, they are not necessarily more closely related to the European alvars than to habitats in adjacent regions of North America that are classified by other names. To the west, alvars grade into dry prairies over limestone or calcareous gravel (Curtis 1959; Erickson, Breener and Wraight 1942). Northward, similar habitats exist within the boreal forest region, where they are referred to as “limestone barren”. The “cedar glades” of Kentucky and Tennessee (e.g., Freeman 1933) are similar, but differ in having more endemic species (Baskin and Baskin 1986, 1988; Catling and Brownell 1995) and a different floristic composition (Catling and Brownell 1995). Nevertheless, the term alvar has been widely adopted, and over the past several years alvars have become a major focus of conservation efforts.

A mosaic of plant associations is a characteristic feature of alvars.