Alvar

An alvar is a flat expanse of limestone bedrock covered by very thin soil or none at all, so that only sparse, grassy, prairie-like vegetation can take hold. The habitat is harsh in both directions: water often floods the surface in spring, and drought dries it out by midsummer, which keeps trees and bushes absent or stunted and leaves mosses and lichens common. Alvars are found chiefly in northern Europe and around the North American Great Lakes, where more than half of the surviving examples lie in Ontario. The word comes from Sweden, and the largest European alvar is on the island of Oland. Because the shallow bedrock, seasonal flooding and grazing or fire keep stronger competitors out, alvars shelter rare plants and animals, but little of this habitat remains and much of it has been degraded by farming and other uses.