An alvar is a biological environment based on a limestone plain with thin or no soil and, as a result, sparse grassland vegetation. Often flooded in the spring, and affected by drought in midsummer, alvars support a distinctive group of prairie-like plants. Most alvars occur either in northern Europe or around the Great Lakes in North America. This stressed habitat supports a community of rare plants and animals, including species more commonly found on prairie grasslands. Lichen and mosses are common species. Trees and bushes are absent or severely stunted.
The relevant geological history began about 450 million years ago when the limestone rocks were deposited in a warm ocean. This was the origin of the limestone and dolomite rocks that underlie much of Manitoulin Island and the rocks that constitute the foundation for the alvar today. That geological history was augmented by the geological processes that marked the end of the last great period of glaciation to affect the area, about 10,000 years ago. During this post glacial period, conditions were such that the alvar pavements were created and washed clean of soils and land bridges with local climate encouraged the influx of many different plant species.
The primary cause of alvars is the shallow exposed bedrock. Flooding and drought, as noted, add to the stress of the site and prevent many species from growing. Disturbance may also play a role. In Europe, grazing is frequent, while in North America, there is some evidence that fire may also prevent encroachment by forest. The habitat also has strong competition gradients, with better competitors occupying the deeper soil and excluding other species to less productive locations.
There are several classifications of alvar types and there are transitions between different alvar types on an island, one type may occur within another. For simplicity, the following are the alvar classifications used:
- Shoreline alvar: bare limestone and dolomite along the lake shore;
- Open alvar pavement; (Flowering Plants of open alvar pavement, inland from shore alvar);
- Open alvar grassland; <25%tree and shrub cover and >50% herbaceous cover with grasses and grasslike plants;
- Alvar Woodland >25%, but <60% trees;
Alvars comprise a small percentage of the Earth's ecosystems by land extent. Although approximately 120 alvars exist in the Great Lakes region, in total there are only about 112 square kilometres (43 sq mi) left across the entire Great Lakes basin, and many of these have been degraded by agriculture and other human uses. More than half of all remaining alvars occur in Ontario. There are smaller areas in New York, Michigan, Ohio, Wisconsin and Quebec.
In order to log this EarthCache as found you have to answer this questions:
- What kind of bedrock is in this area?
- What type of Alvar you can see at this location?
- Do you think this Alvar was formed before Ice Age or after, why?
- Mesure the elevation change between trailhead and the posted coordinates.
- How well do you think water will drain from this area after the rain?
- Post a picteure of yourself with your GPS unit at the posted coordinates with the Alvar landscape at the background. (optional)