Alvar communities may look different from one community to the next due to local influences (fire, grazing, historical land use) though they share key characteristics:
- They occur on flat areas of thin mineral soil over calcareous ( limestone or dolostone) bedrock
- They are naturally open landscapes, with little to no tree cover
- They are subject to seasonal drought and flooding in the summer and exposed to deep frost (heave) in the winter
- They have a distinctive set of plant species
- They may contain many species that are rare elsewhere in the Great Lakes basin
Types of Alvar
Almost all of North America’s alvars are found within the Great Lakes basin. There are several types of alvar, most of which are globally rare and under stress.
Open alvar (alvar pavements and grassland)
- Less than 10% tree cover and less than 25% shrub cover
- High abundance of herbaceous plants or exposed bedrock (may be covered with lichens and mosses).
Shrub alvar (alvar shrublands)
- Less than 10% tree cover with moderate to high cover of shrubs
- Variable amounts of herbaceous, moss, and lichen cover.
Tree alvar (alvar woodlands)
- Partial canopy of trees; savannas 10-25% cover and woodlands 25%-60%
- High cover of shrubs, some trees, herbaceous and moss and lichen cover