Alvar is a grass- and sedge-dominated community, with scattered shrubs and sometimes trees. The community occurs on broad, flat expanses of calcareous bedrock (limestone or dolostone) covered by a thin veneer of mineral soil, often less than 25 cm deep. Alvars are only known from three areas of the world: the Baltic region of northern Europe, Counties Clare and Galway of northwest Ireland, and the Great Lakes region south of the Canadian Shield. In Michigan, most of the sites occur in the Upper Peninsula along the shorelines of Lake Huron and Lake Michigan, in a band from Drummond Island to Cedarville, west to Seul Choix Point on the Garden Peninsula. Alvar also occurs farther west and inland along the Escanaba River. In the Lower Peninsula, alvar occurs on Thunder Bay Island and along the Lake Huron shoreline near Rogers City, Alpena, and Thompson’s Harbor. The plant community is also referred to as alvar grassland.
County Distribution Map
Global Rank: G2? - Imperiled (inexact)
State Rank: S1 - Critically imperiled
In Michigan, alvar is commonly found near northern Great Lakes shores where flat bedrock pavement is exposed. Bedrock types include both limestone and dolostone of Middle and Late Ordovician and Early Silurian origin (405 to 500 million years ago), when shallow, inland seas covered the Lake Michigan and Lake Huron basins. Much of this limestone has been converted through geological processes to dolomite or dolostone, a magnesium-rich form of limestone. Bedrock of Thunder Bay Island dates to the more recent Devonian period (345 to 405 million years ago). Topography of alvar is flat, and horizontal plates of bedrock are sometimes exposed, giving the impression of, and earning the name, “pavement” or “limestone pavement.”
The surrounding uplands typically support areas of limestone glade (alvar glade), boreal forest, and mesic northern forest. Typical lowlands associated with alvar include northern fen and rich conifer swamp.
Alvar soils are characterized by shallow soil over bedrock, with soil depth usually less than 25 cm (10 in). Soil texture is primarily loamy sand or sandy loam. Soil is saturated, or locally inundated in the spring, but becomes droughty later in summer. Thin layers of organic soil may develop in shallow depressions that remain wet for longer periods. Soil is mildly to moderately alkaline.
Alvars are typically subjected to seasonal environmental extremes of soil saturation or inundation in the spring followed by drought in summer. Flooding is less prevalent where there are abundant enlarged cracks (grykes) in the rock, which provide improved internal drainage. The combination of shallow soil and extreme fluctuations in soil-water availability play an important role in controlling the establishment of trees. Historically, fire probably also played an important role in limiting tree establishment and maintaining open grasslands. Where trees do establish, they are typically stunted and windthrow is common due to shallow rooting in the thin soil.
Alvar is dominated primarily by grasses and sedges, with mosses and lichens dominant in the driest areas and on exposed bedrock. Scattered shrubs and occasionally trees may occur in areas where soil depth is greatest or where cracks or grykes provide additional moisture needed by woody vegetation. Dominant grasses and sedges include little bluestem (Schizachyrium scoparium), prairie dropseed (Sporobolus heterolepis, state special concern), and bulrush sedge (Carex scirpoidea, state threatened). Where soil-water availability is greater flattened spike-rush (Eleocharis compressa, state threatened), big bluestem (Andropogon gerardii), mat muhly (Muhlenbergia richardsonis, state threatened), and cordgrass (Spartina pectinata) are often dominant. Additional commonly occurring grasses include ticklegrass (Agrostis scabra), hair grass (Avenella flexuosa), Kalm’s brome (Bromus kalmii), and poverty grass. Sedges common in alvar include Crawe’s sedge (Carex crawei), Richardson’s sedge (C. richardsonii, state special concern), bulrush sedge, and golden-seeded spike-rush (Eleocharis elliptica). Common forbs include small-leaved pussytoes (Antennaria neglecta), wild columbine (Aquilegia canadensis), hairy rock cress (Arabis pycnocarpa), harebell (Campanula rotundifolia), Indian paintbrush (Castilleja coccinea), field chickweed (Cerastium arvense), low calamint (Clinopodium arkansanum), bastard toadflax (Comandra umbellata), prairie cinquefoil (Drymocallis arguta), common peppergrass (Lepidium virginicum), rock sandwort (Minuartia michauxii), wild bergamot (Monarda fistulosa), balsam ragwort (Packera paupercula), early buttercup (Ranunculus fascicularis), and old-field goldenrod (Solidago nemoralis). Common shrubs include shrubby cinquefoil (Dasiphora fruticosa), common juniper (Juniperus communis), choke cherry (Prunus virginiana), fragrant sumac (Rhus aromatica), and snowberry (Symphoricarpos albus). Trees commonly occurring in alvar include northern white-cedar (Thuja occidentalis), white spruce (Picea glauca), white pine (Pinus strobus), and quaking aspen (Populus tremuloides).
Beaver can cause flooding in long, narrow depressions in the bedrock plain, providing conditions for the establishment of black ash (Fraxinus nigra) swamps. Many species of ants are found living in diverse niches in the bedrock landscape. Black bears feed on the ants and other insects that inhabit alvar.
- Allium schoenoprasum var. sibiricum (wild chives, state threatened)
- Asplenium trichomanes-ramosum (green spleenwort, state threatened)
- Astragalus canadensis (Canadian milk vetch, state threatened)
- Astragalus neglectus (Cooper's milk vetch, state special concern)
- Calypso bulbosa (calypso, state threatened)
- Carex concinna (beauty sedge, state special concern)
- Carex richardsonii (Richardson’s sedge, state special concern)
- Carex scirpoidea (bulrush sedge, state threatened)
- Cirsium hillii (Hill's thistle, state special concern)
- Cypripedium arietinum (ram's-head lady’s-slipper, state special concern)
- Danthonia intermedia (wild oatgrass, state special concern)
- Eleocharis compressa (flattened spike-rush, state threatened)
- Geum triflorum (prairie smoke, state threatened)
- Gymnocarpium robertianum (northern oak fern, state threatened)
- Hedysarum alpinum (Alpine sainfoin, state endangered)
- Hymenoxys herbacea (lakeside daisy, state endangered)
- Iris lacustris (dwarf lake iris, federal/state threatened)
- Muhlenbergia richardsonis (mat muhly, state threatened)
- Pellaea atropurpurea (purple cliff-brake, state threatened)
- Pinguicula vulgaris (butterwort, state special concern)
- Piperia unalascensis (Alaska orchid, state special concern)
- Poa alpina (alpine bluegrass, state threatened)
- Scutellaria parvula (small skullcap, state threatened)
- Solidago houghtonii (Houghton's goldenrod, federal/state threatened)
- Sporobolus heterolepis (northern dropseed, state special concern)
- Trichostema brachiatum (false pennyroyal, state threatened)
- Trisetum spicatum (downy oat-grass, state special concern)
- Viola novae-angliae (New England violet, state threatened)
- Viola pedatifida (prairie birdfoot violet, state threatened)
- Catinella exile (land snail, state special concern)
- Flexamia delongi (leafhopper, state special concern)
- Lanius ludovicianus migrans (loggerhead shrike, state endangered)
- Prosapia ignipectus (red-legged spittlebug, state special concern)
- Pyrgus wyandot (grizzled skipper, state special concern)
- Sistrurus c. catenatus (eastern massasauga, federal candidate species and state special concern)
- Vertigo elatior (land snail, state special concern)
- Vertigo hubrichti (Hubricht’s vertigo, state special concern)
- Vertigo morsei (six-whorled vertigo, state special concern)
Biodiversity Management Considerations
Major threats are related to road construction, quarry development, off-road vehicle use, invasive species, and trampling of vegetation. Road construction results in modification of the hydrology by disrupting overland surface flows, typically flooding one side of the road and drying out the other. Road corridors and associated maintenance facilitate the rapid introduction and expansion of invasive plants. Invasive plants that may threaten diversity and community structure of alvar include Canada bluegrass (Poa compressa), Kentucky bluegrass (Poa pratensis), common St. John’s-wort (Hypericum perforatum), spotted knapweed (Centaurea stoebe), rough-fruited cinquefoil (Potentilla recta), common mullein (Verbascum thapsus), timothy (Phleum pratense), ox-eye daisy (Leucanthemum vulgare), curly dock (Rumex crispus), hawkweeds (Hieracium spp.), wild carrot (Daucus carota), blueweed (Echium vulgare), white sweet clover (Melilotus alba), and common buckthorn (Rhamnus cathartica). Monitoring and control efforts to detect and remove invasive species are critical to the long-term viability of alvar. Eliminating illegal off-road vehicle activity is a primary means of protecting the ecological integrity of alvar.
The historical prevalence of fire in Michigan alvar is not well understood, but these level grasslands likely experienced occasional fires due to lightning strikes and anthropogenic causes. Prescribed fire management to maintain open conditions and species diversity should be implemented and if possible, followed by monitoring to assess changes in species composition and structure.
Limestones and dolomites vary in chemical composition, resistance to erosion, and depth and amount of crevice formation, all factors that affect soil development and plant species composition.
Similar Natural Communities
Limestone bedrock lakeshore, limestone bedrock glade, and boreal forest. Although alvar grassland may resemble prairies of southern Michigan in community structure, the climate, hydrology, soil properties, and species composition are much different.
- Albert, D.A. 2006. Natural community abstract for alvar. Michigan Natural Features Inventory, Lansing, MI. 8 pp.
- Albert, D.A., P. Comer, D. Cuthrell, D. Hyde, W. MacKinnon, M. Penskar, and M. Rabe. 1997. The Great Lakes bedrock lakeshores of Michigan. Michigan Natural Features Inventory, Lansing, MI. 218 pp.
- Baskin, J.M., and C.C. Baskin. 1999. Cedar glades of the southeastern United States. Pp. 206-219 in Savannas, barrens, and rock outcrop plant communities of North America, ed. R.C. Anderson, J.S. Fralish, and J.M. Baskin. Cambridge University Press, Cambridge, UK. 480 pp.
- Belcher, J.W., P.A. Keddy, and P.A. Catling. 1992. Alvar vegetation in Canada: A multivariate description at two scales. Canadian Journal of Botany 70: 1279-1291.
- Brownell, V.R., and J.L. Riley. 2000. The alvars of Ontario: Significant alvar natural areas in the Ontario Great Lakes region. Federation of Ontario Naturalists, Don Mills, ON. 269 pp.
- Catling, P.M. 1995. The extent of confinement of vascular plants to alvars in southern Ontario. Canadian Field Naturalist 109: 172-181.
- Catling, P.M., and V.R. Brownell. 1995. A review of alvars of the Great Lakes region: Distribution, floristic composition, biogeography, and protection. Canadian Field Naturalist 109: 143-171.
- Catling, P.M., and V.R. Brownell. 1998. Importance of fire in the maintenance of distinctive, high diversity plant communities on alvars — Evidence from the Burnt Lands, eastern Ontario. Canadian Field Naturalist 112: 662-667.
- Catling, P.M., and V.R. Brownell. 1999. Alvars of the Great Lakes region. Pp. 375-391 in Savannas, barrens, and rock outcrop plant communities of North America, ed. R.C. Anderson, J.S. Fralish, and J.M. Baskin. Cambridge University Press, Cambridge, UK. 480 pp.
- Faber-Langendoen, D., ed. 2001. Plant communities of the Midwest: Classification in an ecological context. Association for Biodiversity Information, Arlington, VA. 61 pp. + appendix (705 pp.).
- Jones, J., and C. Reschke. 2005. The role of fire in Great Lakes alvar landscapes. Michigan Botanist 44(1): 13-27.
- Lee, Y.M., L.J. Scrimger, D.A. Albert, M.R. Penskar, P.J. Comer, and D.L. Cuthrell. 1998. Alvars of Michigan. Michigan Natural Features Inventory, Lansing, MI. 30 pp.
- Reed, R.C., and J. Daniels. 1987. Bedrock geology of northern Michigan. State of Michigan Department of Natural Resources. Map: 1: 500,000.
- Reschke, C., R. Reid, J. Jones, T. Feeney, and H. Potter. 1999. Conserving Great Lakes alvar: final technical report of the International Alvar Conservation Initiative. The Nature Conservancy, Chicago, IL. 241 pp.
- Schaefer, C.A., and D.W. Larson. 1997. Vegetation, environmental characteristics, and ideas on the maintenance of alvars on the Bruce Peninsula, Canada. Journal of Vegetation Science 8: 797-810.
- Stephenson, S.N., and P.S. Herendeen. 1986. Short-term drought effects on the alvar communities of Drummond Island, Michigan. Michigan Botanist 25: 16-27.
For a full list of references used to create this description, please refer to the natural community abstract for alvar.
Kost, M.A., D.A. Albert, J.G. Cohen, B.S. Slaughter, R.K. Schillo, C.R. Weber, and K.A. Chapman. 2007. Natural Communities of Michigan: Classification and Description. Michigan Natural Features Inventory, Report No. 2007-21, Lansing, MI.