. The Canadian field-naturalist. Natural history. 1995 Catling and Brownell: Alvars of the Great Lakes Region 151. Figure 7. Extensive alvar grassland dominated by Sporobolus heterolepis on Little Cloche Island in the North Channel of Lake Huron. Sites like this one are believed to be closely related to the continuous periglacial grasslands that existed in the Great Lakes region until 9000 years ago. Photo by P. M. Cading, June 1991. periodically moist sites but may become a widespread dominant following grazing since it is often avoided by cattle in favour of grasses. Various herbs are someti
. The Canadian field-naturalist. Natural history. 1995 Catling and Brownell: Alvars of the Great Lakes Region 151. Figure 7. Extensive alvar grassland dominated by Sporobolus heterolepis on Little Cloche Island in the North Channel of Lake Huron. Sites like this one are believed to be closely related to the continuous periglacial grasslands that existed in the Great Lakes region until 9000 years ago. Photo by P. M. Cading, June 1991. periodically moist sites but may become a widespread dominant following grazing since it is often avoided by cattle in favour of grasses. Various herbs are sometimes frequent or even co- dominant in alvar grasslands including Long-fruited Anemone {Anemone cylindrica). Low Bindweed {Calystegia spithamea), Cerastium arvense. Bastard Toadflax (Comandra umbellata). Common Strawberry (Fragaria virginiana), Geum triflorum, Seneca Snakeroot (Polygala senega), Prairie Cinquefoil {Potentilla arguta). Heal-all {Prunella vulgaris). Early Buttercup {Ranunculus fascicularis), Scutellaria parvula, Balsam Ragwort {Senecio pau- perculus). Early Goldenrod {Solidago juncea), Gray Goldenrod {Solidago nemoralis). Heath Aster {Virgulus ericoides) and Zigadenus glaucus. A correlation of vegetation pattern, particularly woody species, with both hydrological and other fac- tors, persuaded Reschke (1994*) that both drought and inundation prevent invasion of woody species into alvar grasslands. This combined effect is proba- bly most pronounced in periodically very wet sites such as Deschampsia caespitosa, Carex lanuginosa, or Carex sartwellii associations. Regular summer droughts may be enough to prevent invasion of woody species in the driest sites such as Bouteloua curtipendula or Schizachyrium scoparium associa- tions. Periodic, catastrophic drought effects may be the most important factor in excluding woody species from sites of intermediate moisture level such as many Sporobolus heterolepis or Poa com- pressa grasslands. 2. Pavements and Pavement Edges The
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