. The Canadian field-naturalist. Natural history. Figures 17-20. Distributions of vascular plants in southern Ontario (based on herbarium specimens) in relation to lime- stone plains landscapes (shaded). 17. Geranium carolinianum. 18. Myosotis verna. 19. Pellaea atropurpurea. 20. Carex richardsonii. are in different climatic regions of southern Ontario, climate alone could account for some of this varia- tion (Brown et al. 1980). Numerous species are found in Ontario only on the Stone Road alvar on Pelee Island, or on other alvars of the western Lake Erie region, but these are too restricted t
. The Canadian field-naturalist. Natural history. Figures 17-20. Distributions of vascular plants in southern Ontario (based on herbarium specimens) in relation to lime- stone plains landscapes (shaded). 17. Geranium carolinianum. 18. Myosotis verna. 19. Pellaea atropurpurea. 20. Carex richardsonii. are in different climatic regions of southern Ontario, climate alone could account for some of this varia- tion (Brown et al. 1980). Numerous species are found in Ontario only on the Stone Road alvar on Pelee Island, or on other alvars of the western Lake Erie region, but these are too restricted to be of much use in defining alvar vegetation at the provincial level. Species restricted to this part of the pattern are largely southern and midwestern elements at their northern limits existing in a part of southern Ontario that has an exceptionally mild climate. The presence of the boreal and cordilleran species such as Carex scirpoidea, Piperia unalascensis and Poa alpina, as well as endemics including Cirsium hillii, Hymenoxys herbacea. Iris lacustris and Solidago houghtonii, on the alvars in Bruce Peninsula and Manitoulin sites may be explained with respect to the cool, dry environment and contin- uous availability of disturbed habitats. This situation probably approximates that which existed more than 10 000 years ago along the Wisconsin ice front and in the Picea parkland near to it (Webb 1987). Although some upper Great Lakes sites may have been under water in very early postglacial times, they were probably connected and more extensive by 9500 years ago (Lewis and Anderson 1989), at a time before climatic and vegetation zones had moved substantially northward. Furthermore, they may have been above water to the present since the precursor of Lake Huron was smaller than the pre- sent lake. In addition to vascular plants, there are some bryophytes that are more or less restricted to alvars. Examples include Scorpidium turgescens and Riccia sorocarpa. With a high level o
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