. The Canadian field-naturalist. Natural history. Figures 9-12. Distributions of vascular plants in southern Ontario (based on herbarium specimens) in relation to limestone plains (shaded). 9. SoUdago ptarmicoides. 10. Scutellaria parvula. 11. Trichostema brachiatum. 12. Sporobolus heterolepis. on alvars. Of these, 32 have more than 70% of their Ontario occurrences on alvars, and 19 have more than 86%. Many of these species, including species with a very high level of confinement, occur in three or more of the major alvar regions. The very close correspondance of the distribution of many of th
. The Canadian field-naturalist. Natural history. Figures 9-12. Distributions of vascular plants in southern Ontario (based on herbarium specimens) in relation to limestone plains (shaded). 9. SoUdago ptarmicoides. 10. Scutellaria parvula. 11. Trichostema brachiatum. 12. Sporobolus heterolepis. on alvars. Of these, 32 have more than 70% of their Ontario occurrences on alvars, and 19 have more than 86%. Many of these species, including species with a very high level of confinement, occur in three or more of the major alvar regions. The very close correspondance of the distribution of many of these species to the alvar landscape is shown in Figures 3- 24. As the extent of confinement (Table 1) increases, the extent of association with the alvar landscape on distribution maps also increases. Trichostema brachiatum (Figure 11), with over 95% of its Ontario locations on alvars, is entirely confined to the alvar landscape on the basis of the specimens examined. Although this species sometimes occurs in roadside ditches, it is usually close to natural occurrences in periodically moist depressions in very shallow soil of natural alvar openings. Among the other species with a very high level of confinement (Table 1) within southern Ontario to alvars, and a correspondingly high correlation with the alvar land- scape, are Carex crawei (Figure 15), Geum triflorum (Figure 13), Ranunculus fascicularis (Figure 16), Scutellaria parvula (Figure 10), SoUdago ptarmi- coides (Figure 9), and Sporobolus heterolepis (Figure 12). Few species largely confined to alvars in Ontario are without alternative habitats in the province (Table 1), and most of those have other habitats elsewhere. The only species that is confined global- ly to alvars, and has most of its populations in Ontario, is Hymenoxys herbacea, considered imper- iled in Canada and critically imperiled in the United States (Argus and Pryer 1990). The extent of restric- tion to alvars is generally not a consequence of the scarcit
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