ï~~2005 THE MICHIGAN BOTANIST 133 Leguminosae (legume family), and thirty species of trees. Taken as a whole, most of the range boundaries fall within a northwest to southeast diagonal zone across the state. The width of the zone varies from 10 to 15 miles to 20 to 30 miles. This tension zone, as Curtis noted, is quite similar to the line Knapp defined in 1871 for the limits of certain horticultural crops. This tension zone for plants coincides with the range limits of a number of animal species, including many birds and fishes (Greene 1935). The tension zone can also be traced through Minnesota, Michigan, and Ohio, according to Curtis (1959). He stated that the tension zone is distinct in northwest Minnesota but becomes less clear in the southeast. Presumably, Curtis meant that the tension zone is the eastern deciduous forest vegetation type that separates the northern coniferous forest and tallgrass prairie vegetation types (Wendt and Coffin 1988). These three major vegetation types in Minnesota follow a northwest to southeast diagonal pattern and the eastern deciduous forest zone appears to match the western edge of the tension zone Curtis found in Wisconsin. No detail is given on the tension zone in Michigan. However, Curtis cited Griggs' (1914) work in eastern Ohio as evidence of a wide and diffuse tension zone there. John Adams and others published work in the early part of the 20th century about the distribution of some genera which are of restricted range in Canada's Carolinian Zone (Soper 1962). Soper briefly notes in his paper (1962) that the work of these botanists was instrumental in the development of the tension zone concept in southern Ontario. The work of Margaret Thompson McCann (1979) for her master's thesis provides information on the tension zone in Michigan that broadens Livingston's work to a statewide perspective. McCann located the tension zone in Michigan's Lower Peninsula and examined possible ecological causes for it. The vegetation of the Lower Peninsula has been found to be arranged into two different types in a north-south pattern. Potzger has determined a tension zone about 60 miles wide across the center of the Lower Peninsula (1946). McCann analyzed the distribution of 649 vascular plant species from herbarium and literature records for their range limits and calculated a zone index value for each county. She found evidence of a floristic tension zone (p. 12), "a concentration of range lines of many species." She noted that a floristic tension zone is different from a vegetation tension zone (p. 12), reflecting "a change in the abundance of the dominant species (such as trees)." Using her method of mapping northern and southern range limits and calculating zone indexes, McCann (1979) found that many northern range limits of southern plants (species which are found mainly in the southern portion of the Lower Peninsula) were concentrated in a tension zone in the center of the Lower Peninsula in an east-west orientation. Southern range limits of northern plants (species which are found mainly in the northern portion of the Lower Peninsula) did not show a clear pattern. McCann examined possible reasons for this tension zone, including "soils, topographical barriers, precipitation (amount, season, and kind), evaporation, temperature extremes, frost-free season length, and amount of cold and heat" (1979, p. 69). She compared the plant ranges for each of these possible factors
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