ï~~128 THE MICHIGAN BOTANIST Vol. 44 two balancing forces causing or tending to cause extension." As used by plant ecologists, a tension zone is a transitional area where there is an active relationship between either different plant associations or between the requirements of a plant association and the total environment, including such factors as climate, topography, and soils. There is reasonable agreement on the definition among ecologists. A tension zone is described as a "boundary between floristic provinces (USA)" (Resinger and Gomez Gutierrez 1992, p. 295); a band between two floristic provinces, "which contains some members of each" (Curtis 1959, p. 15); a boundary "between floristic provinces" that coincides "with the distributional limits of many species" (Krebs 1994, p. 446); and "an area where there is much change in vegetation in a comparatively narrow zone" (McCann 1979, p.1). D. J. de Laubenfels (1975) made the distinction that tension zones occur where there are less perceptible boundaries between roughly similar plant communities, such as between savanna and brush, rather than a continuous gradient between extremes, such as between grassland and woodland. Tension zones may occur at different scales-between different plant species and between plant societies (Griggs 1914). However, in the most common use of the term in plant ecology, a tension zone occurs at the level of plant communities on a regional scale. FOUNDATIONAL WORK Although the actual term "tension zone" was not used until the early 1900s, there was considerable discussion of similar ideas in the late 1800s which provided a basis for its subsequent development. Groundwork for the concept was established in Wisconsin with the works of J.G. Knapp (1871a, 1871b) and L.S. Cheney (1894). Both of these writers proposed schemes of vegetation classification for the state which were closely related to agriculture and timber production. At least part of the purpose of these classifications seemed to be educating state residents about which regions were best suited to various types of agriculture and forestry. Knapp published two papers in 1871 through the State Horticultural Society, along with a map of mean temperatures in Wisconsin and suitability areas for growing dent corn, Concord grape, and pear and cherry trees. In "The Native Vegetation of Wisconsin" (1871b), he divided the native vegetation into four divisions, or "vegetable belts": the Canadian, the Ontario, the Michigan, and the Wisconsin. For each belt, Knapp listed the predominant tree and shrub species, and some herbaceous plants. He commented generally on the climate of each belt, including snowfall, rainfall, and temperatures; and also described general soil characteristics. The economic nature of the era is evident in his comments on the potential of each belt for various forms of agriculture and timber production. He correlated the vegetation belts with isothermal and rain lines of the state, saying that they were closely related. In his paper, "The Isothermal Lines of Wisconsin" (1871a), Knapp examined the pattern of mean temperature lines across the state. His map (in the same volume as his two papers) shows these mean temperature lines for January and July
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