Page  127 ï~~2005 THE MICHIGAN BOTANIST 127 THE HISTORICAL DEVELOPMENT OF THE TENSION ZONE CONCEPT IN THE GREAT LAKES REGION OF NORTH AMERICA Barbara J. Andersen Department of Landscape Architecture Art & Architecture Building University of Idaho, Moscow, ID 83844-2481 ABSTRACT John T. Curtis' detailed work with Wisconsin plant communities synthesized the range limits of 182 plant species into a northwest to southeast diagonal zone across the state. This paper combines Curtis' synthesis with similar work from other Great Lakes areas into a regional view of the tension zone concept. The search for spatial pattern helped form early ecological thinking in the U.S. INTRODUCTION The concept of a tension zone or transitional area between two types of vegetation was first discussed in the late 1800s and early 1900s by botanists and ecologists who were studying the boundaries of plant communities. After the Civil War, there was considerable work being carried out in the United States in determining the geographic extent of biological resources as well as climatic zones. Various federal and state agencies conducted biological surveys in the sparsely-populated area west of the Mississippi River as railroads and EuropeanAmerican settlement expanded westward. Some of the first biologists who contributed to the newly developing discipline of ecology were participants in these surveys (McIntosh 1985, pp. 48-49). Botanists and plant ecologists began to study the tension zone concept as a tool for understanding the dynamics and spatial structure of plant communities. The idea of tension, or transition, zones is largely limited to use in the Great Lakes region of North America (Minnesota, Wisconsin, Michigan, Ohio, and southern Ontario), particularly in the boundary between the eastern deciduous forest (with the prairie that replaces it as one moves westward) and the mixed hardwoods forest of the region. The tension zone concept was especially applied to Wisconsin with John T. Curtis' detailed work in mapping the distribution of the plant communities. DEFINITIONS Tension is a dynamic quality, involving a pull of forces and a resultant stress or strain. Merriam-Webster (1993) defines tension as: "the act or action of stretching or the condition or degree of being stretched to stiffness" and as: "either of

Page  128 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

Page  129 ï~~2005 THE MICHIGAN BOTANIST 129 which run in a northwest to southeast diagonal pattern. On the same map, the "Northern Limit of Dent Corn and Concord Grape" is shown as a line in basically the same diagonal fashion. He also showed a line for the "Northern Limit of the Pear and Cherry" which runs from Prairie du Chien through Baraboo and up the eastern side of Oconto County to Washington Island in Door County. Knapp was adamant on the need for horticulturalists to recognize climatic conditions as paramount in their considerations of what plants to grow and where to grow them. He advocated the use of climate information in both adapting to the natural limits and in the planting of masses of trees and shrubs to ameliorate the effects of climate. L.S. Cheney published a paper (1894) titled "Is Forest Culture in Wisconsin Desirable? Is It Practicable?" In this paper, Cheney delineated two major vegetative provinces in Wisconsin: prairie in the southern part of the state with "scattering groves of oak" and the pine region in the northern part of the state. His argument was that the timber cutting practices in Wisconsin needed to be changed from that of exploitation and destruction of the forest to a more long-term forest culture that would restore forest cover through protection of what remained, replanting, and establishment of new forests. He gave examples of profitable forest management both in Europe and in other parts of the United States. On lands which were unsuitable for agriculture, such as "1,365,000 acres of tamarack and cedar swamp land"-basically the northern part of the state (almost 4,000,000 acres by his estimate), he advocated "systematic forest cultivation." Clinton Hart Merriam was a zoologist who became the first head, in 1888, of the Division of Economic Ornithology and Mammalogy of the U.S. Department of Agriculture. Merriam's "Laws of temperature control of geographic distribution" were among the most often cited "laws" of nineteenth century ecology (McIntosh 1985, p.64) and were based on the idea that "animals and plants are restricted in northward distribution by the total quantity of heat during the season of growth and reproduction" (Merriam 1894). In 1898, he published "Life zones and crop zones," a work that was influential with some botanists (Griggs 1914). This bulletin was based on the data he collected on expeditions across the country, especially through Arizona and the West in 1889. He collected information on the distribution of plants and animals at different elevations and the relation between temperature and distribution. Each "life zone" he described had characteristic species of plants and animals. He believed that temperature was the main cause, particularly during the growing season, of the distribution of vegetation types (Barbour et al. 1980). Perhaps the major contribution of these early ecological studies by Merriam, Knapp, and Cheney is that they gathered large amounts of data and attempted to see the larger picture-characterizing ecological zones at broader scales than had been done previously in the United States. Although Merriam's work was influential at that time and still is useful in the mountainous regions of the west, it was later discounted for much of the country and found to be based on inadequate theoretical foundations (Egerton 1976). Pre-1900 plant and animal ecology characterized communities as relatively static and falling within discrete classifications. In the late 1800s, this view began to give way to ecological perspec

Page  130 ï~~130 THE MICHIGAN BOTANIST Vol. 44 tives which recognized the dynamic nature of the environment, including the tension zone concept. THE TENSION ZONE CONCEPT Conway MacMillan was one of Charles Bessey's students at the University of Nebraska, where he earned his bachelor's and master's degrees in 1885 and 1886 respectively. MacMillan joined the botany faculty at the University of Minnesota in 1897 and was state botanist of Minnesota (Egerton 1976). He was influenced by the phytogeographical tradition of Alexander von Humboldt, Oscar Drude, and other German geographical botanists and their work with developing zonal classifications of vegetation. He published several ecological surveys of Minnesota vegetation. His book, The Metaspermae of the Minnesota Valley (1892), was praised by Bessey as a pioneering investigation of an area with natural boundaries, rather than political or scientific ones. However, some of MacMillan's work was criticized by the botanists Russell Pound and Frederic Clements (1897, cited in McIntosh 1985) as being too focused on the physical environment. In The Metaspermae of the Minnesota Valley (1892), MacMillan provided an exhaustive listing of 1174 plant species and where they had been found to occur in North America and in the Minnesota River valley. In a section titled "Distribution of the forest and prairie," MacMillan wrote of the intermediate area between the forest and prairie. In another section called "The dynamic inter-relations of plants," he wrote of plants competing dynamically, saying: We no longer permit ourselves to look at a grove standing in the midst of the prairie as stable or even as quiescent, but we picture to ourselves the complex condition of strain which exists in varying degree and under different degrees of organization, between the different plants, species and groups of species. (p. 584) MacMillan referred to the expansive movement of northern groups of plants as "south-bound" and of those with a southern range as "north-bound." In the same book (1892), MacMillan wrote in a general way of "Pressures and Tensions" in describing the physical and biological forces shaping vegetation of the Minnesota River valley. He compared these factors to forces in physics with the "the weaker plants of a formation crowded to its periphery where they meet and struggle with the weaker plants of an adjacent formation." This transitional area he defined as "a zone of plants not perfectly established in either forest or prairie." The tensions he wrote of in this section were mostly continental in scope, such as climate and topography, but he did not refer much to tensions between vegetation types. He does refer to a general tension between plants in his summary but this idea was not developed fully in this book. MacMillan coined the term "tension-line" (1893) in describing a transitional area between the two plant formations: the forest and the prairie in the eastern portion of the United States. This region he defined as "a very narrow strip of plants imperfectly developed in either forest or prairie" and believed it to be

Page  131 ï~~2005 THE MICHIGAN BOTANIST 131 found anywhere two different formations met. Tension-line characteristics which MacMillan noted were: typical tension-line groups of species distinct from the typical prairie groups and typical forest groups, greater richness of species and more numerous individuals than in the neighboring formations, movement of the tension-line as formations advance and retreat, effects of the tension-line have influence at some distance into the adjacent formations as one formation encroaches upon another, and the movement of plants from the formations bordering the tension-line as they "work out" into the tension-line area. The term "tension-line" is a little misleading since MacMillan repeatedly refers to this concept as not necessarily a thin line but as a region, a "thicker or thinner band of tension-line species" or "the tension-area." In his 1899 book on Minnesota vegetation, MacMillan stated (p. 10) that plant populations of two regions are in "a state of tension [his emphasis], and the line between them is necessarily slowly shifting and irregular." MacMillan wrote of tensions at different temporal scales. The tension between forest and prairie, he termed a continental tension. Minor tensions at a finer scale, such as between knolls and ravines, were described as similar but differing in their shorter history. The term tension zone was first used by an early ecologist, Burton Edward Livingston (1903), in describing a space where different plant communities reached their boundaries. He referred to the "zone of tension" between the upland plant societies he studied in Kent County, Michigan. Livingston described an area along a "wavy east and west line" through the county that marked the limits of five different plant societies. His conclusion was that soil was the ultimate cause of the vegetation pattern, along with climatic, historical, and physiographic influences. In 1914, Robert F. Griggs utilized the tension zone concept in his paper about vegetation in the Sugar Grove region of Fairfield County, Ohio. In this area, there is a high proportion of species at the edges of their ranges. Griggs questioned why plant boundaries occur in certain areas and what factors decide the limits of their ranges. From his data on species abundance, he concluded that competition with different species was more influential than climatic factors in determining range limits, although he said that climatic conditions were also an important factor. His study at least partially disproved the idea (attributed to Blytt) that species at the limits of their ranges are restricted to areas with the most favorable conditions. Griggs pointed out that the tension zones between plant species apparently resemble the tension zones between plant societies. In the former, some species advance at the same time as other species are being forced back. He emphasized the importance of studying tension zones to ascertain the geographical movements of plant societies, stating that "the exceedingly slow movements of vegetation may be readily detected by observations of the tension zones between the ranges" (p. 49). Griggs' observations show the dynamic nature of the Sugar Grove vegetation, with boreal species pushing other species west and south. In the conclusion to his paper, Griggs stated that the tension zone concept is useful for learning more about this shifting interplay of species at the edges of their zones.

Page  132 ï~~132 THE MICHIGAN BOTANIST Vol. 44 John T. Curtis and Robert P. McIntosh (1951) examined the upland hardwood forests in the prairie-forest border area of Wisconsin and their relationships with each other and with the environmental factors of soil acidity, nutrients, and moisture. They pointed out the 1894 work of Cheney in classifying Wisconsin's vegetation into two floristic provinces, the prairie and hardwood forest region in the southwest part of the state and the hardwood and coniferous forest region in the northeastern part. Applying Griggs' tension zone idea, they remarked that the diagonal boundary zone between these two provinces contains members of each province. Using a method developed by Frederick Clements in his text (1905), Curtis and McIntosh stated that the location of the tension zone (which Clements termed a "limiting line or ecotone of a... province") could be delineated by combining the territorial limits of principal species and verifying these against the range limits of typical species of the contiguous vegetation. The southwest zone they called the prairie-forest province and the northeast zone the northern hardwoods province. Curtis and McIntosh sampled the vascular species (herbs, shrubs, and trees) from a random sample of 95 forest stands in 29 counties. They also sampled the soils in most of the stands. In determining the environmental factors of each stand, Curtis and McIntosh calculated importance values, constancy, and average frequency for species. These attributes were the basis for assigning a relative value to each species, which they called a "climax adaptation number." Multiplying the adaptation number by the importance value of a species in a particular stand produced a suite of weighted numbers added together giving a single number for that stand, which they termed the "vegetation continuum index." By this method, they found support for their hypothesis that the upland hardwood forests of southwestern Wisconsin "represent a continuum in which no clearly defined subdivisions are discernable but in which a definite gradient is exhibited." This idea of continuum built upon Henry Gleason's "individualistic concept of the plant association" and was in distinct contrast with the view prominent in ecology up until then: the discrete view of associations championed by other ecologists, primarily Frederick Clements. In Chapter One of his text, Vegetation of Wisconsin (1959), Curtis traced the concept of the tension zone through the earlier work of J. S. Knapp, L. S. Cheney, B. E. Livingston, and R. F. Griggs. Curtis made distribution maps for Wisconsin plant species based upon the notes and species maps of Cheney for tree species and the work done by Norman Fassett and his students from 1929 to 1953 for non-tree species (Fassett 1929, 1930, 1931, 1939, 1943, 1944, 1945, 1951, Fassett & Calhoun 1952). Curtis gave a cautionary note about the range maps derived from Fassett's work: that since the distribution maps were derived only from herbarium specimens, the choice of species may have been somewhat biased. Taxonomists often collect more unusual specimens at the expense of more commonly occurring ones. Nevertheless, the distribution maps are a valuable source of information and give range limits for a large number of Wisconsin species. Curtis' tension zone across Wisconsin is a synthesis of the range limits for 182 species of plants. Curtis compiled composite maps based on groups of species, such as sixteen species of Gramineae (grass family), eleven species of

Page  133 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

Page  134 ï~~134 THE MICHIGAN BOTANIST Vol. 44 134 THE MICHIGAN BOTANIST Vol. 44 MN FIGURE 1. Tension zone in Great Lakes region, adapted from Curtis 1959, Wendt & Coffin 1988, McCann 1979, Livingston 1903, Potzger 2946, & Thaler & Plowright 1973. and concluded that, for most of the plants, insufficient heat (as indicated by growing degree day data) determines the range limits of southern species. In other words, it is too cold for these plants to live north of the tension zone. She found additional support for this conclusion by comparing growing degree day isolines for Wisconsin and southern Ontario with the tension zones found there by Curtis (1959) and Thaler and Plowright (1973) respectively. In both cases, she found the tension zones followed the pattern of the growing degree data, although in southern Ontario, the pattern shows a "weakly developed tension zone" (Thaler and Plowright 1973). Figure 1 shows how the tension zone information of Curtis (1959), Wendt and Coffin (1988), McCann (1979), Livingston (1903), Potzger (1946), and Thaler and Plowright (1973) combined into a regional view encompassing Minnesota, Wisconsin, Michigan, and part of Ontario. Soper (1962) published a thorough survey and analysis of Carolinian flora, in which he examined some of the genera which have restricted range in southern Ontario. He termed the area a transition zone "between the Deciduous Forest and the Mixed Forest" (p. 54). He recognized the permeability of species distribution in this transition zone and wrote that it is not as though all southern species end their ranges at "a magic line" (p. 54) on a map. Instead, some southern species disappear gradually as one travels from south to north and others extend only into the southern fringes of the transition zone. Some northern species are distributed similarly, only in a north to south direction.

Page  135 ï~~2005 THE MICHIGAN BOTANIST 135 CONCLUSION: THE RELATION BETWEEN THE TENSION ZONE CONCEPT AND THE ECOTONE CONCEPT From a regional term used to describe a transitional area between floristic provinces, ecologists have broadened the tension zone concept into the idea of ecotones. Ecotones operate at a variety of spatial scales, from the microecotone level where a moss cushion meets the soil to the macroecotone level of different ecosystems (such as desert and tropical forest) connected by mesoecotones (such as a grassland-spruce forest ecotone) (Rusek 1992). The tension zone concept and the concept of the ecotone differ in their levels of complexity. The tension zone concept describes a large-scale phenomenon across 10-60 miles, inclusive of a variety of smaller scale vegetation differences within the zone, while the term ecotone is used to mean abrupt change happening between two communities in a particular place either at small scales or as a larger-scale boundary (Barbour et al. 1980). An ecotone is generally defined as a "transition zone between two plant communities" (Krebs 1994); and a "transition line or strip of vegetation between two different communities which has characteristics of both kinds of neighboring vegetation as well as characteristics of its own (e.g., forest-meadow ecotone)" (Resinger and Gomez Gutierrez 1992, pp. 97-98). Barbour (Barbour et al. 1980) described an ecotone as "an intermediate habitat" in which the vegetation can change abruptly. Ecotone approaches synonymy with tension zone, but without the sophistication of the latter. So a tension zone or transition zone is a concept incorporating a more complex understanding of competing forces than the more simplistic "ecotone" or sharp change. Both ecotones and tension zones can vary in width and may consist of changes in vegetation structure, such as grasses to trees, as well as changes in species composition. Although the two terms are associated, ecotone is a more broadly applied term than is tension zone. Ecotones have a variety of causes while a tension zone is more narrowly used to apply only to the range limits of plant species. Causes of ecotones include "disturbance (e.g., fire, human activities)" and "natural edaphic boundaries (e.g., soils, hydrology, climate)" (Johnston et al. 1992). Scales of ecotones range from the level of ecoregions or biomes to community-level and finer (Johnston et al 1992). There is an emerging consensus among ecologists that ecotones are critical for their ecological roles. Ecotones are an inherent part of most landscapes. Research documents increasing habitat fragmentation within terrestrial landscapes. As fragmentation increases, ecotones are created, expanded, and moved, and may play a role in increasing local extinctions (Merriam and Wegner 1992). Building on the basic work of botanists and ecologists in the late 1800s and early 1900s with the tension zone concept, ecologists are examining ecotones more closely to determine their ecological functions and roles in landscapes. Based upon the earlier understanding of the tension zone concept, ecologists today consider ecotones to be important areas in which to focus studies on changes in ecosystems. The study of ecotones in ecology has been inactive until recently (Rusek 1992). Recent research suggests that ecotones have important functions in landscapes (di Castri et al. 1988, Furley et al. 1992, Gosz 1993,

Page  136 ï~~136 THE MICHIGAN BOTANIST Vol. 44 Hansen and di Castri 1992, Holland et al. 1993, Risser 1995, and Ward and Wiens In Press). These functions include serving as frontiers for successional change (Rusek 1992), governing ecological flows and patterns (Wiens 1997), and preservation of biodiversity (Yoon 1997). It is important to understand how and why the tension zone concept developed because it is part of one of the major themes in ecology-the search for pattern or distribution in space of species and individuals and understanding their relationships with other organisms and the environment (McIntosh 1976), and understanding change and uniformity within ecosystems (Hagen 1992). This search has been going on since the time of Darwin. In On the Origin of Species (1859), Darwin presented the contrasting views of change brought about by the struggle of competition and "a high degree of uniformity, stability, and interdependence" (Hagen 1992) that nature's complex web sometimes produces. ACKNOWLEDGMENTS The author gratefully acknowledges the comments provided by A. A. Reznicek and two anonymous referees on earlier drafts of this work. LITERATURE CITED Barbour, M. G., J. H. Burk, and W. D. Pitts. 1980. Terrestrial Plant Ecology. Menlo Park, California: Benjamin/Cummings Publishing Company. Cheney, L. S. 1894. Is forest culture in Wisconsin desirable? Transactions of the Wisconsin State Horticultural Society 24: 163-170. Clements, F. E. 1905. Research Methods in Ecology. Lincoln, Nebraska: University Publishing Company. Curtis, J. T. 1959. The Vegetation of Wisconsin. Madison, Wisconsin: University of Wisconsin Press. Curtis, J. T. and R. P. McIntosh. 1951. An upland forest continuum in the prairie-forest border region of Wisconsin. Ecology 32: 476-496. Darwin, C. R. 1859. On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life. London: John Murray. de Laubenfels, D. J. 1975. Mapping the World's Vegetation: Regionalization of Formations and Flora. Syracuse, New York: Syracuse University Press. di Castri, F., A. J. Hansen, and M. M. Holland (Eds.). 1988. A new look at ecotones: emerging international projects on landscape boundaries (special issue 17). Biology International. Egerton, F. N. 1976. Ecological studies and observations before 1900. In Issues and Ideas in America (B. J. Taylor and T. J. White, eds.), pp. 311-351. Norman, Oklahoma: University of Oklahoma Press. Fassett, N. C. 1929. Preliminary reports on the flora of Wisconsin. Transactions of the Wisconsin Academy of Sciences, Arts, and Letters 24: 249-68. Fassett, N. C. 1930. The plants of some northeastern Wisconsin Lakes. Transactions of the Wisconsin Academy of Sciences, Arts, and Letters 25: 157-168. Fassett, N. C. 1931. Notes from the herbarium of the University of Wisconsin. VII. Rhodora 33: 224-228. Fassett, N. C. 1939. The Leguminous Plants of Wisconsin. Madison, Wisconsin: University of Wisconsin Press. Fassett, N. C. 1943. Another Driftless Area endemic. Bulletin of the Torrey Botanical Club 70: 388-399. Fassett, N. C. 1944. Vegetation of the Brule Basin, past and present. Transactions of the Wisconsin Academy of Sciences, Arts, and Letters 36: 33-56. Fassett, N. C. 1945. Juniperus virginiana, Juniperus horizontalis and Juniperus scopulorum, IV. Hy

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