Up until the 1940s, plant ecology in Europe and North America was dominated by the community-unit theory—the concept that plants grow together in definite, repeating communities. These natural kinds of vegetation, generally termed “associations” or “formations,” were held to constitute the proper subject matter for ecological study, just as the plant species provided the proper object of study for taxonomic botany. The units were defined by the American ecologist George Nichols in his 1923 article “A Working Basis for the Ecological Classification of Plant Communities.” Thus, “the association may be described as a vegetation-unit characterized by its essentially constant physiognomy and by its essentially constant floristic composition” (p. 17). Between 1917 and 1945, only one American botanist dissented from the general consensus surrounding the community-unit theory—Henry Allan Gleason (b. 1882–d. 1975). In a key article, “The Individualistic Concept of the Plant Association” (1926), Gleason championed an alternative view, the “individualistic hypothesis,” arguing that the association was “not an organism, scarcely even a vegetation unit, but merely a coincidence” (p. 16). In his view, the phenomena of the plant community depended entirely upon the behavior of individual plants. Thus, associations or formations did not constitute basic units of which all vegetation was composed. The association was a classifier’s category, produced by the activity of classification rather than by the essential reality of vegetation, and it varied according to the priorities and interests of each classifier. Throughout the 1910s and early 1920s, Gleason was one of the most active field ecologists in North America. Moreover, the time of his maximum involvement in ecological research was also the period during which plant ecology became institutionalized as an academic specialty in the United States. Gleason was also an accomplished taxonomic botanist. In 1919 he was appointed assistant director of the New York Botanical Garden. (It should be noted that his work in taxonomic botany is not directly covered in this bibliography.) From the mid-1920s onward, his involvement in ecological research in the field diminished, as he established himself as one of America’s leading systematic botanists. However, several of his most important theoretical papers were produced in this latter period. After the Second World War, the broad unanimity as to the fundamental nature of vegetation began to break down. A number of younger American ecologists expressed dissatisfaction with the prevailing state of ecological science and called for a cautious reexamination of Gleason’s ideas. In the 1950s, a series of major field studies, notably by John Curtis and his students in Wisconsin, and by Robert Whittaker, who worked on the vegetation of the Great Smoky Mountains of the southeastern United States, revived the individualistic hypothesis. Gleason came to be regarded as one of the most important plant ecologists of the 20th century. However, his views have often been misrepresented or misunderstood, by both scientists and historians of science. This bibliographic survey seeks to document and clarify the development of his ecological ideas and to situate them in the context of plant ecological science in America in his time.
Plant Ecology and Geography in the United States before Gleason
This section identifies the founding texts of American plant ecology, the publications by which the exemplar of ecological research that had been developed in Europe was transmitted to the universities of the Midwest. They form the beginning of a distinctive research tradition within which Gleason participated. Macmillan 1897 was the first large-scale ecological study published in the United States. Henry Chandler Cowles, at the University of Chicago, was one of the most influential American teachers of ecology. His first major paper (Cowles 1899) is a comprehensive application of Warming’s work on the shoreline vegetation of Denmark to the sand dunes of Lake Michigan. Like Macmillan, Cowles emphasized that the relatively rapid topographical changes taking place within the dune systems make them favorable sites for vegetational research. In a general statement of the importance of topography in governing the character of vegetation (Cowles 1901b), he set out the principles of “physiographic ecology.” Cowles presented his system as “genetic and dynamic,” implying that it was based upon an understanding of vegetational succession and its relation to erosion and deposition. Cowles’s physiographic perspective conveyed a powerful sense of ubiquitous vegetational change, and of an underlying predictable regularity within that change. It promised a privileged insight into the history of vegetation. By walking through a dune system, from the newest dunes toward the mature ones, an observer could see laid out horizontally the developmental stages of the deciduous forest that was now established on the most stable, humus-covered dunes. Cowles 1901a is a forthright expression of monoclimax theory that all the vegetation of a given region is tending toward an ultimate common destiny, which, in the temperate United States, is mesophytic forest. Cowles’s work established the pride of place for successional studies within American ecology that they were to occupy until after World War II. Clements 1916 was the paradigmatic expression of Clements’s concept of the plant community. Clements held the formation to be a “complex organism,” alluding to the high degree of integration and orderly development he identified within it. Succession was “the basic organic process of vegetation, which results in the adult or final form of the complex organism” (p. 6). Retrogressive succession—vegetational changes that lead away from the climatic climax—was impossible. Gleason’s reading of Plant Succession (Clements 1916) was the stimulus for his first theoretical paper (Gleason 1917, cited under Development of the Individualistic Hypothesis). Plant Succession is a useful guide to the differences of opinion among early ecologists as to how to define the plant community, and it provides the background against which Gleason’s conception of the plant community should be understood. Charles Adams’s papers (Adams 1902, Adams 1905) combine floristic, ecological, and biogeographical interpretations and were an important influence on Gleason.
Adams, C. C. 1902. Southeastern United States as a center of geographical distribution of flora and fauna. Biological Bulletin 3:115–131.
Much of the biota of the central part of the northern United States is derived from the southeastern region. After the last Ice Age, southeastern species migrated north along three major routes: the Mississippi valley, the plain of the Atlantic seaboard, and the southern Appalachians. The distribution of biota should be interpreted “dynamically and genetically” in terms of the relationship between centers of origin and highways of dispersal.
Adams, C. C. 1905. The postglacial dispersal of the North American biota. Biological Bulletin 9:53–71.
The present distributions of biota are largely the product of past conditions. Climatic and habitat factors are important, but the character of a region’s flora is also determined by where its species originated. Analogy is made between the movements of flora northward into the land exposed by the retreating glaciers and vegetational successions observed in the present day. Successional processes may be ecologically similar in regions of different floral composition.
Clements, F. E. 1905. Research methods in ecology. Lincoln, NE: Univ. Publishing Company.
Clements sets out the key techniques and principles of field ecological research, including, in particular, the quadrat and transept methods. He also provides an elaborate terminology for the discipline, but not all his suggestions were adopted.
Clements, F. E. 1916. Plant succession: An analysis of the development of vegetation. Washington, DC: Carnegie Institution.
Plant Succession provides a detailed interpretation of the vegetation of western North America, setting out the principles of Clements’s theory of vegetation and his research methods. The formation is held to be a “complex-organism,” characterized by orderly development and high degrees of integration. Succession is “the basic organic process of vegetation, which results in the adult or final form of the complex organism” (p. 6). Retrogressive succession is impossible.
Cowles, H. C. 1899. The ecological relations of the vegetation on the sand dunes of Lake Michigan. Botanical Gazette 27:95–117, 162–202, 281–308, 361–391.
Cowles’s interpretation of the sand dune vegetation is structured around the theme of topographic and vegetational development. While the topography influences the vegetation, the vegetation also modifies the topography. Cowles charts the historical stages of the sand features, from the dunes of the beach, through actively moving dunes, to mature, stationary dunes upon which a climax, mesophytic forest eventually establishes itself.
Cowles, H. C. 1901a. The influence of underlying rocks on the character of the vegetation. Bulletin of the American Bureau of Geography 2:163–176, 376–388.
While acknowledging that the physical and chemical nature of the underlying rock influences vegetation, Cowles asserts that physiography is more important than the underlying rock type. The flora of youthful limestone topography is more like the flora of a similar stage in sandstone than it is like that of mature limestone. The vegetation growing on clay hills today will eventually colonize granite hills, as erosion proceeds and humus accumulates.
Cowles, H. C. 1901b. The physiographic ecology of Chicago and vicinity: A study of the origin, development, and classification of plant communities. Botanical Gazette 31:73–108, 145–182.
Cowles subordinates every aspect of the habitat to a single variable—topography—which determines the water content of the soil and exposure to light and wind. As the general trend of physiographic change is toward the peneplain, so the general trend of vegetational change, in temperate regions, is toward mesophytic woodland. Cowles’s physiographic perspective conveyed a powerful sense of ubiquitous vegetational succession, and of a predictable regularity within that change.
Macmillan, C. 1897. Observations on the distribution of plants along shore at Lake of the Woods. Minnesota Botanical Studies 1:949–1023.
A detailed account of the “plant formations” of the dunes and shorelines of the lake. The comparatively rapid processes of vegetational change observable in bogs and dunes make those locations valuable for ecological research. Macmillan interprets the distribution of the various forms of vegetation in terms of the environmental and topographic conditions, emphasizing that many factors have to be taken into account to explain why a particular vegetation type grows where it does.
Nichols, G. 1923. A working basis for the ecological classification of plant communities. Ecology 4:11–23, 154–179.
A definitive exposition of the community-unit theory, as employed in ecological research in the United States. The concrete association—a particular stand of plants of “definite floristic composition” and “uniform physiognomy”—is distinguished from the abstract association, a higher category into which the individual stands may be classified. The abstract association is analogous to the taxonomic species. It is defined by floristic composition and physiognomy but, contra Clements, not by habitat.
Pound, R., and F. E. Clements. 1898. A method of determining the abundance of secondary species. Minnesota Botanical Studies 2:19–24.
Pound and Clements introduced the quadrat method into the American botanical literature, and thus provided Gleason (and many of his contemporaries) with an essential tool for ecological field research.
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