From his observations of the vegetation of Nebraska and the western United States, Clements developed one of the most influential theories of vegetation development. Vegetation composition does not represent a permanent condition but gradually changes with time. Clements suggested that the development of vegetation can be understood as a unidirectional sequence of stages resembling the development of an individual organism. After a complete or partial disturbance, vegetation grows back towards a stable" climax state," which describes the vegetation best suited to the local conditions. Though any actual instance of vegetation might follow the ideal sequence towards stability, it can be interpreted in relation to that sequence, as a deviation from it due to non-ideal conditions. In these studies, he and R. Pound developed the widely-used method of sampling using quadrats around 1898. Clements's climax theory of vegetation dominated plant ecology during the first decades of the twentieth century, though it was criticized significantly by ecologists Henry Gleason and Arthur Tansley early on, and by Robert Whittaker mid-century, and largely fell out of favor. However, significant Clementsian trends in ecology re-emerged towards the end of the twentieth century.
In his 1916 publication, Plant Succession, and his 1920 Plant Indicators, Clements metaphorically equated units of vegetation, with individual organisms. His observations were that some groups of species were repeatedly associated together. He is frequently said to have believed that some species were dependent on the group, and the group on that species, metaphorically as organs and the animal containing them mutually depend on each other. However, this interpretation has been challenged by the argument that Clements did not assume mutual dependence as an organizing principle of formations or plant communities. Clements observed little overlap in kinds of species from type to type, with many species confined to just a single type. Some plants were widespread over vegetation types, but the areas of geographical overlap was narrow. His view of a community as a distinct unit was challenged in 1926 by Henry Gleason, who viewed vegetation as a continuum, not a unit, with associations being merely coincidental, and that any support by observations or data of clusters of species as predicted by Clements' view was either an artifact of the observer's perception or a result of defective data analysis.
Lamarckism
Clements was an advocate of neo-Lamarckian evolution. Ecologist Arthur Tansley wrote that because of his support for Lamarckism, Clements "never seemed to give proper weight to the results of modern genetical research." Science historian Ronald C. Tobey has commented that:
believed that plants and animals could acquire a wide variety and range of characteristics in their struggle to survive and adapt to their environment, and that these features were heritable. In the 1920s, he conducted experiments to transform plant species native to one ecological zone into a species adapted to another, higher, zone. Clements was quite convinced of the validity of his experiments, but this experimental Lamarckism fell to experimental disproof in the 1930s.
Clements spent much time trying to demonstrate the inheritance of acquired traits in plants. By the late 1930s scientists had provided Darwinian explanations for the results of his transplant experiments.
Honors
In 1903, the flowerClementsia rhodantha, a stonecrop, was named in honor of Frederic Clements.
Writings
Among his works are:
The Phytogeography of Nebraska
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Flower Families and Ancestors
Plant Competition. An Analysis of Community Functions
