Clements & Gleason - Two contrasting views of vegetation associations

Clements 1936 – Nature and Structure of the Climax

Introduction 

        Clements published this foundational paper in the Journal of Ecology in 1936. From Clements position at the Carnegie institute he was one of the leading botanical-ecological researchers of his time. He had publications examining plant community ecology, plant development, and several on succession and climax. His previous work had taken him across North America to examine different plant communities of all the major biomes. The Nature and Structure of Climax corroborates all of Clements observations into one foundational paper addressing the major factors and vocabulary involved in defining and understanding the climax state. As defined by Clements, “the climax constitutes the major unit of vegetation and as such forms the basis for the natural classification of plant communities.” He goes on to point out the intimate relation of climax and climate as the overriding thesis of the bodywork.

Defining Climax 

        The climax did not come to be rather it evolved out of a preceding climax state just as species arise. Much as a species is defined by a common trait a climax is defined by common species or even close relatives. Hence comparing dominant species can test a climax. Clements expands on his climax definition by adding that animals must also be considered in the climax and thus when referring to both plants and animals climax and biome are synonymous. However when referring to plants of an area alone he proposes emphasizing climax due to the more intimate relations of plants and climate versus more transient animals that have some ability to modify or escape the effects of climate. Climax is a circular state for plants one where they both respond to and indicate climate. Clements proposes that plants, by nature of their sessile state, directly and fully integrate physical factors through their growth forms. This utility of the plant climax is illustrated by comparing the different and similar dominant plant forms of the prairie as latitude and longitude change emphasizing the associated temperature and precipitation changes. Stability seems to be the hallmark of climax, yet Clements disputes this noting that in the absence of human intervention change is constantly at work within a climax and is neither destructive nor beneficial to it.

Layers of Climax

        The climax comes to be through a series of states intervening to reach a state most well fit to a particular climate. Any of these states may seem to be a climax on the human time scale.  The proclimax is defined as any state resembling the climax that is gradually replaced by the climax in the absence of disturbance. The proclimax is defined by four types: The subclimax precedes the climax, a notable example being Aspen in the Rocky Mountains preceding the coniferous climax state. The disclimax is the community resulting from the disturbance or replacement of the true climax brought about by human intervention and/or mass migration. The encroachment of Salsola on western range and cropland as a result of overgrazing is a notable example of disclimax. Preclimax may be best defined as a position similar to sub climax but as a result of unusual edaphic factors such as valleys and canyons providing extra water or on the margins of two adjacent climaxes. Such edaphic factors buffer the area from the climate that pushed the surroundings to full climax. The postclimax is similar to the preclimax but differs in that these areas are where “new” climate change is buffered due to unique edaphic factors that have kept the old climax from transitioning to the new.

Community of climax

        The major functions of climax serve to select and group organisms, while the minor functions affect abundance and visibility. The most abundant species is known as the dominant. A perdominant may be a type that is found throughout many similar climax types such as the shrub of chaparral and desert. The eudominant form refers to a dominant unique to its particular association. Subdominants are life forms subject to control by the dominant. The term influent applies to animal members of the biome that exert an influence back onto the community, similar per, eu, and sub “fluents” apply as above. Within a climax there are lower divisions meant to more specifically address unique local or regional characteristics that modify the climax form. In descending order of hierarchy these are at a community level: association, consociation, faciation, location, and at a society level, sociation, lamination, and clan. These organizational units run from regional (association) to layers of the forest floor (lamination).

Conclusion

        Clements presents his current knowledge of the organization of the biological word in this treaty. Though his view is almost wholly plant centric, it makes many generalizations that apply to a more well-rounded view of the biological world including microbes and animals. The plant-centric nature of his piece is not without justification, but neglects the key role of microbes in facilitating plant-abiotic interactions.  Nevertheless Clements defines and illustrates many traits of succession and biomes that hold true today and served as a catalyst for research further refining his proposals.

Gleason 1926 – The Individualistic Concept of the Plant Association

        Gleason begins by pointing out the high degree of disagreement among plant ecologists in how to classify vegetation associations, and suggests there may be something fundamentally wrong with a classification system that tries to pigeon-hole every community into a strictly defined type. While acknowledging that plant associations are real and a useful concept, he argues that the rules invented to define them may be overgeneralizations and that the amount of variation within plant associations indicates that plant communities might be better understood as loose associations changing continuously in space and time, rather than discrete, precisely-defined units. He emphasizes that in addition to plant-plant relations, the physical environment (climate, soils, physiography) is quite important in determining the composition of communities. To support these argument, Gleason cites a number of examples, including transition zones with continuous change in vegetation (i.e. ecotones), inter-annual variation in species’ relative abundance (i.e. non-equilibrium), associations that are fragmentary in space and time, the effect of broad-scale environmental gradients on plant associations, similar associations occupying different environments, and distinct associations in the same environment.

 Gleason goes on to explain his alternative theory of how the sorts of vegetation assemblages we observe could come to be through his individualistic concept of community assembly. According to this view, the abiotic environment and a species’ physiological tolerances are paramount in determining the presence and abundance of plants, with biotic effects from other vegetation mattering secondarily and only in so much as they modify the physical environment favorably or unfavorably. Since species tend to have slightly different physiological limits, their relative abundance will vary idiosyncratically in space and time. Equally important, according to Gleason, are the processes of dispersal and migration, including the proximity of seed-producing parent plants to potential habitat, species’ typical dispersal mode and distances, chance long-distance dispersal events, and incumbent effects giving an advantage to individuals who arrive in a habitat first. Gleason emphasizes in particular, with several examples, how “various accidents of dispersal” can largely determine community composition in many cases. He is careful to acknowledge that associations and succession are real phenomenon, that there is environmental sorting, but argues that it is not as predictable or deterministic as others have assumed. Gleason finishes by dismissing the “usual” (i.e. Clementsian) concept of vegetation units resembling species or organisms, stating instead the “each species of plant is a law unto itself,” with its distribution determined by migration and environmental selection, and that resemblances between communities are due to similarities in these underlying factors, not some intrinsic affiliation of groups of species for one another. 

Gleason articulates quite well a number of concepts that have been further developed and supported in more recent ecological research, including the competing role of niche and neutral processes in community assembly (i.e. environmental sorting and chance dispersal, in his terms). Are there other concepts used by Gleason that are still active areas of research in ecology today? What are some shortcomings in Gleason’s ecological worldview?

Gleason uses the example of dune succession to illustrate the role of chance dispersal events (pgs. 19-20). How does this story compare to that told by Cowles? What parts of The Individualistic Concept would Cowles likely agree or disagree with?

Gleason makes some strong arguments against the Clementsian view of plant associations. Are there areas where you think Gleason was not entirely convincing? Where is the more traditional notion of plant associations still useful, or at least more useful than the individualistic concept?

Overall questions to consider for Clements and Gleason

        What sorts of evidence do Clements and Gleason each use to make the case for their concepts of vegetation associations? What are the strengths and weaknesses of their respective positions? Is one more convincing? Why?

        To what extent, and in what contexts, do the Clementsian and Gleasonian views of vegetation association still hold today? Has one or the other won out? Where were they each right and/or wrong?

Note: Please don't feel bound by the summaries, discussion points, and questions written here. We'd be interested in hearing your own questions, thoughts, and observations that we may have missed.