PLANT COMMUNITY CONCEPTS

VEGETATION AND PLANT COMMUNITIES

1. Attributes of vegetation

a. Physiognomy

b. Floristic composition

c. Structure (spatial arrangement and sizes of plants and plant parts) as characterized by abundances, density, heights, stem diameters, cover, biomass, spatial pattern, etc.

2. The debate over community concepts: What is the nature of a plant community?

In the debate over "plant community concepts" there are two fundamental questions:

a. Can discrete (separate) units of vegetation be clearly defined? Or, is there continuous, gradual variation over space in the attributes of vegetation? This is the question of "spatial variation."

b. How integrated is a plant community? Does a plant community function as a coordinated, harmonious whole? This is the question of interdependence or functional integrity.

TRADITIONAL CONCEPTS OF PLANT COMMUNITIES

1. Organismal view-- Frederick Clements

a. High degree of integration in a plant community.

b. A plant community is an organic entity (e.g. a complex organism).

Implications of the organismal view:

a. Discrete, repeatable vegetation units can be recognized and classified.

b. Succession constitutes the series of life history stages of the complex organism.

2. Quasi-organismal view-- Alex Tansley

A plant community differs from a true organism in the following ways:

a. lack of clear delimitation;

b. lack of genetic unity;

c. same community type may have different "life histories" (i.e. successional pathways);

d. lack of coordinated reproduction;

e. lack of structural integrity.

However, Tansley saw a strong enough similarity between an organism and a community so that an analogy is valid.

3. Individualistic concept of the plant association--Henry Gleason

Based on three simple premises:

a. Dispersal of propagules occur at different rates and therefore different sites arrive at different times at a bare site.

b. The site (operational environment) acts as a filter so that only certain species can survive at a particular site.

c. Sites vary over space and time.

Therefore, species composition at any particular site will be unique because of chance dispersal and the independent distribution of each species.

Implications:

a. Vegetation varies gradually in space and discrete boundaries between different vegetation units are rare.

b. Due to the uniqueness of associations, vegetation cannot be perfectly classified

Supporting evidence for validity of Gleason's concept:

< quantitative studies of floristic composition and vegetation gradients.

< independent migration of species as shown by paleoecological studies.

4. The continuum/gradient concept

Continuum concept-- J.T. Curtis and R.P. McIntosh, 1951

Gradient concept-- R.H. Whittaker, 1951