Grisebach, A. 1838. Ueber den Einfluss des Climas auf die Begranzung der naturlich Floren. Linnaea 12:159-200.

I would term a group of plants which bears a definite physiognomic character, such as a meadow, a forest, etc., a phytogeographic formation. The latter may be characterized by a single social species, by a complex of dominant species belonging to one family, or finally, it may show an aggregate of species, which, though of various taxonomic character, have a common peculiarity; thus, the alpine meadow consists almost exclusively of perennial herbs.

Mobius, K. 1877. Die Auster and die Austerwirtschaft. [The Oyster and the Oysterbed.] Wiegumdt, Hempel and Parey, Berlin.

A group of living organisms corresponding in composition (i.e., in numbers of species and individuals) with a particular set of environmental conditions. These organisms are bound together by mutual dependence upon one another and are able to maintain themselves, through reproduction, permanently in a particular locality ... if any of the conditions are changed for any length of time, the structure of the community will be changed. [From R. Dajoz. 1976. Introduction to Ecology. Crane, Russak and Co., Inc., New York.]

Clements, F.E. 1916. Plant Succession: An Analysis of the Development of Vegetation. Carnegie Inst. Wash. Publ. 242. [Also see Weaver and Clements 1938, Plant Ecology, McGraw-Hill, New York.]

Vegetation is the sum total of plants covering an area ... vegetation is more than the mere grouping of individual plants. It is the result of the interactions of numerous factors. The effects of the plants upon the place in which they live and their influence upon each other are especially significant ... a study of vegetation reveals that it is an organic entity and that, like an organism, each part is interdependent upon every other part ... the development of vegetation consists of a number of closely related processes (migration, which includes all movements by means of which plants move from one place to another; ecesis, involving germination, growth to maturity and reproduction; aggregation, the grouping of individuals as a result of propagation; competition, which occurs when plants are so aggregated that the demand for energy or materials exceeds the supply; reactions, or the effects of plant on their environment; and stabilization, which occurs when vegetation is in equilibrium with its climate) ... thus, like all organisms, vegetation arises, develops, matures, reproduces, and may eventually die ... vegetation, like all organisms, not only undergoes development but also possesses structure. Depending upon climate, composition or structure varies (e.g., grassland, forest, tundra); these large units are formation. A formation is a fully developed or climax community of a natural area in which the essential climatic relations are similar or identical. Each formation is a complex and definite organic entity with a characteristic development and structure. It is a product of the climate and is controlled by it. The climax (or climax formation) is not merely the response to a particular climate, but is at the same time the expression and indication of this climate. The major subdivisions of the climax formation are called associations; each corresponds to a subdivision of the climate within the formation (e.g., shortgrass prairie, mixed prairie, true prairie), and is marked by one or more dominants peculiar to it. Faciations are concrete subdivisions of the associations characterized by the grouping of dominant species (e.g., the blue grama-buffalograss faciation of the shortgrass prairie association) and corresponding to real but small differences in a particular regional climate. Societies are local communities within the faciation where certain subdominant species exert local control. A family is a small group of plants belonging to the same species, usually occupying recently disturbed areas. It must not be supposed that one can distinguish the units of vegetation of any given area by superficial examination ... the study of a local area of vegetation should always be supplemented so far as is feasible by examination of adjacent areas. Only in this way is it possible to gain a broad viewpoint for a proper perspective of the local communities. Until sufficient examination has been made by means of ecological methods to warrant placing any area of vegetation in a particular formation, association, faciation, or smaller unit, each should be called a community, a term which implies no definite rank ... the term community should be used as an inclusive term for any and all units of vegetation from the formation to the family.

Gleason, H.A. 1917. The structure and development of the plant association. Bull. Torrey Bot. Club 43:463-481. [Also see Gleason 1926, The individualistic concept of the plant association, Bull. Torrey Bot. Club 53:7-26 and Gleason 1939, The individualistic concept of the plant association, Amer. Midl. Nat. 21:92-110.]

Towards a concept of association, the following points must be considered:

Granted, of the actual existence of definable units of vegetation there is no doubt that these units have describable structure, that they appear, maintain themselves, and eventually disappear are observable facts.

General principles in explanation of the usual phenomenon of vegetation:

1. Vegetation, in its broader aspects, is composed of a number of plant individuals. The development and maintenance of vegetation is therefore merely the resultant of the development and maintenance of the component individuals ... according to this view, the phenomena of vegetation depend completely upon the phenomena of the individual. It is in sharp contrast with the view of Clements that the unit of vegetation is an organism, which exhibits a series of functions distinct from those of the individual and within which the individual plants play a part as subsidiary to the whole.
2. In the same limited region, that is, with the same surrounding population, areas of similar environment, whether continuous or detached, are therefore occupied by similar assemblages of species. Such an assemblage is called a plant association.
3. On of the most important features of the environment is the control or modification of the original physical factors by the plant population itself ... the physical factors of the environment generally vary gradually in space ... such gradual and progressive variation of environment would normally lead to equally gradual and progressive changes in the vegetation and to the establishment of broad transition zones between adjacent associations, in which the species of both mingle. (In fact, the difficulty of distinguishing associations in many regions is to be expected when the vegetation depends upon environmental selection of favored individuals; this same difficulty is hard to reconcile with the belief that the plant association is an organic entity.)
4. The general tendency of the population of an association to migration tends to produce uniform distribution of each species within it, and consequently uniformity of the association as a whole. This uniformity is one of the most characteristic visible features of the association, and has by some been taken as the basis for a definition of it.
5. Because of differences in the surrounding plant population, from which the inhabitants of an area are drawn; because of accidents of migration and the time available for it; and because of environmental differences, no two areas need have identical populations, measured by component species and the relative number of individuals of each ... whether any two areas, either contiguous or separated, represent the same plant association, detached examples of the same one, developmental stages of one, or different associations, and how much variation of structure may be allowed within an association without affecting its identity, are both purely academic questions, since the association represents merely the coincidence of certain plant individuals and is not an organic entity of itself. While the similarity of vegetation in two detached areas may be striking, it is only an expression of similar environmental conditions and similar surrounding plant populations. If they are for convenience described under the same name, this treatment is in no wise comparable to the inclusion of several plant individuals in one species.

Egler, F.E. 1942. Vegetation as an object of study. Phil. Sci. 9:245-260.

Vegetation, through emergent evolution, has become an organism, an organized whole, which is social insofar as it is organic in nature.... The characteristics of vegetation are something more than the mere summation of the characteristics of its parts. The characteristics of the parts are present in the whole to a greater or less degree. New characteristics, typical of the whole, have emerged. This social organism is self-sufficient, stable, and tends to perpetuate itself essentially unchanged in nature.

McIntosh, R.P. 1958. Plant communities. Science 128:115-120.

After reviewing evidence derived from gradient analysis studies on the basic question: whether or not species populations occur as groups which are discontinuous with other groups and recognizable as a series of discrete classes, the author states that when plant populations are analyzed according to their quantitative distribution along an environmental gradient, each species forms a bell-shaped curve. The curves are distributed along the gradient as a series with overlapping ranges, but no two curves have identical ranges or optima. No clusters of species are found, indicative of similar species behavior relative to the gradient and leading therefore to recognition of a series of discrete units. This suggests that species populations form a shifting series of combinations along environmental gradients and leads to the interpretation of vegetation as a complex and largely continuous population pattern ... the order in nature is not one of clear-cut cause and effect relationships resulting in well-defined aggregations of species into clearly bounded and readily pigeonholed units as objective natural entities. Much recent evidence points toward the concept of communities as an ordered pattern of species, individually distributed in space and time and most effectively considered in terms of orders and gradients.

McIntosh, R.P. 1960. Natural order and communities. The Biologist 42:55-62.

A community is thus visualized as a continuous, orderly pattern of species individually distributed in time and space and most effectively considered in terms of gradients, orders and probabilities. Species having ecological amplitudes over similar sectors of the gradient have greater probabilities of being represented in frequent combinations in nature ... each curve has its own optimal point, these being distributed uniformly along a gradient, and as additional species are studied, their optima are interspersed among the others, seemingly leaving no niche unfilled.

McIntosh, R.P. 1963. Ecosystems evaluation and relational patterns of living organisms. Amer. Sci. 51:246-267.

The concept of the ecosystem includes a group of organisms of different kinds (the biotic community) with reciprocal relations to the nonliving environment and especially having mutual relations of varying kinds and degrees among themselves. Each species population is presumed to occupy a particular niche in the complex, which is determined by its relations to the physical environment, to the other living components of the system and to its role in securing, transforming, and transferring energy in the system ... the ecosystem is not chaotic but is subject to restrictions due to the environment, the biological properties of the organisms, and the interactions among them which impose limits upon the aggregations of organisms and the patterns of relations which occur in nature.

McIntosh, R.P. 1970. Community, competition and adaptation. Quart. Rev. Bio. 45:259-280.

A community is a multispecies aggregation with varying degrees of integration, a more or less specific composition, and some degree of repeatability and consistency from place to place (hence a community type).

Kimmins, J.P. 1987. Forest Ecology. Macmillan Publ. Co., New York.

... each community is characterized by a particular species composition, vertical structure, patterns of change over time, biomass, energy flow, and nutrient cycling. It is the biotic component of the ecosystem. Community has no implicit definition of spatial extent or boundaries.

Orloci, L. 1988. Detecting vegetation patterns. ISI Atlas of Science: Plants & Animals, 1:183-177.

The primary objective of vegetation analysis is to explain what brings a plant community into existence and makes it function as an organic unit.

Wilson, D.S. and E. Sober. 1988. Reviving the superorganism. J. Theor. Biol.

... groups and communities can be organisms in the same sense that individuals are. Furthermore, superorganisms are more than just a theoretical possibility and actually exist in nature.

Drake, J.A. 1990. Communities as assembled structures: do rules govern pattern? Trends Ecol. Evol. 5:159-164.

... a community is defined as the ensemble of species in some area whose limits are determined by the practical extent of energy flow. The key to determining community limits is to identify boundaries, manifest as interspecific interactions broadly defined, by documenting where the population dynamics of a species in an ensemble (including indirect and cascading effects) are unaffected by each other.... Such a definition may include a large number of species, so much so that critics might plead unwieldy complexity. However, nature proceeds without regard to human logistical and analytical sophistication.

Modern discussions of the community concept include the following elements:

1. Vegetation and climate are inseparable

   Greek philosopher Hippocrates (400-370 B.C.) recognized effect of environment on plants

   Biggest advocate was Clements, but even Gleason agreed with this concept

   Clements defined climax as a product of, and controlled by, climate (organismic view)

   Shreve, then Gleason and Raminsky--species vary gradually along environmental gradients

   We can extend the concept of inseparability to environment (vs. merely climate)

   Extrinsic factors--environment per se

   Intrinsic factors--environment as influenced by plants

   
   
   
   
   

   Jack Major (1951, J. Ecol. 32:392-412) modified Hans Jenny's equations of state for soil-forming factors to formalize the relationship between vegetation and environment: veg = f(organisms, climate, soil, time) (i.e., veg = f (environment)

2. Species in community interact--at least some of the species, some of the time (degree of interaction is focus of considerable debate)

   Demonstrating interaction does not infer that the interaction shapes community structure

   However, to the extent that interactions dictate resource allocation, these interactions can contribute to the organization of plants into communities

   Westhoff (1951, Synthese 8:194-206) summarized these two concepts (with a Clementsian slant):

   The notion of the plant community as a collection of individuals that are more or less regularly grouped in space is not only a statement of fact derived from experience, but it is also an hypothesis: that plants in such groupings are interrelated and that their combination is not merely the result of a selection by an extrinsic environment.

3. Communities are delimitable in space and time; thus, communities have boundaries

   Two interpretations of community

   ~ stand (concrete): acc. to proponents of the individualistic view, no two are identical; therefore, community as an organism does not exist

   ~ association (abstract): class concept, of which members are stands (examples) of the concept

   
   
   
   

4. Communities are characterized by some degree of structural homogeneity

   Disturbance and/or development --> inability to recognize the community

   Usu. based only on spp. composition and abundance, but Drake (1990 TREE 5:159-164) indicates that energy flow should be the basis. Drake acknowledges this will be difficult to measure, but writes: "nature proceeds without regard to human logisitical and analytical sophistication."

Data collection for plant communities

Desirable qualities of community samples:

1. Appropriate:
   • to the character of the community

     
     

   • to investigator's research purposes

     
     

   • to plans for subsequent data analysis

     
     

   • in intensity and breadth to support research conclusions

     
     

2. Homogeneous in structure
   • scale is problematic, because plants are rarely distributed randomly--they are patterned on several scales

     
     

   • units of community ecology (i.e., communities) are not naturally delimited--subjective decisions must be made about what area to include or exclude

     
     

   • sites vary in degree of homogeneity and in type and severity of disturbances

     
     

   It has been suggested that the problem of obtaining reasonable sample homogeneity can be resolved along two lines:

   1. subjective procedures are generally adequate (judgments of experienced ecologists tend to coincide)

   2. if classifying communities into "types" is the goal, then the critical matter is homogeneity within sample sites relative to overall variation in the data set; if considerable variability exists in the data set relative to "within-community", then clear patterns will emerge

3. Objective and standardized

   there are many different sampling procedures, so selection of one is subjective

   once selected, sampling procedure should be applicable in an objective, standardized way; it should also be unambiguous and operational

   allows comparisons across treatments, years (data sets?)--long-term data collection often needed

4. Efficient

   maximum amount of information per unit of time and effort

   
   

Standard community sampling procedures

Selection of sampling procedures should consider at least the following:

1. kinds of communities sampled

2. kinds of environmental and historical data needed to complement (corroborate?) vegetation data

3. scope, accuracy, and purposes of the study

4. requirements to allow comparison w/ other studies

5. requirements for valid application of anticipated data analysis methods

6. practical limitations

Typical sampling methods

Some measure of abundance of individual taxa (not necessarily species, though this is usu. attempted)

Quadrat sampling

Determination of quadrat and sample size:

1. Species-area method

   nested plots, each 2 x previous --> species-area curve

   
   

   various cutoff rules devised to determine where curve is "flat"

   
   

2. Statistical methods

   compute standard errors of the mean for dominant (or common) species for various quadrat sizes and number

   
   

3. Convention

Attributes commonly used to signify importance:

frequency

density (w/ easily-distinguishable individuals)

biomass (aboveground)

cover (usu. foliar)

usu. visual estimation of precise cover or assignment to cover classes (or both, w/ former preceding latter)

various scales are used for cover classes; results depend on scale used

Data collection limitations:

1. Plant distributions and environmental factors involve many spatial scales, so any sample (quadrat) size may be appropriate for some species, too large for others, too small for others

2. Accuracy usu. can be improved by increasing size and number of quadrats (to a point), w/ cost as a trade-off

Typical data management methods:

1. Transformations to reduce heterogeneity of variance

   W/ ecological data, variance usu. incr. w/ incr. in mean cover, density, or biomass --> log transformation

   y_new = log_e(y_old)

   or, w/ values of 0, y_new = log_e(y_old + 1)

   van der Maarel's cover/abundance scale is comparable to log-transformed data for cover values (5-9); abundance values (1-4) all unrealistically represent identical cover values

2. Remove rare species (5% frequency)

   more on the effects of this later

3. Eliminate outliers