These authors define the concept of a plant community through discussion of its evolution. They start by defining the term ‘vegetation’ in a way that may surprise some readers because it excludes plants growing in certain situations. To be considered vegetation, plants need to emerge spontaneously.
Vegetation, the central object of study in vegetation ecology, can be loosely defined as a system of largely spontaneously growing plants. Not all growing plants form vegetation, for instance, a sown corn field or a flower bed in a garden do not. But the weeds surrounding such plants do form vegetation. A pine plantation will become vegetation after some years of spontaneous growth of the pine trees and the subsequent development of an understory [van der Maarel 2013: 1].
Two competing schools of thought regarding the nature of a stand of plants growing together geographically are represented by two early 20th Century botanists. H.A. Gleason observed “that species are ‘individualistically’ distributed along omnipresent environmental gradients and thus cannot form bounded communities” [van der Maarel 2013: 2]. By contrast, E. Clements compared plant community with an integral organism, where the whole was greater than the sum of its parts. During the same time period, the Braun-Blanquet approach was developed, which “paid much attention to the relations of plant communities with the environment and the interactions within communities, which is now incorporated in the concept of ecosystem” [van der Maarel 2013: 2].
The authors state that while individual plant species are distributed according to abiotic environmental conditions, the fact of being co-located with particular sets of other species in a particular environment results in interspecies interactions, which are in fact ecosystem processes (emergent properties).
In conclusion, a plant community is generally recognized as a relatively uniform piece of vegetation in a uniform environment, with a recognizable floristic composition and structure, that is relatively distinct from the surrounding vegetation. Even if the populations of the participating species are usually distributed individualistically in the landscape, they may well interact within the community and build up an integrated unit with emergent properties. At the same time, plant communities can be convenient units for conveying information about vegetation and its environment [van der Maarel 2013: 4].
Van der Maarel, Eddy & Janet Franklin, 2013, Vegetation ecology: historical notes and outline, Vegetation Ecology 2nd ed., John Wiley & Sons, Ltd., https://onlinelibrary.wiley.com/doi/book/10.1002/9781118452592