A plant community can be described floristically (the species it contains) and/or physiognomically (its physical structure ). For example, a forest (a community of trees ) includes the overstory, or upper tree layer of the canopy, as well as the understory, further subdivided into the shrub layer, herbaceous layer, and sometimes also moss layer. In some cases of complex forests there is also a well-defined lower tree layer. A plant community is similar in concept to a vegetation type, with the former having more of an emphasis on the ecological association of species within it, and the latter on overall appearance by which it is readily recognized by a layperson.
A plant community can be rare even if none of the major species defining it are rare. This is because it is the association of species and relationship to their environment that may be rare. An example is the Sycamore Alluvial Woodland in California dominated by the California sycamore Platanus racemosa.
