Is there a standard format for plant community composition data?
There does not appear to be a current standard, but Veg-X is a standard that has been developed to facilitate exchange of plot-based vegetation data, and may provide the closest to what you are looking for.

Veg-X is described in Wiser et al 2011 and the project home page is http://wiki.tdwg.org/Vegetation/

From the abstract:


  The exchange standard for plot-based vegetation data (Veg-X) allows
  for observations of vegetation at both individual plant and aggregated
  observation levels. It ensures that observations are ﬁxed to physical
  sample plots at speciﬁc points in space and time, and makes a
  distinction between the entity of interest (e.g. an individual tree)
  and the observational act (i.e. a measurement). The standard supports
  repeated measurements of both individual organisms and plots, allows
  observations of entities to be grouped following predeﬁned or
  user-deﬁned criteria, and ensures that the connection between the
  entity observed and taxonomic concept associated with that observation
  are maintained.




Wiser, S.K., N. Spencer, M. De Caceres, M. Kleikamp, B. Boyle, &amp; R.K. Peet. 2011. Veg-X -- An international exchange standard for plot-based vegetation data. Journal of Vegetation Science 22:598-609.