We discussed this at length in French to English a while ago:

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Note added at 7 mins (2011-09-19 09:47:23 GMT)
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This applies even if we're talking about trees.

"The hierarchy levels, from most general to most specific, are: system, class, subclass, group, formation, alliance, and association. The lowest level, or association, is thus the most precisely defined, and incorporates the names of the dominant one to three (usually two) species of the type. An example of a vegetation type defined at the level of class might be "Forest, canopy cover > 60%"; at the level of a formation as "Winter-rain, broad-leaved, evergreen, sclerophyllous, closed-canopy forest"; at the level of alliance as "Arbutus menziesii forest"; and at the level of association as "Arbutus menziesii-Lithocarpus densiflora forest", referring to Pacific madrone-tanoak forests which occur in California and Oregon, USA."
http://en.wikipedia.org/wiki/Vegetation

Rather along the same lines as Phil's answer (but arrived at independently, I assure you).

Googles well:

"The Ozarks are a key transition between the western-most reach of oak-hickory forest and the Cherokee Prairie dominated by Big bluestem and Indiangrass with associated flora of Bluestem, Tall dropseed as well as composite forbs."
http://www.eoearth.org/article/Oak

"Associated flora of Koelreuteria paniculata (a tree) in the wild?"
http://www.asktheplantlady.com/associated-flora-of-koelreute...

make up/form part of the suite of species

Although limited in extent, the chalk grassland habitats host a rich suite of plants including wild liquorice and horseshoe vetch. Other rarities include ground pine and broad-leaved cudweed.
http://www.plantlife.org.uk/nature_reserves/ranscombe_farm_r...

Is this because species phenology are all shifted earlier, so that the same species would flower ~50 d earlier there? Or is it just a suite of species that flower earlier are found at Chinnor, but late-flowering species are not?
http://wildplantspost.blogspot.com/2010_10_01_archive.html

From an initial focus on the trees, and especially Scots pine, many projects and various conservation groups are now looking at the much broader suite of species that make up the pinewood ecosystem.

http://www.treesforlife.org.uk/tfl.biodiversity_restoration....

This is a synonnym for 'plant association', but I actually think it's more common in English.

plant association
A grouping of plant species, **or a plant community** that recurs across the landscape. Plant associations are used as indicators of environmental conditions such as temperature, moisture, light, etc.
http://www.biology-online.org/dictionary/Plant_association

http://en.wikipedia.org/wiki/Plant_community
Plant community or "phytocoenosis" (American spelling "phytocenosis", rarely used) is a collection of plant species within a designated geographical unit, which forms a relatively uniform patch, distinguishable from neighbouring patches of different vegetation types. The components of each plant community are influenced by soil type, topography, climate and human disturbance. In many cases there are several soil types within a given phytocoenosis[1].
Alpine Heathland plant community at High Shelf Camp near Mount Anne, Tasmania, Australia



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Note added at 7 hrs (2011-09-19 17:12:53 GMT)
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Another definition:

http://polyland.calpoly.edu/overview/archives/derome/communi...
VI. PLANT COMMUNITIES
A plant community is a recognizable and complex assemblage of plant species which interact with each other as well as with the elements of their environment and is distinct from adjacent assemblages. A plant community is not a static entity: rather it may vary in appearance and species composition from location to location and also over time. What makes each of these communities distinguishable to us is its general physiognomy or physical structure. This overall appearance is created by the particular species present, as well as their size, abundance, and distribution relative to one another. Dominant species, those whose presence most influences the community environment and composition, are often the largest or the most abundant and may be a single species or several codominant species. Dominance may also be sociologic, expressed in the form of allelopathogens, chemical compounds manufactured by some plants that inhibit the growth and development of other species and/or seedlings of the same species within a certain distance. Community structure and distribution are dictated by the delicate balance of environmental factors: soils, climate, topography, geography, fire, time, and humans and other living beings.