Plant Communities of the Midwest - NatureServe

define the type. Other lower strata, whatever their cover, are somewhat less critical in terms of
identifying dominant species to define the type.
Since dominant species lend themselves to delimiting types, they may also be characteristic
species, especially where quantitative data on all plant species present in a community are not available.
But characteristic species often have at least three other interrelated features. First, they may be fairly
constant in any stratum in a community type. Second, because of their geographic range relative to
community types with similar dominant species, they may help distinguish the geographic limits of a
type. For example, flowering dogwood (Cornus florida) is more typically abundant in unglaciated,
southern oak forests compared to glaciated, northern oak forests. Third, characteristic species may be
particularly restricted to certain environmental conditions. For example, chinquapin oak (Quercus
muehlenbergii) is typically found on thin soils over alkaline bedrock and is thus characteristic of oak
types in those settings. A species may contain multiple features that make it characteristic of a type. The
choice of characteristic species, then, is partly made in relation to the definition of other types; it is chosen
because it helps discriminate one type from another. Information on characteristic species was often
developed from the characteristic species listed by a local or state classification which, upon rangewide
review, also held up regionally.
Characteristic species identified as such from a regional and rangewide perspective may, from a
local or state perspective, either be rather widespread and not helpful in discriminating types or,
conversely, be rather rare and not constant enough in stands of a particular type. These issues can be
difficult to resolve given the different geographic perspectives between regional or global definitions and
state or local definitions. This is one of the reasons that state Natural Heritage programs sometimes prefer
to develop their own names for types within a state, even when those types are conceptually equivalent to
those in the USNVC.
Finally, in addition to the more primary floristic and physiognomic criteria, the association
concept also uses criteria based on “uniform habitat conditions.” This component has also proven to be of
value in the qualitative stages of development. Habitat criteria are helpful when using studies that have
assessed vegetation patterns in relation to environmental gradients, or when working with state Natural
Heritage program classifications based on “natural community” concepts that rely as much on abiotic and
geographic criteria as on vegetation criteria. For example, the Missouri Natural Heritage Program
recognizes two kinds of alkaline glade types in the state: “Dolomite Glades” and “Limestone Glades”
(Nelson 1985). These units, named and identified primarily through physical habitat features,
nevertheless also contain some floristic and geographic differences that suggest they may represent two
different associations: the Schizachyrium scoparium – Bouteloua curtipendula – Rudbeckia missouriensis
– Mentzelia oligosperma Wooded Herbaceous Vegetation (on limestone), and the Schizachyrium
scoparium – Sorghastrum nutans – Bouteloua curtipendula – Rudbeckia missouriensis – Hedyotis
nigricans Wooded Herbaceous Vegetation (on dolomite). The 2 types share many species in common,
but at least at the outset, the requirement for uniform habitat conditions led them to be recognized as
distinct associations. It is hoped that future collection and analysis of quantitative plot data from these
two habitats can be used to clarify the distinction at the association level.
The habitat criteria are particularly helpful for more sparsely vegetated community types, such as
some cliffs, talus, or gravel bar habitats, where species assemblages may be highly variable or not well
described across their range. In some cases, associations are often named based entirely on habitat
criteria (e.g., the Basalt-Diabase Cobble-Gravel Great Lakes Shore Sparse Vegetation). Emphasizing
habitat over vegetation criteria is perhaps an interim solution, and when better data are collected on these
types, vegetation criteria may play a more predominant role in their definition.
Occasionally, associations form distinctive spatial aggregations that lend themselves to being
defined as complexes. In the Midwest, several of these complexes have been described in order to
facilitate conservation applications of the associations. Complexes include the patterned (string and flark)
PLANT COMMUNITIES OF THE MIDWEST – 2001
17