Plant Communities of the Midwest - NatureServe

Midwestern and Southeastern regions of ABI (see Figure 3). That is, all associations found in the two
regions are uniquely placed into a single agreed-upon ecological group at this level. Higher levels of
organization at Levels 1-3 are undergoing more extensive review. The hierarchy of ecological groups for
Levels 1-3 in the Midwest is shown in Table 6.
In this report, only those associations found in ABI’s Midwestern region are reported for an
ecological group, but there may be other associations in that group that are found in other regions. Thus,
some ecological groups that appear to contain few member associations would be much larger if a
complete list of associations were included. (For example, there is only one association in the Midwest
reported for the Rocky Mountains Rich Fens group, but there are many other fen associations in the
Rocky Mountains of the Western United States).
Illustration of the Ecological Groups Approach
The process of developing ecological groups to portray the ecological relationships among
associations is an iterative one. That is, the member associations help determine the criteria for ecological
groups, and the groups in turn highlight some of the major ecological characteristics that are shared
among the associations. An illustration of how ecological groups and associations can be determined may
help demonstrate this relationship.
Tallgrass prairies were a dominant feature of the midwestern landscape. These prairies are
defined by the one to two meter tall grass physiognomy and the dominance of a number of grass species,
including big bluestem (Andropogon gerardii), Indian grass (Sorghastrum nutans), little bluestem
(Schizachyrium scoparium), switchgrass (Panicum virgatum), prairie dropseed (Sporobolus heterolepis),
porcupine grass (Hesperostipa spartea) and others. Tallgrass prairies also form a complex ecotone with
midwestern oak forests, woodlands, and savannas (see Anderson 1983, Sims and Risser 2000).
Numerous studies of tallgrass prairies have interpreted the vegetation patterns using a topographic
moisture and substrate gradient (Curtis 1959, Dix and Smeins 1967, Nelson and Anderson 1983, White
and Glenn-Lewin 1984, Faber-Langendoen and Maycock 1987, Umbanhowar 1992, Robertson et al.
1997), and many midwestern state Natural Heritage programs have used similar approaches in their
classifications (White and Madany 1978, Nelson 1985, Minnesota Natural Heritage Program 1993).
These studies helped inform the association units recognized here and suggest some useful ecological
group categories that might clarify the underlying abiotic gradients.
To demonstrate how these ecological groups can help bridge the association concept and an
ecological-type perspective, we can use Nelson’s (1985) natural community types in Missouri. Tallgrass
prairie types in Missouri were defined using moisture and substrate. Nelson provides a schematic
illustration of the range of types from dry prairie to wet-mesic prairie vegetation along a topographic
gradient of substrate and moisture (Figure 5). Associations in the Midwest were defined in part using this
information, and the “cross-walk” links can be readily shown (see also Table 3). The associations and
natural community are then categorized into two ecological groups, the “Midwestern Deep Soil Tallgrass
Prairies” and the “Midwestern Thin Soil Tallgrass Prairies,” which in turn are grouped into “Midwestern
Tallgrass Prairies” (Figure 5). Thus, the original habitat criteria used for the associations and natural
community types (substrate and physiognomy) are used to define their ecological group membership.
The overall presentation of ecological groups is familiar to many ecologists, and the linkage of these
groups to both associations and state types clarifies the scale of resolution provided by the associations.
PLANT COMMUNITIES OF THE MIDWEST – 2001
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