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Northern Prairie Wildlife Research Center

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Floristic Quality Assessment for Plant Communities of
North Dakota, South Dakota (excluding the Black Hills),
and Adjacent Grasslands


Assignment of Coefficients

At an initial meeting of scientists and land managers in November 1999, we determined that the development of a species list with standardized C values for our region's flora would improve efforts to assess the quality of remnant natural areas and aid evaluations of restoration and management efforts. A panel, the Northern Great Plains Floristic Quality Assessment Panel, was formed to develop this checklist list using the "Atlas of the Flora of the Great Plains" and the "Flora of the Great Plains" (Great Plains Flora Association 1977, 1991) as primary reference sources. In January, 2000, the panel met for a 3-day workshop at the North Dakota State University herbarium, in Fargo, North Dakota, where we assigned a C value to each taxon on the checklist (Appendix).

We attempted to include all species naturally occurring in plant communities of the Dakotas on our checklist. However, the listing of taxa is not intended to serve as an exhaustive flora of the region; we developed this checklist for the primary purpose of facilitating use of floristic quality assessment in our area. For ease of use, we followed the botanical nomenclature provided in the region's most widely used reference, Flora of the Great Plains (Great Plains Flora Association 1991). We acknowledge that some of the taxonomy is outdated, but given the current state of flux in botanical nomenclature, we decided to standardize our checklist with this widely available printed source which includes keys and detailed descriptions of each taxon. In addition, we are developing an Internet-based version of our checklist to be served from the U.S. Geological Survey's Northern Prairie Wildlife Research Center World-Wide Web site ( where we intend to provide updated synonyms and species codes standardized with the U.S. Department of Agriculture's PLANTS database (USDA, NRCS 1999).

We assigned C values (0 to 10) to each taxon of the region's native flora. In assigning coefficients, we followed the guidelines set forth by Taft et al. (1997), which are a slight modification of earlier efforts by Swink and Wilhelm (1979, 1994). The C values we assigned represent our collective knowledge of the patterns of occurrence of each plant species in the Dakotas and our confidence that a particular taxon is natural area dependent. We supplemented our combined field experience by examining range maps and reviewing habitat comments in several references for some taxa (Stevens 1963, Van Bruggen 1976, Great Plains Flora Association 1977, 1991, Larson 1993). We assigned a coefficient of 0 to weedy species that can flourish in the most highly disturbed habitats (freshly tilled fields, gardens, new road cuts, etc.). Although native to our region, these weedy taxa provided us with 0 confidence that a specimen brought into the herbarium was collected from a remnant natural area. We assigned coefficients ranging from 1 to 4 to taxa that occur in natural areas but also in highly degraded habitats. Thus, a taxon receiving a 3 provided only about 30% confidence that this plant came from a remnant natural area. Taxa to which we assigned coefficients ranging from 5 to 9 are usually found in natural areas but have decreasing degrees of tolerance to disturbance. A taxon with a coefficient of 5 likely came from a natural area, but it could have been a very disturbed natural area, and a taxon receiving a 9 likely came from a relatively undisturbed or otherwise high quality natural area. We reserved the coefficient 10 for those taxa virtually restricted to natural, undisturbed habitats in the Dakotas (i.e., we could be nearly 100% confident that a sample brought into the herbarium was collected from a high quality natural remnant). In Table 1, we provide examples of species that characterize each C value.

Table 1.  Plant species characteristic of coefficients of conservatism (C values) ranging from 0 to 10.

C value Example Species
0 Amaranthus retroflexus, Conyza canadensis, Panicum capillare
1 Beckmannia syzigachne, Draba reptans, Solidago canadensis 
2 Aster ericoides, Potentilla anserine, Verbena stricta 
3 Bidens cernua, Ratibida columnifera, Rosa arkansana 
4 Agropyron smithii, Carex eleocharis, Ribes missouriense
5 Andropogon gerardii, Cornus stolonifera, Helianthus maximiliana 
6 Eriogonum annuum, Iris missouriensis, Sorgastrum avenaceum
7 Astragalus crassicarpos, Erysimum inconspicuum, Koeleria pyramidata
8 Heuchera richardsonii, Liatris aspera, Stipa spartea 
9 Amorpha nana, Lespedeza capitata, Mertensia lanceolata 
10 Cypripedium spp., Trillium spp., Vallisneria americana

As expected, the differences in plants receiving consecutive coefficients (e.g., 2 and 3) are sometimes slight and hardly discernable. However, concern over whether a plant is assigned a 2 and not a 3 or a 1, for example, is mitigated by averaging values in the actual application of this system (see below). We assigned an asterisk to all non-native taxa as they are typically excluded from the Floristic Quality Assessment calculations described below. Taxa identified as native on our checklist, but that also occur in our region as escapees from cultivation, should be ranked as non-native when found in these situations (e.g. Juniperus scopulorum, Juniperus virginiana, and Pinus ponderosa.)

Mean C Values

By applying the C values provided in the Appendix to a plant species list assembled for an area of interest, a mean C value () can be calculated by summing the C values for each native species present in the survey and dividing the summation by the total number of species present (N):

Thus, represents the average conservatism of the plant community. By repeating surveys and calculating over periods of time, temporal changes in floristic quality can be evaluated. If habitat quality of an area is degrading, the first plants lost from the plant community will be conservative species (i.e., those with the highest C values). These conservative species will be replaced by less conservative species (i.e., those with lower C values), non-native weeds, or no plants at all and will decline. Alternatively, if habitat quality is improving, the number of conservative species will increase and be reflected by an increase in .

Non-native species are not used in calculations to estimate the floristic quality of the native plant communities. Because these species did not develop in our region, it is difficult to assess the relationship of their occurrence to habitat integrity. Additionally, some non-native species such as Bromus inermus and Poa pratensis are virtually ubiquitous in the region. It is better to measure the effect that these species have on the native plant communities by considering them as a disturbance (i.e., measuring their effect indirectly through changes in of the native plant community). If desired, a calculation of can be conducted that includes the non-native species. In these cases, non-native species should be assigned a C value of 0 and a separate calculation using only the native species should also be conducted and reported.

Floristic Quality Index

If ranking sites in order of their floristic quality, a method is needed to discriminate among areas with similar s but that differ significantly in the number of native species each supports. This can be accomplished by calculating a floristic quality index (FQI). FQI is obtained by multiplying by the square root of the number of native species present (N):

Thus, FQI is simply a weighted species richness estimate that uses a square root transformation of N to limit the influence of area alone on species richness (Swink and Wilhelm 1979, 1994). Two sites may have similar s but different FQIs or similar FQIs but different s if the number of native species each supports differs greatly. Calculating values for both and FQI facilitates better comparisons among sites.

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