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Wisconsin Plant of the Week

Coefficients of Conservatism and Floristic Quality Assessment

Floristic Quality Assessment

Floristic quality assessment is a standardized tool for natural area assessment developed by Floyd Swink and Gerald Wilhelm (1994).  The method replaces very subjective measures of quality, such as “high” or “low”, with a still somewhat subjective, but more dispassionate and quantitative index.  This “Floristic Quality Index” allows comparison of the floristic quality among many sites and tracking changes at the same site over time.  It is not intended to be used as a stand-alone method, but rather to complement and corroborate other methods of evaluating the natural quality of a site.

The method assigns a Coefficient of Conservatism to each native plant species based on that species tolerance for disturbance and fidelity to a particular pre-settlement plant community type.  The aggregate conservatism of all the plants inhabiting a site determine its floristic quality.  Refer to Swink and Wilhelm (1994) for a thorough discussion of the method and how to calculate a Floristic Quality Index.

Coefficient of Conservatism, C

The concept of species conservatism is the foundation of floristic quality assessment. Each native species is assigned a coefficient of conservatism (C) following the methods described by Swink and Wilhelm (1994) and Wilhelm and Masters (1995).  Coefficients of conservatism range from 0 to 10 and represent an estimated probability that a plant is likely to occur in a landscape relatively unaltered from what is believed to be a pre-settlement condition.  For example, a C of 0, is given to plants such as Acer negundo, box elder, that have demonstrated little fidelity to any remnant natural community, i.e. may be found almost anywhere.  Similarly, a C of 10 is applied to plants like Potentilla fructicosa (shrubby cinquefoil) that are almost always restricted to a pre-settlement remnant, i.e. a high quality natural area.  Introduced plants were not part of the pre-settlement flora, so no C value is applied to these.

While C values are assigned based on collective extensive experience with the flora though out an area the assignments are still somewhat subjective. The conceptual difference between a value of 0 and a value of 1, or between 9 and 10, is slight, while the difference between a value of 0 and a value of 3 is more distinct. 

Also certain species are known to exhibit varying degrees of conservatism over their range.  For example Thuja occidentalis (northern white cedar) in southern Wisconsin is restricted to relatively few habitats and may justify a C of 8 or 9.  In the north the same species is found over a broad range of natural communities and even disturbed sites, so it might justify a C of 1 or 2. For such cases an intermediate C value may be assigned. Concerns over any particular C value are usually compensated within the floristic quality assessment method since it requires the average C value of all the individual species that occur at a site.

Numerical values assigned to a particular species are based on the observed behavior of populations within a defined geographic area. As one travels away from this region these locally assigned values may be less valid.  The further away one goes the more likely these values do not reflect local conditions.  In addition to the Chicago Region Coefficients of Conservatism have been assigned for Northern Ohio, Missouri and Michigan. 

Coefficients of Conservatism specifically for Wisconsin plant species are being developed and, as of October 2001, will soon be available at the UW Herbarium website.  This site lists the C value assigned for the Chicago Region (S&W), for the State of Michigan (MI) and, now that we have our own, for Wisconsin (WI).  These often differ due to the varying degrees of conservatism over a wide geographic range as explained above. Where Wisconsin C values are not yet listed the Chicago Region C values apply to those Wisconsin counties within the Chicago Region -- Racine, Kenosha and Walworth Counties -- and generally in the southern third of Wisconsin or south of the Tension Zone.  The Michigan C values are more applicable to the north. In the extreme southwest part of Wisconsin both C values are less appropriate, since the flora in that region of Wisconsin is influenced by species that occur in the Great Plains. 


Swink, F. and G. Wilhelm (1994). Plants of the Chicago Region, 4th ed., Indiana Academy of Science, Indianapolis, 921 pp.

Wilhelm, G. S. and L. A. Masters (1995). Floristic Quality Assessment in the Chicago Region and Application Computer Programs, Morton Arboretum, Lisle, IL. 17 pp. + Appendices.

Herman, K.D., L. A. Masters, M. R. Penskar, A.A. Reznicek, G. S. Wilhelm, and W. W. Brodowicz (1996). Floristic quality assessment with wetland categories and computer application programs for the State of Michigan.  Michigan Department of Natural Resources, Wildlife Division, Natural Heritage Program.  Lansing, MI. 21 pp. + Appendices.


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