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Floristic Quality Assessment Index (FQAI)

Floristic Quality Assessment (FQA) is a bioassessment method that uses characteristics of the plant community to derive an estimate of nativity or habitat quality (Swink and Wilhelm 1979, 1994).  FQA is based on the premise that individual plant species have evolved varying degrees of tolerance to disturbance or environmental stress (Odum 1985, Chapin 1991, Hobbs and Huenneke 1992), and exhibit varying levels of fidelity to specific habitat integrity (Herman et al. 1997, Mushet et al. 2002). This combination of tolerance and fidelity is expressed numerically as a coefficient of conservatism or C-value (Swink and Wilhelm 1979, 1994).

The C-value is a number between 0 and 10 assigned to individual plant species by a panel of experts with knowledge of the native flora of a particular region (Andreas and Lichvar 1995, Alix and Scribailo 1998, Nichols 2001). Plant species with high C-values typically occur in high quality habitats, while species with low C-values occur in a wide variety of conditions and generally are highly tolerant of disturbance (Wilhelm and Ladd 1988, Matthews 2003).

Some examples of taxa along the range of C values:
Scirpus cyperinus
C-value = 2
Verbena hastata
C-value = 3
Sarracenia purpurea
C-value = 10

By its broadest definition, FQA encompasses a number of plant-based metrics (Taft et al. 1997). Most applications, however, have focused on the Floristic Quality Index (FQI), a metric the uses both the aggregate conservatism and species richness of the plant community to derive a measure of habitat condition or quality. The index is calculated according to the following equation (Swink and Wilhelm 1979, 1994):

where  represents the average coefficient of conservatism for native species, and N is native species richness. Like C values, high-quality habitats typically have high FQI scores, while low-quality habitats have low scores.

Once FQI scores have been calculated, sites can be ranked by condition. This is typically done by regressing FQI scores against some measure of anthropogenic disturbance.  The resultant “dose-response curve” graphically depicts the relationship between plant community conservatism and disturbance and gives an estimate of habitat quality.
Dose-reponse curve for headwater floodplain wetlands in central Pennsylvania. By knowing the Rapid Assessment (disturbance) and FQI scores for each wetland, we can readily classify them into different categories of condition.

Using this approach, researchers have found that FQI effectively differentiates wetlands across disturbance gradients in Pennsylvania (Miller and Wardrop 2006) and Virginia (Nichols et al. 2006, DeBerry 2006). 

Recent emphasis on advancing floristic assessment at the regional scale has led to a collaborative effort to develop comprehensive C-values for the Mid-Atlantic region.  In 2009, the Mid-Atlantic Regional Floristic Quality Assessment Project was initiated as a joint effort between EPA Region 3 and its member states.  Click HERE for more information on the Mid-Atlantic Regional Floristic Quality Assessment Project.

One of the products of the Mid-Atlantic Regional Floristic Quality Assessment Project was an interactive calculator to assist in determining FQA metrics. Use the following steps to determine the floristic quality of your site based on a plant list from the site. For the most accurate assessment of floristic quality, it is recommended that you have a comprehensive list of plants at your site prior to using the calculator.
  1. Select the ecoregion you are working in from the drop-down box at the top of the page. The regional list is divided into the five main ecoregions in the Mid-Atlantic region: Appalachian Plateau, Glaciated Plateau (Pennsylvania only), Ridge & Valley, Piedmont and Coastal Plain.
  2. Enter plants either by cutting and pasting an exiting list (such as an Excel Spreadsheet) into the box provided or by adding plants individually using the Search tab. To use the Search tab, simply begin typing in any part of the scientific name or symbol and search-assist will provide selectable suggestions. When you've located your plant, hit Add to ad it to the list.
  3. Press the Submit List button. A pop-up box will appear that shows accepted plants and those that cannot be located in the database. Unrecognized plants are generally due to either misspellings or differences in nomenclature and will need to be reconciled since they will not appear in the final list. The calculator is based on the USDA Plants database and all lists should be reconciled to that authority prior to submittal.
  4. To revise the list, hit the Cancel tab. You can then correct misspellings and nomenclature, delete entries and add entries by hand. You can also delete entries by hand once they've been submitted by clicking on the red "x" next to the plant name. If you choose to make corrections to your original list, hit Continue to submit the list and then Clear List to start over.
  5. Once your list is correct and you hit Continue, the calculator provides the metrics below. The calculator also provides graphic depictions of the percent native and non-native species present and the tolerance rankings of the plant community.
    Metric Description Notes
    FQI Uses only native species
    Adjusted FQI Includes non-native species (A)
    Total Mean C Average (CNative and CNon-Native) Mean coefficient value for native and non-native species
    Total N No. of Native species + No. of Non-native species Total number of species present
    Native Mean C Average (CNative) Mean coefficient value for native species
    Native N No. of Native species Total number of native species present
  6. The metrics may then be saved in pdf format using the print tab. In this part of the calculator, you can give your site a unique identifier and specify the person completing the form. The calculator does not store site data, so all information should be printed prior to exiting the program.


Literature Cited:
  • Alix, M. S. and R. W. Scribailo. 1998. Aquatic plant species diversity and floristic quality assessment of Saugany Lake, Indiana. Proceedings from the Indiana Academy of Science 107:123-139.
  • Andreas, B. K. and R. W. Lichvar. 1995. Floristic Index for establishing assessment standards: a case study for Northern Ohio. U.S. Army Corps of Engineers, Waterways Experiment Station, Technical Report WRP-DE-8, Washington, DC.
  • Chapin, F. S., III. 1991. Integrated responses of plants to stress. Bioscience 41:29-36.
  • DeBerry, D. A. 2006. Floristic Quality Index: ecological and management implications in created and natural wetlands. Ph.D. Dissertation, College of William and Mary, Williamsburg, VA.
  • Herman, K. D., L. A. Masters, M. R. Penskar, A. A. Reznicek, G. S. Wilhelm, and W. W. Brodowicz. 1997. Floristic quality assessment: development and application in the state of Michigan (USA). Natural Areas Journal 17:265–279.
  • Hobbs, R. J. and L. F. Huenneke. 1992. Disturbance, diversity, and invasion: implications for conservation. Conservation Biology 6(3):324-337.
  • Matthews, J. W. 2003. Assessment of the floristic quality index for use in Illinois, USA,
  • wetlands. Natural Areas Journal 23:53-60.
  • Miller, S. J. and D. H. Wardrop. 2006. Adapting the floristic quality assessment index to indicate anthropogenic disturbance in central Pennsylvania wetlands. Ecological Indicators 6:313-326.
  • Mushet, D. M., N. H. Euliss, Jr., and T. L. Shaffer. 2002. Floristic quality assessment of one natural and three restored wetland complexes in North Dakota, USA. Wetlands 22:126-38.
  • Nichols, S. A. 2001. Long-term change in Wisconsin lake plant communities. Journal of Freshwater Ecology 16:1-13.
  • Nichols, J. D., J. E. Perry, and D. A. DeBerry. 2006. Using a Floristic Quality Assessment technique to evaluate plant community integrity of forested wetlands in southeastern Virginia, USA. Natural Areas Journal 26: 360-369.
  • Odum, E. P. 1985. Trends expected in stressed ecosystems. Bioscience 35:419-422.
  • Swink, F. and G. S. Wilhelm. 1979. Plants of the Chicago Region, 3rd ed. Morton Arboretum, Lisle, IL. 922 pp.
  • Swink, F. and G. S. Wilhelm. 1994. Plants of the Chicago Region, 4th ed. Indiana Academy of Science, Indianapolis, IN.
  • Taft, J. B., G. S. Wilhelm, D. M. Ladd, and L. A. Masters. 1997. Floristic quality assessment for vegetation in Illinois: a method for assessing vegetation integrity. Erigenia 15: 3–95.
  • Wilhelm, G. and D. Ladd. 1988. Natural area assessment in the Chicago Region. Transactions of the North American Wildlife and Natural Resources Conference 53:361-375.