Floristic Quality Assessment

Regions with C-values assigned to their flora (as of 2019)[6]

Each plant species in a region is assigned a coefficient of conservatism, also known as a C-value, ranging between 0 and 10.[1] A plant species with a higher score (e.g. 10) has a lower tolerance to environmental degradation such as overgrazing or development and therefore is naturally restricted to undisturbed, remnant habitats. Non-native plants are either assigned a C-value of 0 or are excluded from assessments.[7] In the Chicago Region, 84% of the native plant species have a C-value of 4 or greater.[8] Plants with a C-value of 4 or greater rarely naturally move from a remnant area to surrounding degraded land.[8] For example, the federally endangered Dalea foliosa has a C-value of 10.[9]

C-values are assigned within specific ecological and geographic regions by botanical experts familiar with the species' autecology within the respective regions.[10] As of February 2018, there were more than 50 different FQA databases ranging from the Gulf Coastal Plain to western Washington, though most databases represented regions in the eastern and central United States and Canada.[11][6]

The mean C-value (${\displaystyle {\bar {C}}}$) is calculated based on an inventory of plants. An area with a native mean C-value of 3.5 or higher likely has "sufficient floristic quality to be of at least marginal natural area quality."[2] Remnant natural areas with mean C-values of 4.0 or greater are unmitigable.[8]

The Floristic Quality Index (FQI, or Rating Index according to Swink and Wilhelm[2]^: 855) is calculated by multiplying the mean C value by the square root of the total number of species:

${\displaystyle {\bar {C}}{\sqrt {n}}}$

For example, the FQI for Nelson Lake Marsh was 78 in 1994 [4]^: 186 and that for Russell R. Kirt Prairie was about 30 in 1999.[12]

• Universal FQA