Floristic Quality Index

An Explanation of the Floristic Quality Assessment Summary Report
See attachments below

Most natural areas within the city parks, and some other natural areas in Ann Arbor, have been inventoried by Natural Area Preservation (NAP). The report is arranged in descending order of their Floristic Quality Index (FQI) values, from highest to lowest quality sites.

  • Total Species is the total number of different plant species (native and non-native) identified within each site since NAP started its plant inventories in 1994.
  • Native Species is the number of species considered indigenous, or native, to Michigan.
  • E-T-SC is the number of species listed by the State of Michigan Department of Natural Resources as being endangered, threatened, or special concern in the state. Endangered and threatened plants have a protected legal status.
  • Ave Rating is the average “Coefficient of Conservatism” for all the plant species at the site.

A coefficient of conservatism is assigned to each native Michigan plant species as part of the Michigan Floristic Quality Assessment System developed by botanists and the Michigan Department of Natural Resources. The 0-10 scale generally describes how confident you can be that you are in a high-quality natural plant community – mimicking pre-European settlement conditions – when you encounter a particular species. It is tempting to think of the coefficient as the same as a measure of rarity. Although it is often true that rare plants also have a high coefficient, sometimes they don’t, and sometimes a plant with a high CoC is fairly common. It is important to remember that the coefficient is a measure of how faithful a plant is to a certain undisturbed ecosystem, not how rare it is. The Ave Rating is one of the numbers used to calculate the Floristic Quality Index (FQI) for a site. But the Ave Rating itself provides information about the botanical quality of a site because over 85% of Michigan’s total native flora has a coefficient of 4 or greater1. Non-native plant species are not assigned a Coefficient of Conservatism, and thus do not directly contribute to the Ave Rating, or to the Floristic Quality Index (FQI). But indirectly, non-native plant species effect these numerical values by displacing the native species that would otherwise grow there.
The Flor. Qual. Index for a site is calculated by multiplying the Ave Rating by the square root of the number of Native Species. So, a site with 100 native plant species (square root of 100 = 10) and an average rating of 4.0 has an FQI of 40 (10 x 4). When used in a consistent manner to compare similar sites, the FQI is a standard method to quantify the botanical, or floristic, quality of a site. A higher number may indicate a higher quality or more diverse site – at least in terms of its plant communities. Most undeveloped land in Michigan has an FQI of less than 20. In contrast, areas with an FQI of higher than 35 are generally considered to be floristically important enough to merit protection and ecological land management. In fact, the authors of the reference article cited at the end of this document suggest “that areas with known high floristic quality (FQI > 35) cannot be routinely restored to their original floristic quality and therefore are unmitigable.”1 Areas rated 50 or higher are rare and may harbor an appreciative amount of Michigan’s native biodiversity. The Floristic Quality Assessment System does not take into account how important the site is for other biological or social reasons such as wildlife, open space, passive recreation, scenic vistas, linkage corridors, etc…
Certainly Ann Arbor has some extremely biologically rich natural areas. Our restoration and stewardship efforts are being targeted primarily in those sites in an attempt to preserve their ecological integrity.

Updated Sept 28,2006

As someone who has compiled and used floristic quality assessments, as well as researched impacts of deer on vegetation, I would like to add a caveat about using FQI for evaluating deer impacts or assessing effects of deer management. The FQI is based on simple presence/absence data, rather than abundance or population size. Therefore, one could observe (as I have, although I haven’t done measurements) that the abundance of flowering trilliums in Bird Hills Park has declined from hundreds or thousands of flowers carpeting the hillside along Huron River Drive in the early 1990s to just a few clumps of flowers on the steepest slopes and under the shelter of fallen trees or branches in 2014–but this would have no effect on the FQI. The only thing that would change the FQI is if the plant disappeared altogether (or if a new species colonized the site).

The FQI offers valuable botanical information as as an indicator of the presence of rare and “conservative” plants at a given point in time, but was not intended as a tool for monitoring species populations or viability.

Jacqueline Courteau, Ph.D.
University of Michigan
Program in the Environment

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