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Plant Stewardship Index

The Plant Stewardship Index is a standardized assessment tool that calculates a numerical index reflecting the quality of native plant communities for a given area. It indicates the impacts of invasive plants and can also be used to monitor the efficacy of land management practices.


A common goal of land stewardship projects is to preserve and protect those remnants of the natural environment that remain relatively intact and to protect these sensitive areas from being adversely impacted by human activities. A standardized assessment tool, such as the Plant Stewardship Index (PSI) provides land stewards with the information they need to establish stewardship priorities and set management goals. The PSI helps land stewards answer two questions: What is the naturalness of any site? How have land management practices (or their absence) affected that naturalness over time?


The PSI is based on the observation that plants may act as generalists, which are not particular about where they grow and might grow very well on roadside ditches or in dooryards, as opposed to conservative species, which may grow only in specialized habitats, such as serpentine barrens or the pine barrens of New Jersey. For all of New Jersey and the Piedmont region of Pennsylvania, over 2000 plants have been catalogued and assigned a number from zero to ten by local experts and botanists. Zero represents the most generalist species, tolerant of disturbance and includes invasive or introduced nonnative species. Ten represents the most conservative species and includes many rare and endangered state-listed native plants that require special habitats and do not regrow after disturbance.

This database of plants and associated numerical values are available to users through Bowman's Hill Wildflower Preserve's PSI Calculator. Once a user inputs a list of plants found on a particular site, the calculator automatically computes: (1) the Mean C, which is the average of all the assigned numbers of plants found on a site, and (2) the Index number, which is the Mean C multiplied by the square root of the total number of plant species.


Assigning Coefficients of Conservatism Overall Assessment of Analysis by Habitat Type or Management Unit

Prior to undertaking a PSI, stewards must decide whether they are interested in an overall assessment of floristic quality, or in a more detailed analysis by habitat types or management units, in which case individual inventories of each habitat or unit would be compiled. For example, a property might consist of an upper meadow and riparian corridor adjacent to one another, but requiring potentially different management regimes. In this case, separate inventories could be compiled for each section, allowing each area's quality to be tracked separately. The Bowman Hill Wildflower Preserve's PSI calculator can then run an aggregate computation for the total PSI values of the two areas considered together, thus yielding an overall index value.

To determine if floristic quality is distributed more or less evenly throughout the site, or if some areas are more degraded than others, quadrat plots may be sampled, either randomly or along a transect. Commonly, a cover category or percent cover is assigned to each species within a quadrat, so that importance values may also be calculated for each of the species present. Native and total mean C values may fluctuate along the transect, as may percent native, relative importance and FQAI/PSI, all of which will reflect changes in quality across the site. Another approach to the question of uniform floristic quality across a site would be to establish permanent sampling plots in representative portions of a site with surveys that are repeated on a regular basisfor the management of these public trusts.

Assigning Coefficients of Conservatism

To apply the PSI methodology, an inventory of the study area is done and each native taxon is assigned a coefficient of conservatism (C), an integer from 0 to 10, termed a coefficient of conservatism (C), which combines its tolerance to disturbance and its fidelity to a pristine or undisturbed habitat (remnant). Species given a C-value of 0-1 are taxa adapted to severe disturbances, while species ranked with a C-value of 2-3 are associated with somewhat more stable, though degraded, environments. Those species with coefficients of 4-6 include many dominant or matrix species for several habitats; they have a high consistency of occurrence within given community types. Species with C-values of 7-8 are taxa we mostly associated with natural areas, but that can be found persisting where the habitat has been somewhat degraded. Those species with coefficients of 9-10 are considered to be restricted to high-quality natural areas.

Coefficients of Conservatism are assigned by panels of experts familiar with their regional vascular flora. Coefficients have been assigned to the vascular flora of the Piedmont area of Pennsylvania and to the Upper Penn's Creek area in central PA, as well as to Ohio, Colorado, Florida, Mississippi, the Dakotas, Illinois, Michigan, Virginia, West Virginia, Missouri, Michigan, Iowa and southern Ontario.

Coefficients of Conservatism for the Piedmont area of Pennsylvania are available to download* (No longer available at Bowman's Hill Wildflower Preserve, 2017)

The Mean Native C value is a meaningful measure of habitat quality. It is not dependent on size of inventory area and is thus a meaningful measure of native species richness. (Rooney + Roger. 2003) A difference between native mean C and total mean C that is greater than 5 suggests that native flora has been compromised by non-native species. (Rothrock and Homoya 2005) Introduced plants, by their very nature, were uninvolved in the pristine native landscape, so Coefficients of Conservatism are not applied to them. All non-native or adventive plants are given a C value of 0.

Calculation of the PSI

Calculation of the PSI formulas are difficult and may be beyond the skills of some users. To address this concern, the free, accessible Bowman's Hill Wildflower Preserve web-site automatically calculates the PSI and FQAI values of any entered plant inventory, and allows the account-holder to print out complete reports of the findings. An overview of the process used to calculate the PSI follows: The naturalness of the total plant community is calculated by adding in all non-native species and giving them a C value of 0. Thus Total Mean C can be computed, and the PSI value is calculated by multiplying the Total Mean C by the square-root of N, where N is native species richness. The FQAI of an area is computed by multiplying the Native Mean C by the square-root of N, where N is the native species richness.

Use of the PSI in Monitoring Habitat Restorations

When monitoring habitat restorations, if the management is efficacious, the Native Mean C either stabilizes or gradually obtains its original value (Wilhelm + Taft. 2004) Tracking the quality of restorations and management efforts is well within the capabilities of preserve and federal land wildlife managers when using the FQAI and PSI. As an example, if a wetland preserve ecologist were implementing different techniques for restoring previously disturbed wetlands, changes in Native Mean C values or FQAI over time could aid in determining which method is more effective in repopulating the areas with desirable species.(Herman et al. 2006)

Available Resources

The database and PSI calculator are available to the public at no charge on Bowman's Hill Wildflower Preserve's website (*now available at and as a library item at Accounts are free and account-holders may compile and save site lists, modify or update lists, track lists print reports and view all other users site lists.

Individuals or organizations without the time, expertise or resources to conduct PSI surveys may hire Bowman's Hill Wildflower Preserve to conduct a half-day or full-day inventory on a fee-for-service basis. Most sites may be assessed with a professional half-day or a full-day survey, or a complete floristic inventory may be produced by repeated surveys at various times throughout the year.

Individuals or organizations may attend educational courses at Bowman's Hill Wildflower Preserve to learn more about implementation of the PSI methodology or to enhance plant ID skills. These classes: Introduction to the Plant Stewardship Index, and PSI Field Training workshop are offered repeatedly throughout the year. For a full list of current prgram offerings see

Benefits of Using the PSI

  • If the FQAI and the PSI of an area are the same, there are no non-native plants on the site. Thus, the numerical difference between FQAI and PSI can reveal the extent of non-native impacts.
  • The PSI/FQAI ratio is simple(based on either an inventory survey or a transect study), and as such it is an excellent rapid assessment of biological integrity. (EPA Office of Water 2002)
  • The PSI can be used to determine the extent to which management (or lack thereof) is having an effect on the floristic quality of a site.
  • FQAI/PSI can be effective in tracking vegetation changes in restoration, reconstruction, or control situations. (Taft et al. 1997)
  • Land trusts and landowners can use these assessments to guide their stewardship on the ground, while governmental oversight officials might use this data to set buffers and mitigation ratios. (In Michigan and Illinois, High Quality wetlands (FQI ~ 35) must be mitigated at higher rates; 1:3 or 1:5 for example.)
  • The PSI is not meant to be a stand-alone test of habitat quality, but should be used in conjunction with other measures of habitat quality. (Taft et al.1997; Hermann et al 1997) Floristic Quality Assessment is a richness- weighted index of natural quality where natural quality is combination of tolerance to disturbance, and habitat fidelity to pristine or undisturbed habitat. These two metrics have particular relevance to conservation decision-making.
  • PSI inventory or transect will yield data used to calculate the following parameters: 1) species richness both native and introduced 2) floristic quality index 3) plant stewardship index ( FQAI with introduced species) 4) mean native coefficient of conservatism 5) mean total coefficient of conservatism 6) guild diversity 7) species relative importance 8) percent native and 8) wetness characteristics.

Use of FQAI and PSI in Permitting, Performance Standards, and Mitigation

FQAI and PSI can be useful in the application of resource protection laws. Two examples follow:

Detroit Airport Expansion

The following is an example excerpted from Herman (1994) where the FQAI system was used as a performance standard and for establishing mitigation criteria in an endangered species permit for the Detroit Metropolitan Wayne County Airport expansion:

In 1989, expansion of the Detroit Metro Airport was expected to result in the on-site loss of three plant species listed as threatened under the M-ESA. The three species were Aristida longispica, Juncus brachycarpus (short fruited rush), and Ludwigia alternifolia(seed box). The three species were found within remnant lakeplain wet-mesic prairies and mesic sand prairies. The statutory requirements of Part 365 (Endangered Species Protection) of P.A. 451 of 1994 were enhanced by making compliance with this act a condition of the state wetland permit. The provisions of the endangered species permit allowed the translocation of affected plants and seed bank to an off-site location, which had been excavated and graded to match the land contours and hydrology of the airport site. Unaffected areas on the airport were required to be protected and monitored as a baseline to compare the success of the translocated plants. Hydrology, soil moisture, and vegetation are being monitored for ten years.

The criteria for success, required by the permit, states that at the off-site mitigation location, populations of threatened plants must be at least as large and viable as populations eliminated by the airport expansion. In addition, the mitigation area is required to be free of aggressive weeds such as Lythrum salicaria (purple loosestrife), the species diversity index should be stable or show an increase in native species diversity throughout the monitoring period, and it should show a stable or increasing floristic quality index and mean coefficient of conservatism.

DuPage County Stormwater and Flood Plain Ordinance

DuPage County, Illinois in implementing its stormwater and flood plain ordinance uses a C= 3.5 as a criteria for identifying critical wetlands and requires mitigation for the loss of these wetlands in the form of 3:1 acre wetland replacement (DuPage County Stormwater Management Committee,1992). Administrative rules to the Illinois Wetland Policy Act of 1989 (20 ILCS 830, 17 Ill. Adm. also requires a 5.5:1 replacement ratio for mitigating loss of wetlands with a native floristic quality greater than 20 (FQI 20) or a mean coefficient of conservatism greater than or equal to 4 (C 4.0). Wilhelm (1991, 1992, 1993) suggests, based on monitoring data obtained from Chicago region restoration sites, that areas with known high floristic quality (FQI 35) cannot be routinely restored to their original floristic quality and therefore are immitigable. Conversely, lower quality wetlands registering FQAI in the teens and twenties may be mitigable.


The PSI methodology was created in the 1970s in the Chicago region by Floyd Swink and Dr. Gerould Wilhelm and has been developed in many other states as the Floristic Quality Assessment Index (FQAI).

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Thanks to Leslie Jones Sauer, Tama Matsuoka, Ann Rhoads, Jack Holt, Janet Ebert, Linda Kelly, Bill Olson, Emile DeVito, Mary Leck, Karl Anderson, David Snyder, Kathleen Walz and Gerould Wilhelm for their generous assistance and expertise in helping to guide the development of the Plant Stewardship Index by Bowman’s Hill.


Nothing contained in this or any other document available at is intended to be relied upon as legal advice or to create an attorney-client relationship. The material presented is generally provided in the context of Pennsylvania law and, depending on the subject, may have more or less applicability elsewhere. There is no guarantee that it is up to date or error free.

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