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| A multi agency project funded by US EPA's STAR Program | ||||||||
Results: Wetland Indicators
Multitaxa Wetland Vegetation Indices
What is it? Indices that use a few selected plant species to evaluate wetland condition.
What is measured? Average cover by species in the wetland, measured in plots spanning the moisture gradient within emergent vegetation stands.
When should measurements be made? When vegetation is at maximum growth stage in July or August.
Equipment needed: Sticks to define a unit area of wetland (e.g., a 1 m x 1 m square) within which plant cover is visually estimated, plant identification guides
Expertise needed: Ability to distinguish the ten wetland plant taxa from other wetland plant taxa.
The ten-taxa index: SUM_INDEX = 2.141 - 0.029(CAST8) - 0.027(CALAA) - 0.352(CIBU) - 0.040(EQFL) + 0.013(LEMI) + 0.161(LYTH2) + 0.025(LYSA2) + 0.020(PHAU7) + 0.039(POAM8) + 0.017(invasive_Typha)
The four-taxa index: SUM_INDEX = 2.239 - 0.034(CAST8) - 0.030(CALAA) + 0.015(PHAU7) + 0.019(invasive_Typha)
where: SUM_INDEX is a cumulative index of anthropogenic stress representing the major threats to coastal ecosystems in the U.S. Great Lakes developed by Danz et al. (in press), for which values range from 0.4 (lowest stress) to 4.0 (highest stress)
CALAA = mean percent wetland cover by Carex lasiocarpa var. americana
CAST8 = mean percent wetland cover by Carex stricta
CIBU = mean percent wetland cover by Cicuta bulbifera
EQFL = mean percent wetland cover by Equisetum fluviatile
LEMI = mean percent wetland cover by Lemna minor
LYSA2 = mean percent wetland cover by Lythrum salicaria
LYTH2 = mean percent wetland cover by Lysimachia thyrsiflora
PHAU7 = mean percent wetland cover by Phragmites australis
POAM8 = mean percent wetland cover by Polygonum amphibium
invasive_Typha = mean percent wetland cover by Typha angustifolia or Typha x glauca
Why it works: All of the taxa have widespread ranges throughout the Great Lakes, and all are sensitive to stress. Four of the plant taxa (CIBU, EQFL, and the two Carex species) decrease in abundance as anthrogenic stress increases, and six of the taxa (LEMI, LYSA2, LYTH2, PHAU7, POAM8, invasive_Typha) increase in abundance. Invasive_Typha, LYSA2, and PHAU7 are invasive plant taxa.
How reliable is it? The ten-taxa index has an r2 of 0.61 with SUM_INDEX. The four-taxa index has an r2 of 0.50. The four-taxa index only works if one or more of the four taxa is present within the wetland, but that condition was true for all of the 90 wetlands that we sampled. The ten-taxa index utilizes more species, and is therefore more reliable.
How transferable is it? These indices can be used throughout the Great Lakes coast, but they are not applicable to salt marshes on oceanic coasts, where species diversity is much lower. The concept is probably transferable to other freshwater wetlands, but new formulas would have to be derived based on empirical data sets from other regions.
Maximum Canopy Height
What is it? An index of plant biomass, which increases with increasing anthropogenic disturbance.
What is measured? The maximum height of the herbaceous plant canopy in the wetland. Only one measurement is needed, but it should be made in the tallest stand of plants within the wetland. It would help to first view the wetland from an elevated spot nearby to scope out areas with tall vegetation.
When should measurements be made? When vegetation is at maximum growth stage in July or August.
Equipment needed: A meter stick
Expertise needed: No special expertise needed
The index: SUM_INDEX = 0.820 + 0.069(cover_hgt_max)
The greater the value of the index (on a scale from 0.4 to 4), the more degraded the wetland.
Why it works: (1) fertilization by nutrients contributed by nonpoint-source pollution increases plant growth, (2) invasive plants tend to be taller than non-invasive plants, and (3) tall plants shade out other plants, reducing the biodiversity of the plots in which they occur.
How reliable is it? The index has an r2 of 0.41 relative to the SUM_INDEX of anthropogenic stress. Using an average value for multiple canopy height measurements within the wetland yielded less reliable results (r2 of 0.375), and requires more effort.
How transferable is it? Although this index has not been tested outside of Great Lakes coastal wetlands, we believe that it may work in any wetland, including salt marsh wetlands, because the underlying mechanisms of fertilization and species invasion are universal.
1. Measuring maximum canopy height in a stand of invasive Phragmites |
2. The three tallest plant species, invasive cattails and Phragmites, shade out other plant species in their plots. |
Species Dominance Index
What is it? The Species Dominance Index (SDI) indicates ecological integrity by identifying dominant species and categorizing their behavior as one of seven forms of dominance.
What is measured? Average cover by species in the wetland, measured in plots spanning the moisture gradient within emergent vegetation stands.
When should measurements be made? When vegetation is at maximum growth stage in July or Aug.
Equipment needed: Sticks to define a unit area of wetland (e.g., a 1 m x 1 m square) within which plant cover is visually estimated, plant identification guides
Expertise needed: Ability to identify all wetland plants.
The index: SDI combines three related attributes of dominance to score a particular species’ dominance in much the same way Curtis and McIntosh used three related measures of tree abundance to calculate their importance value. These three attributes, mean cover (MC), mean species suppression (MSS), and tendency toward high cover (THC) measure the abundance of a potentially dominant species, the number of species associated with a potentially dominant species, and the likelihood that the species is abundant when it occurs, respectively. The attributes for which a dominant species has high values determines its dominance form.
The use of SDI involves three steps;
1) creating a list of potential dominants,
2) computing the SDI to identify the dominant species, and
3) classifying the dominance forms.
To be considered potentially dominant and subsequently subjected to the SDI, a species must occur with a minimum frequency (a third of plots when aggregated per wetland) and have >25 % absolute cover and the most cover of any species in at least one plot. These characteristics are required for dominance rather than variable attributes of a dominant species, which are the focus of SDI.
To compute SDI, the value of each attribute must be calculated for each potentially dominant species. Attribute values can range from 0 to 1. MC is calculated by averaging the mid-point of recorded cover classes of that species. Values of zero are used when a species does not occur in a plot. MSS is the mean of the inverse of the number of species (1/number of species) in a plots where the potentially dominant species has >25 % absolute cover and the most cover of any species. THC is the ratio of the number of times a potentially dominant species has >25 % absolute cover and the most cover of any species in a plot to the number of times it is present in a plot. The attributes values for each species in a wetland or lake are averaged together to yield the SDI score (SDI = (MC + MSS + THC)/3). Dominant species are those with above average SDI scores. Average SDI was 0.241.
Dominance form of a dominant species is assigned based on which attributes have values above the mean value of each attribute. The mean values were 0.197, 0.187, and 0.339 for MC, MSS, and THC, respectively. Using mean values to differentiate between “high” and “low” attribute values (and dominant vs. not dominant species, as above) is appropriate because samples came from a large number of wetlands distributed across the environmental gradient. The cut-off values used here should be used for determining dominant species and dominance forms in any future sampling efforts that use similar methods.
Why it works: Because a dominant species controls its habitat and the presence and performance of other species, its behavior, which may be variable depending on the environmental conditions, provides insight into the community as a whole. By examining the behavior of a dominant species in addition to its identity, SDI also acknowledges that invasive species do not always act invasive and native species sometimes do.’
How transferable is it? The concept is transferable to any wetland. The index has proven useful in characterizing changes in dominance in a restoration site at the UW-Madison Arboretum, in mesocosms where reed canary grass has invaded wet prairie, and in both natural and restored salt marshes of southern California.
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