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Organic Soil Flats_Michigan Interim Wetland Assessment Model


Interim Wetland Functional Assessment Model


This interim functional assessment model was developed by an interagency team within Indiana, Ohio, and Michigan. The model represents a specific set of conditions within a specified location. You may “alter” the model in any manner to fit the settings of conditions within your locale. However, if this is done, rename the model before distributing it. ANY CHANGE TO THIS MODEL MEANS ANOTHER MODEL HAS BEEN DEVELOPED, AND SHOULD BE IDENTIFIED AS SUCH.

Subclass Description

This subclass of wetlands evolved under anaerobic soil conditions due to saturation and/or ponding. Organic wetlands are found on a variety of landforms including postglacial lake plains, coastal marshes, closed depressions, riverine, or on steep slopes with seepage. These wetlands are located throughout the northern midwest and predominate in northern Michigan, Wisconsin and Minnesota. They are typically refered to as peatlands, bogs, moors, peats, fens, histosols, or mucks. They are distinguished by very low topographic gradients and poorly drained and very poorly drained organic soils with more than half of the upper 80 cm consisting of organic materials.

The primary water source is precipitation for ombtrotrophic peatlands such as bogs. Groundwater is the primary water source for minerotrophic peatlands such as fens. Some sites have minor overland runoff. Peatlands typically have high water tables from fall to early summer. Water tables will fluctuate and are influenced by evapotranspiration and periodic rainfall. Surface ponding may be extensive or may be limited just to micro-depressions within the flat area. Ponded water typically does not run off laterally. The primary direction of water movement is vertical in the upper part of the soil profile, but water movement is horizontal in shallow organic soils which are underlain by slowly permeable subsoil layers.

This model is based on systems dominated by woody vegetation such as trees or shrubs; this is the predominate vegetation on these wetlands in some areas. In some of the organic flats the vegetation is dominated by emergent herbaceous, sedges and rushes. Some of the organic flats are dominated by shrubs. Many organic wetlands are dominated by a combination of herbaceous and woody vegetation. This model can be used where sites are dominated by trees or shrubs, but not intended to be used on wetland subclasses that are managed as grassland or cropland or where sedges and rushes and emergent vegetation predominate.

Distinguishing Characteristics

This subclass differs from mineral soil flats in the region in that most of the mineral wetlands have a predominance of forest vegetation, consist of primarily mineral soils, and vary in soil biochemical conditions. The lack of significant surface inflow distinguishes them from depressional wetlands. The reliance upon precipitation as opposed to groundwater, and the low topographic gradient, distinguishes flats from slope wetlands. As opposed to riverine wetlands, the flats are primarily dependent upon precipitation with the primary surface water movement being vertical fluctuation as opposed to unidirectional horizontal flow. In some cases, wetlands in these other classes may appear similar to flats and may even function similarly. For example, direct precipitation may be equal to surface runoff as a primary water source for very large depressional wetlands.

Important Functions

Due to their flat gradient and fluctuating water tables, the primary hydrologic function of organic flats is to store subsurface water. Surface water storage is a minor component and depends upon the amount of surface ponding. The fluctuating water tables are conducive to rapid biogeochemical nutrient cycling. Large amounts of organic carbon may be stored in the woody biomass or organic soils present in mature ecosystems of this type. These wetlands usually occur as a portion of extensive wetlands which provided important habitat for a variety of plant and animal species. Pools within the woods serve as important amphibian breeding sites; several migratory birds use these as nesting areas.

Current Conditions

Organic soil flat wetlands originally existed as a portion of extensive wetlands in many areas. Due to the difficulty in clearing and draining them, they were often the last areas settled. These wetlands still exist as large contiguous tract in many areas. Many forested areas are managed for timber production and wildlife habitat. Extensive drainage in the surrounding landscape may have altered the original hydrologic regimes within the wetlands of this subclass. Many areas have been drained and cleared for agricultural production including carrots, onions, potatoes, mint, and cranberries. Some areas are managed for pasture. Some organic flats are mined for their peat.


HYDROLOGICAL FUNCTION 1 - Surface Water Storage

Definition: Capacity of a wetland to temporarily store (detain) surface water for long durations; associated with standing water not moving over the surface. Sources of water are direct precipitation, and to a lesser extent, overland flow and lateral subsurface flow.

Effects On-site: Replenishes soil moisture; detains water for chemical transformations; maintains vegetative composition; maintains seasonally-ponded habitat for herptiles and aquatic invertebrates; and influences soil characteristics.

Effects Off-site: Improves water quality and maintains seasonal flow distribution.

Variables: Vduration


Index of Function: (Vduration )(V%_ponded )

HYDROLOGICAL FUNCTION 2 - Retention of Water in Soil

Definition: Availability of water storage beneath the wetland surface. Storage capacity becomes available as periodic drawdown of water table or reduction in soil saturation occurs.

Effects On-site: Short- and long- term water storage; influences biogeochemical processes in the soil; retains water for establishment and maintenance of biotic communities.

Effects Off-site: Soil moisture recharge; maintains base flow; maintains seasonal flow distribution.

Variables: Vwater_table


Index of Function: (Vwater_table + Vavail_H2O)/2


Definition: Abiotic and biotic processes that convert nutrients and other elements from one form to another; primarily recycling processes.

Effects On-site: removes sediments, nutrients, and contaminants; Net effects of recycling are elemental balances between gains through import processes and losses through hydraulic export, efflux to the atmosphere, and long-term retention in persistent biomass and sediments.

Effects Off-site: To the extent that nutrients are held on-site by recycling, they will be less susceptible to export downstream. This reduces the level of nutrient loading off-site.

Variables: Vstrata Vturnover


Index of Function: The lesser of: (Vstrata + Vcanopy)/2 or Vturnover

BIOGEOCHEMICAL FUNCTION 2 - Long-term Storage of Organic Compounds

Definition: The storage of organic carbon in the living and dead biomass above the soil and within the soil profile.

Effects On-site: The net effect of carbon storage is long-term retention in persistent biomass and in organic matter in the soil profile.

Effects Off-site: Nutrients are held on-site and are less susceptible to transport.

Variables: Vbasal_area Vturnover

Vtree_density Vsoil_OM

Index of Function: (Vbasal_area + Vtree_density + Vturnover + Vsoil_OM)/4

BIOGEOCHEMICAL FUNCTION 3 - Long Term Storage of Nutrients

Definition: The long term storage of nutrients in the living and dead biomass above the soil and within the soil profile.

Effects On-site: The net effect of nutrient storage is long-term retention of persistent biomass and maintain productivity of the soil profile.

Effects Off-site: Nutrients are held on site and are less susceptible to transport.

Variables: Vbasal_area Vturnover

Vtree_density Vsoil_OM

Index of Function: (Vbasal_area + Vtree_density + Vturnover + Vsoil_OM)/4

HABITAT FUNCTION 1 - Maintain Characteristic Plant Community

Definition: Species composition and physical characteristics of living plant biomass. The emphasis is on the dynamics and structure of plant community as revealed by the dominant species of trees, shrubs, saplings, and ground cover, and by the physical characteristics of vegetation.

Effects On-site: Converts solar radiation and carbon dioxide into complex organic compounds that provide energy to drive food webs. Provides seeds for regeneration. Provides habitat for nesting, resting, refuge, and escape cover for animals. Creates micro-climatic conditions that support completion of life histories of plants and animals. Provides organic matter for soil development and soil related nutrient cycling processes. Creates both long- and short- term habitat for resident or migratory animals.

Effects off-site: Provides a source of propagules to maintain species composition and/or structure of adjacent wetlands and supplies propagules for colonization of nearby degraded systems. Provides food and cover for animals from adjacent ecosystems. Provides corridors (migratory pathways) between habitats, enhances species diversity and ecosystem stability, and provides habitat and food for migratory and resident animals.

Variables: Vcomp Vcanopy

Vstrata Vmaturity


Index of Function: (Vcomp + Vstrata + Vturnover + Vcanopy + Vmaturity)/5

HABITAT FUNCTION 2 - Maintain Wildlife Habitat Structure

Definition: The capacity of a wetland to support animal populations and guilds by providing characteristic habitats.

Effects On-site: Provides potential feeding, resting, and nesting sites for vertebrates and invertebrates. Regulates and moderates fluctuations in temperature. Provides characteristic habitat to support a diverse assemblage of organisms dependent upon this wetland type. Affects all ecosystem processes.

Effects Off-site: Provides habitat heterogeneity to landscape, provides habitat for wide-ranging and migratory animals, provides a corridor for gene flow between separated populations, and allows progeny to exploit new areas.

Variables: V%_ponded Vstrata

Vsnags Vmaturity


Index of Function: (V%_ponded + Vsnags + Vcomp + Vstrata + Vmaturity)/5

HABITAT FUNCTION 3 - Maintain Habitat Interspersion, Quality, and Connectivity

Definition: The capacity for all organisms to be provided with diverse and contiguous areas of food and cover.

Effects On-site: Provides maintenance of habitat, species, and gene pool diversity.

Effects Off-site: Provides habitat heterogeneity to landscape, provides habitat for wide-ranging and migratory animals, provides a corridor for gene flow between separated populations, and allows progeny to exploit new areas.

Variables: Vcore Vconnect


Index of Function: [(Vcontiguous)2 + Vcoree + Vconnect]/4


For each of the variables listed below, choose the described condition that most nearly matches the site under consideration. Record the answers in the “TABLE 1” on page 13. Use this point value in the Index of Function formulas.

V1 Vduration

Presence or indication that the wetland surface is ponded for a period of >7 days continuously. Possible field indicators of hydrology include: bare soil areas due to inundation; water stained leaves; water rings on trees; obligate hydrophytic vegetation; mucky surface layer or thin mucky layer on surface; wrack lines; direct measurements; and direct observations (circle all that apply).

Off-site mapping conventions can be used along with the above on-site information to determine seasonal/annual trends in water storage. Information from the county soil survey can also be used to support the field indicators listed above.

A. Two or more indicators show evidence of ponding >7 days. 1.0
B. Ponding occurs for 7 days or less. One or fewer indicators are absent in undisturbed sites. 0.5
C. Hydrology has been altered to prevent or reduce ponding. Examples include evidence of surface ditches, tile drains, and fill. 0.1
D. No historic evidence of ponding; soil redoximorphic features are absent. 0.0

V2 V% ponded

Variable is scored by measurement or estimate of the extent of ponding on the wetland surface which indicates ponding for >7 days. The variable is also used in Habitat Function #2 as an indicator of canopy gap.

A. >50% of area is ponded. 1.0
B. 10-50% of area is ponded. 0.5
C. <10% of area is ponded. 0.1
E. No evidence of ponding. 0.0

V3 Vwater_table

Fluctuations in the water table as indicated to the depth of the seasonal high water table. The water table of mineral flat systems undergo drawdowns due to evapotranspiration and drainage, and rises due to precipitation. Drawdowns of the water table combine with pore space to provide potential volume for water storage below the ground surface.

A. Seasonal high water table to 0.5-18 inches of surface and redoximorphic features below “A” horizon. 1.0
B. Seasonal high water table at depth of 0.0-18 inches and dominant soil matrix of chroma 1 or less below “A” horizon. 0.5
C. Seasonal high water table at depth of 0.0-12 inches duration and gleyed below the “A” horizon. 0.1
D. Soils are saturated to the surface for very long duration. 0.0

V4 Vavail_H2O

Available Water Capacity (AWC) is the volume of water available to plants. It is the amount of water held between field capacity and wilting point, with corrections for salinity, fragments, and rooting depth. AWC is based on an average of the top 40” of soil or to a limiting layer.

A. Available Water Capacity is > 0.18 in./in. 1.0
B. Available Water Capacity is 0.10-0.18 in./in. 0.5
C. Soils compacted, or AWC < 0.10 in./in. 0.1
D. Soils replaced by substrate with negligible pore space (e.g. concrete, asphalt). 0.0

V5 Vstrata

Mature forested wetlands are usually vertically stratified. The number of strata in mature mineral flat forests generally ranges between three and seven in temperate climates. Forest organisms are primarily associated with a specific strata. Differences in structure between sites likely represent differences in animal composition between the sites as well. In fact, more spatially stratified communities often contain more species.  

A. At least three vegetative strata are present (trees, shrubs/vines, herbaceous). 1.0
B. Two vegetative strata present (undisturbed sites). 0.75
C. Two or less vegetative strata present (disturbed sites). 0.5
D. Area converted to agricultural land; restoration is possible. 0.1
E. Area converted to Agricultural land; restoration not possible. 0.0

V6 Vcanopy

This is a measure of the continuity of the upper layers of the forest canopy. It is assumed the more complete the canopy cover the more efficient the wetland is at nutrient cycling regardless of the maturity of the stand. Stands with canopy coverage of 80-100% provide optimal habitat for forest wildlife.

A. Tree canopy cover >80%. 1.0
B. Tree canopy cover 50-80%. 0.5 0.5
C. Tree canopy <50%, or site lacks living biomass due to clearing with the potential for recovery. 0.1
D. No tree canopy; no potential for recovery. 0.0

V7 Vturnover

Detrital stocks are represented by down and dead woody debris, and organic debris on the forest floor. Standing stocks of detritus are assumed to be proportional to annual turnover. Stocks include snags, down and dead woody debris, leaf litter and humus layers. Most of the annual nutrient cycling occurs at the surface and subsurface levels, therefore, organic debris found on the ground as coarse woody debris and as part of the soil profile are indicative of the active biomass fraction and are representative of annual detrital turnover in the system.

A. The following indicators are found throughout the evaluation area: snags, woody debris, leaf litter, high organic matter on soil surface.. 1.0
B. One of the above indicators are absent due to disturbance (i.e. fire, grazing, etc.) 0.75
C. Two or more of the above indicators are absent due to disturbance 0.5
D. Area barren of woody debris (intensive cropping), low organic matter; potential for recovery. 0.1
E. Area barren of all elements; no potential for recovery. 0.0

V8 Vbasal area

This is a measurement of the average tree basal area within the evaluation area. Trees are defined as greater then 4 inches DBH. Basal area is proportional to above ground biomass of trees and is a dependable indication of forest maturity. In mineral flat forests, basal area and stem density both increase in early succession. Thereafter, tree density decreases, and, as the forest reaches maturity, the rate of increase of basal area diminishes to steady-state conditions.

A. Basal area >100 sq. ft. 1.0
B. Basal area 65-100 sq. ft. 0.5
C. Basal area <65 sq. ft.; restoration possible. 0.1
D. No trees present; restoration not possible. 0.0

V9 Vsoil_OM

This variable is defined as the organic matter (OM) content of the upper 12 inches of the soil profile.

A. Soil OM content is >70%. 1.0
B. Soil OM content is 40-70. 0.5
C. Soil OM content is <40%. 0.1
D. No soil OM present. 0.0

V10 Vcomp

This variable is defined using 1/10th acre plots for the tree strata to determine the dominant tree species.

Typical tree species for this sub-class include:Silver Maple, Black Ash, Red Maple, Green Ash, Tamarack, Black Spruce, White Cedar, Balsam Fir, Aspen, White Birch, Swamp White Oak, White Spruce, Hemlock, White Pine, Jack Pine.

A. >4 of the species above are present with specimens over 3” DBH. 1.0
B. 2 to 4 of the species above are present. 0.5
C. <2 species above present, restoration possible. 0.1
D. <2 species present, restoration not possible. 0.0

V11 Vmaturity

Standing and mature or dying trees provide nesting habitat for a variety of animal species, including invertebrates, birds, reptiles, amphibians, and mammals. Mature trees provide a significant amount of food for wildlife species as well as leaf/woody debris for the nutrient cycle.

A. >5 trees/ac. larger than 16 inches DBH. 1.0
B. 2-5 trees/ac. larger than 16 inches DBH. 0.5
C. < 2 tress/ac. larger than 16 inches DBH. 0.1
D. No mature trees within evaluation area; restoration not possible. 0.0

V12 Vsnags

This variable is defined as the density of standing dead trees (>4 inches DBH and >6 feet tall). Standing dead trees (snags) are a normal component of forested wetlands. The density of standing dead trees relates to the suitability of a site as wildlife habitat due to the large number of species that forage on, and nest and den in snags. It may also provide an insight into the hydrology of the site. More standing dead tress than a typical site might show an increase in ponding on the site.

A. >5 snags/ac. 1.0
B. 2-5 snags/ac. 0.5
C. <2 snags/ac. 0.1
D. No snags present; restoration not possible. 0.0

V13 Vcore

The interior core area is defined as the area of the wetland 100 meters (110 yards) from the wetland boundary that is bordered by non-forested habitat. Essentially this variable addresses tract shape (i.e. round tracts have grater “interior core areas” than linear tracts). Forest tracts with a high amount of core area are assumed to be beneficial especially to forest interior bird species which are adversely affected by the creation of edge habitat.

This variable is determined by dividing the amount of the wetland which is 100 m from the tract boundary by the area of the total wetland tract. The variable can be directly estimated by measuring the core area from tan NWI or topographic map, or from aerial photos.

A. >75% of the wetland tract is 100m from non-forested boundary. 1.0
B. 50-74% of the wetland tract is 100m from non-forested boundary. 0.6
C. 25-49% of the wetland tract is 100m from non-forested boundary. 0.4
D. <25% of the wetland tract is 100m from non-forested boundary. Restoration possible. 0.1
E. <25% of the wetland tract is 100m from non-forested boundary. Restoration not possible. 0.0

V14 Vcontiguous cover

Heterogeneity in distribution and abundance of organisms is inherent at all scales in every natural ecosystem. Any measure of ecosystem attributes must consider the appropriate scale and sample size in which to measure those attributes in order to understand ecosystem processes. Patchiness of vegetation (lack of contiguous cover types) affects the types and abundance of trophic interactions, energy flow, and competitive interactions of animals. These processes in turn affect animal populations.

A. Wetland is part of a larger block (>100 acres) of contiguous upland or wetland forest which is non-fragmented and has few non-natural breaks. 1.0
B. Wetland is part of a 40-100 acre block as above. 0.75
C. Wetland is part of a 20-39 acre block as above. 0.5
D. Wetland is part of a block < 20 acres, or is part of a highly fragmented larger block; restoration is possible. 0.1
E. Wetland is part of a block < 20 acres, or is part of a highly fragmented larger block: restoration is not possible (Example: urban area). 0.0

V15 Vconnect

This variable is defined as the distance from the wetland’s perimeter to other “suitable” habitat for wildlife. Many species of wildlife that are common inhabitants of forested wetlands also may use other habitat such as upland forests.

This variable is assessed by measuring the distance from the wetland to other types of wetlands, to upland forest habitat, or to other communities capable of providing habitat for at least some of the species typically associated with forested wetlands. At least 20% of the perimeter of the wetland being evaluated must meet the described distance measurements.

A. A direct connection of continuous woody or shrubby cover not less than 20’ in width is present. Examples include hedge rows, fence rows, windbreaks, etc. 1.0
B. A direct connection does not exist, but suitable habitat is < 1/4 mile from the wetland perimeter. 0.75
C. A direct connection does not exist, but suitable habitat is 1/4-1/2 mile from the wetland perimeter. 0.4
D. A direct connection does not exist, but suitable habitat is 1/2-1 mile from the wetland perimeter. 0.1
E. A direct connection does not exist, and suitable habitat is > 1 mile from the wetland perimeter. 0.0

Was a previous Functional Assessment completed for this site? Check one: YES _____ NO _____.

IF yes, list the base value: ________. Use this value as the Base Value and do not complete another base condition for this assessment.

Note: Base condition is the current situation if completing assessment for planned activities OR the conditions prior to conversion for violations. Planned conditions represent the conditions after the planned activity being evaluated or the current conditions for violations prior to 7/3/96.


VARIABLES Base Conditions Planned Conditions
V1 Vduration    
V2 V%_ponded    
V3 Vwater_table    
V4 Vavail_H2O    
V5 Vstrata    
V6 Vcanopy    
V7 Vturnover    
V8 Vbasal_area    
V9 Vsoil_OM    
V10 Vcomp    
V11 Vmaturity    
V12 Vsnags    
V13 Vcore    
V14 Vcontiguous_cover    
V15 Vconnect    


HYDROLOGICAL 1: (Vduration + V%_ponded)/2

BASE - (_____ + _____)/2 = _____

PLANNED - (_____ + _____)/2 = _____

HYDROLOGICAL 2: (Vwater_table + Vavail_H2O)/2

BASE - (_____ + _____)/2 =

PLANNED - (_____ + _____)/2 = _____

BIOGEOCHEMICAL 1: The lesser of: (Vstrata + Vcanopy)/2 or Vturnover

BASE - (_____ + _____)/2 = _____, or Vturnover = _____

PLANNED - (_____ + _____)/2 = _____, or Vturnover = _____

BIOGEOCHEMICAL 2: [( + Vturnover)/2) + Vsoil_OM]/2

BASE - [(_____ + _____)/2) + _____]/2 = _____

PLANNED - [(_____ + _____)/2) + _____]/2 = _____

BIOGEOCHEMICAL 3: [( + Vturnover)/2) + Vsoil_OM]/2

BASE - [(_____ + _____)/2) + _____]/2 = _____

PLANNED - [(_____ + _____)/2) + _____]/2 = _____

HABITAT 1: (Vcomp + Vstrata + Vturnover + Vcanopy + Vmaturity)/5

BASE - (_____ + _____ + _____+ ____ + _____)/5 = _____

PLANNED - (_____ + _____ + _____+ ____ + _____)/5 = _____

HABITAT 2: (V%_ponded + Vsnags + Vcomp + Vstrata + Vmaturity)/5

BASE - (_____ + _____ + _____ + _____ + _____)/5 = _____

PLANNED - (_____ + _____ + _____ + _____ + _____)/5 = _____

HABITAT 3: [(Vcontiguous)2 + Vcore + Vconnect]/4

BASE - [(_____)2 + _____ + _____]/4 = _____

PLANNED - [(_____)2 + _____ + _____]/4 = _____


FUNCTIONAL INDEX Base Conditions Planned Conditions Net Change in Functional Index
HABITAT 1      
HABITAT 2