Zooming Out: Methods and Future Directions in Landscape‐Scale Functional Assessment of Inland Wetlands

Chapter 2.2.3 - Assessing Streamflow Maintenance Functions in Wetlands of the Blackfoot River Subbasin in Montana, United States

Chapter 2.2.1 - A Landscape-Level Wetland Functional Assessment Tool: Building a Framework for Watershed-Based Assessments in the United States

Federal and state regulatory programs in the Chesapeake Bay watershed require wetland mitigation to obtain a permit for wetland impacts. The loss of wetlands in a watershed through permitted activities diminishes its capacity to perform important habitat, water retention and dissipation, and biogeochemical cycling functions. Past attempts at wetland mitigation have produced wetlands that meet the three parameters (a predominance of wetland plants, wetland hydrology, and wetland soils) of a jurisdictional wetland, but little thought was put into whether the mitigation site was providing similar wetland functions to those wetlands impacted. Current research on wetland hydrogeomorphic (HGM) setting as well as HGM functional assessment models and other functional assessment approaches have provided incite on important characteristics that allow wetlands to provide certain functions. These characteristics are starting to be incorporated into wetland restoration and creation site design in hope that the mitigation site will not just mitigate for acreage, but also mitigate for loss of wetland function. Criteria for proper wetland mitigation design incorporate appropriate HGM setting, ability to reproduce hydroperiod and hydrodynamics, inputs of organic matter in forms that are partially decomposed and as coarse woody debris, macro- and micro-topography, and other characteristics that affect specific wetland functions. Mitigation criteria can be adjusted to address problems in a watershed by concentrating on characteristics that affect wetland functions that can help alleviate that problem.

Chapter 2.2.5 - An Automated Procedure for Extending the NWI Classification System for Wetland Functional Assessment in Minnesota, United States

To achieve a goal of “no net loss” of wetland functions as well as acreage, it is important to know which functions wetlands in a specific region or watershed may individually provide, at what level. Although methods exist for assessing functions of Oregon wetlands (ORWAP; Adamus et al. 2009), site visits and hours of data collection are required to apply these to any wetland. Moreover, property access restrictions prohibit their use on many wetlands. Thus, when there is a comprehensive objective to assess all mapped wetlands in a region or watershed, time and manpower constraints require that spatial data be processed automatically using GIS, even though assessments of functions using such an approach have much lower levels of certainty. The primary object of this project was to define criteria that would allow only the use of existing spatial databases (and only those with comprehensive coverage of the ecoregion) to identify the HGM class of mapped Willamette Ecoregion wetlands, and then apply those criteria automatically and systematically to every mapped wetland in the ecoregion, resulting in an HGM label for each mapped wetland. An ancillary goal was the creation of a database that describes over 200 attributes of each mapped wetland in the Willamette ecoregion, i.e., a regional wetland “profile.”

Seven vernal pool complexes consisting of numerous shallow wetland depressions were sampled in Sacramento County, California, during the spring and early summer of 1998. Data were obtained to characterize ecological conditions within each complex and to develop models for assessing wetland disturbance and functions. Degree of disturbance, topographic features, soil profiles, and plant species composition and percent cover were examined. Pool area, volume, perimeter, maximum depth, distance from pool to pool, and percent of sample area were computed for 265 vernal pools. Additional detailed topographic and vegetative data were obtained at 68 vernal pools and soil profiles characterized at 64 vernal pools. Disturbance was computed quantitatively by integrating the type of disturbance and proximity to the pool into a single numeric index. This disturbance quotient provided a relative measure of vernal pool alterations. Data were analyzed using correlation analysis, stepwise discriminant analysis, and discriminant analysis to construct a model sensitive to disturbance. Results of the discriminant analysis indicated that three variables (disturbance quotient, maximum depth, and percent native to nonnative plant species) provided the best combination of factors to assess relative disturbance. A predictive model was developed using these three variables to accurately assign 92.8 percent of the pools to particular wetland areas indicative of different levels of alteration. Other variables that also related to disturbance included soil depth to the durapan and slope at the edge of the vernal pool. Five wetland functions were identified as being relevant to vernal pools and ecological models were developed for each function. These models were calibrated with data collected from the vernal pools to provide a relative measure of the functional capacity of vernal pool wetlands in the Central Valley of California. The ecological models can be used to assess the capacity of wetlands to perform different functions, calculate project impacts on those functions, compute mitigation requirements to offset unavoidable impacts, and assess mitigation success.

The hydrogeomorphic (HGM) approach to wetland classification and functional assessment is becoming more widespread in the United States but its use has been limited by the length of time needed to develop appropriate data sets and functional assessment models. One particularly difficult aspect is the transferability among geographic regions of specific models used to assess wetland function. Sharing of models could considerably shorten development and implementation of HGM throughout the United States and elsewhere. As hydrology is the driving force behind wetland functions, we assessed the comparability of hydrologic characteristics of three HGM subclasses (slope, headwater floodplain, mainstem floodplain) using comparable long-term hydrologic data sets from different regions of the United States (Ridge and Valley Province in Pennsylvania and the Willamette Valley in Oregon). If hydrology by HGM subclass were similar between different geographic regions, it might be possible to more readily transfer extant models between those regions. We found that slope wetlands (typically groundwater-driven) had similar hydrologic characteristics, even though absolute details (such as depth of water) differed. We did not find the floodplain subclasses to be comparable, likely due to effects of urbanization in Oregon, regional differences in soils and, perhaps, climate. Slight differences in hydrology can shift wetland functions from those mediated by aerobic processes to those dominated by anaerobic processes. Functions such as nutrient cycling can be noticeably altered as a result. Our data suggest considerable caution in the application of models outside of the region for which they were developed.

The Hydrogeomorphic (HGM) approach is used for developing and applying models for the site-specific assessment of wetland functions. It was initially designed for use in the context of the Clean Water Act Section 404 Regulatory Program permit review process to analyze project alternatives, minimize impacts, assess unavoidable impacts, determine mitigation requirements, and monitor the success of compensatory mitigation. However, a variety of other potential uses have been identified, including the design of wetland restoration projects, projecting ecological outcomes, developing success criteria and performance standards, and adaptive monitoring and management of wetlands. This guidebook provides an overview of the HGM approach including classification and characterization of the principal alluvial riverine wetlands identified in the Piedmont physiography. Eight potential subclasses of Piedmont wetlands, including Headwater, Low- and Mid-gradient Riverine, Floodplain Depression, Footslope Seeps, Flats, Precipitation Depressions, and Fringe wetlands were recognized. However, the occurrence of Flats, Precipitation Depressions, and Fringe wetlands in the Piedmont, are uncommon and not generally associated with alluvial riverine systems which is the subject of this Guidebook. Detailed HGM assessment models and protocols are presented for the five most common Piedmont riverine subclasses: Headwater, Low- and Mid-gradient Riverine, Floodplain Depression, and Footslope Seep. For each wetland subclass, the guidebook presents (a) the rationale used to select the wetland functions considered in the assessment process, (b) the rationale used to select assessment models, and (c) the functional index calibration curves developed from reference wetlands used in the assessment models. The guidebook outlines an assessment protocol for using the model variables and functional indices to assess each wetland subclass. The appendices provide field data collection forms. In addition, an automated spreadsheet model is provided to make calculations.

The hydrogeomorphic (HGM) approach for developing "rapid" wetland function assessment methods stipulates that the variables used are to be scaled based on data collected at sites judged to be the best at performing the wetland functions (reference standard sites). A critical step in the process is to choose the least altered wetlands in a hydrogeomorphic subclass to use as a reference standard against which other wetlands are compared. The basic assumption made in this approach is that wetlands judged to have had the least human impact have the highest level of sustainable performance for all functions. The levels at which functions are performed in these least altered wetlands are assumed to be "characteristic" for the subclass and "sustainable." Results from data collected in wetlands in the lowlands of western Washington suggest that the assumption may not be appropriate for this region. Teams developing methods for assessing wetland functions did not find that the least altered wetlands in a subclass had a range of performance levels that could be identified as "characteristic" or "sustainable." Forty-four wetlands in four hydrogeomorphic subclasses (two depressional subclasses and two riverine subclasses) were rated by teams of experts on the severity of their human alterations and on the level of performance of 15 wetland functions. An ordinal scale of 1-5 was used to quantify alterations in water regime, soils, vegetation, buffers, and contributing basin. Performance of functions was judged on an ordinal scale of 1-7. Relatively unaltered wetlands were judged to perform individual functions at levels that spanned all of the seven possible ratings in all four subclasses. The basic assumption of the HGM approach, that the least altered wetlands represent "characteristic" and "sustainable" levels of functioning that are different from those found in altered wetlands, was not confirmed. Although the intent of the HGM approach is to use level of functioning as a metric to assess the ecological integrity or "health" of the wetland ecosystem, the metric does not seem to work in western Washington for that purpose.

Hydrologic data are essential for understanding relationships between wetland morphology and function and for characterizing landscape-scale patterns of wetland occurrence. We monitored water levels in 45 wetlands for three years to characterize the hydrology of wetlands in the vicinity of Portland, Oregon, USA and classified wetlands by hydrogeomorphic (HGM) class to determine whether hydrologic regimes differed in wetlands in different HGM classes. We also compared hydrologic regimes in naturally occurring wetlands (NOWs) and mitigation wetlands (MWs) and in wetlands with/without a human-made water-retention structure to determine whether and how human modifications are changing the hydrology of wetlands. We found no relationship between hydrologic attributes and land use, soil association, or wetland area. We did find significant differences related to presence of a water-retention structure and to wetland type (NOW or MW). Water levels were higher and had less temporal variability and more extensive inundation (as % wetland area) in MWs and in wetlands modified to include a retention structure. HGM class was very effective for characterizing wetland hydrology, with significant differences among, HGM classes for water level and for extent and duration of inundation. For three regional classes, we found the lowest water levels and lowest extent/duration of inundation in slope wetlands, intermediate conditions in riverine wetlands, and the highest water levels and greatest extent and duration of inundation in depressions. In “atypical” classes (Gwin et al. 1999), average water level and extent of inundation were similar to conditions in depressions, but the within-site variability in water levels in depressions-in-slope-setting and in-stream-depressions was significantly smaller than in the regional classes (p ≤ 0.001). Results highlight the importance of both geomorphic setting and wetland structure in defining wetland hydrology and support the use of HGM for wetland classification. Because hydrology is an important determinant of many wetland functions, resource managers using restoration and mitigation to offset wetland losses should strive for project design and siting that re-establish the hydrogeomorphology of natural wetlands to improve the likelihood of replacing wetland functions.

As anthropogenic disturbance continues to degrade wetland condition in many geographic areas, it is imperative to inventory wetland functions to monitor potential loss of associated ecosystem services. Field-based functional assessments are resource intensive, prohibiting their widespread application at landscape scales. This obstacle can be avoided by basing functional assessments on publicly available remote sensed data. This pilot study examined the use of Watershed-based Preliminary Assessment of Wetland Function (W-PAWF) in the assessment of wetland restoration sites. W-PAWF was used to assess 15 depressional wetlands in the US Mid-Atlantic Coastal Plain. These sites spanned a human alteration gradient (i.e., natural wetlands, restored wetlands, and prior-converted croplands) to determine the sensitivity of the assessment method to variation in the assemblage and performance of wetland functions. Field-based rapid assessment methods were used to verify the W-PAWF assessment and detect potential functional gaps of importance to wetland restoration. Results indicate that W-PAWF can differentiate varying levels of restoration condition, but refinement will be necessary to assess functional restoration goals related to biogeochemistry and water quality. An evaluation of the field-based methods and an alternate remote functional assessment system suggest the potential for these functional characteristics to be incorporated in future iterations of the W-PAWF.

Wetland functions are the characteristic activities (biological, chemical, and physical processes) that occur in these ecosystems. Functional assessment models are used to quantify the functional capacity of individual wetlands. The Hydrogeomorphic (HGM) Approach to functional assessment of wetlands classifies wetlands to the regional subclass level and then creates a model for an individual subclass in a given geographic area. An HGM model is essentially a functional profile that describes the physical, chemical, and biological characteristics of wetlands in the regional subclass, identifies those functions that are most likely to be performed on a sustained basis, and addresses landscape and ecosystem attributes that can be expected to influence each targeted function. The HGM Approach does not assign absolute values to functional capacity. Instead it rates functional capacity for a given wetland relative to reference standards-the highest level of functional capacity exhibited by wetlands in the regional subclass. Reference standards are established by assessment of wetlands that exhibit those attributes consistent with maximum sustained functional capacity. The HGM Approach is presently being applied to Piedmont slope wetlands in the Mid-Atlantic Region.

Wetland assessment methods have been used to estimate the capacity of wetlands to perform certain functions and to determine potential changes in wetland condition as a result of anthropogenic impacts. In this chapter, we describe the Hydrogeomorphic (HGM) Approach, a wetland functional assessment method that was developed to alleviate some of the shortcomings of other wetland assessment methods. The HGM approach is a reference-based assessment method that was developed to estimate change in wetland condition by quantitatively comparing ecosystem functions of altered wetlands to unaltered wetlands. The HGM approach involves a developmental phase and an application phase, but the focus of this chapter is on the application phase. Specifically, we describe how to conduct a wetland functional assessment using the HGM approach. The components of the HGM approach that are described include classifying wetlands using HGM classification, collecting data representing individual variables in functional models, calculating the functional capacity of the wetland, and analyzing the functional capacity results to determine potential impacts on wetland functions from proposed projects.

Over 300,000 ha of forested wetlands have undergone restoration within the Mississippi Alluvial Valley region. Restored forest successional stage varies, providing opportunities to document wetland functional increases across a large scale restoration chronosequence using the Hydrogeomorphic (HGM) approach. Results from >600 restored study sites spanning a 25 year chronosequence indicate that: 1) wetland functional assessment variables increased toward reference conditions; 2) restored wetlands generally follow expected recovery trajectories; and 3) wetland functions display significant improvements across the restoration chronosequence. A functional lag between restored areas and mature reference wetlands persists in most instances. However, a subset of restored sites have attained mature reference wetland conditions in areas approaching or exceeding tree diameter and canopy closure thresholds. Study results highlight the importance of site selection and the benefits of evaluating a suite of wetland functions in order to identify appropriate restoration success milestones and design monitoring programs. For example wetland functions associated with detention of precipitation (a largely physical process) rapidly increased under post restoration conditions, while improvements in wetland habitat functions (associated with forest establishment and maturation) required additional time. As the wetland science community transitions towards larger scale restoration efforts, effectively quantifying restoration functional improvements will become increasingly important.

Over 300,000 ha of forested wetlands have undergone restoration within the Mississippi Alluvial Valley region. Restored forest successional stage varies, providing opportunities to document wetland functional increases across a large-scale restoration chronosequence using the Hydrogeomorphic (HGM) approach. Results from >600 restored study sites spanning a 25-year chronosequence indicate that: 1) wetland functional assessment variables increased toward reference conditions; 2) restored wetlands generally follow expected recovery trajectories; and 3) wetland functions display significant improvements across the restoration chronosequence. A functional lag between restored areas and mature reference wetlands persists in most instances. However, a subset of restored sites have attained mature reference wetland conditions in areas approaching or exceeding tree diameter and canopy closure thresholds. Study results highlight the importance of site selection and the benefits of evaluating a suite of wetland functions in order to identify appropriate restoration success milestones and design monitoring programs. For example, wetland functions associated with detention of precipitation (a largely physical process) rapidly increased under post restoration conditions, while improvements in wetland habitat functions (associated with forest establishment and maturation) required additional time. As the wetland science community transitions towards larger scale restoration efforts, effectively quantifying restoration functional improvements will become increasingly important.