Richard D. Rheinhardt

The Role of Reference Wetlands in Functional Assessment and Mitigation

Compensatory mitigation for damages to wetlands in the United States occurs largely without explicit analysis and replacement of wetland functions. We offer an approach to standardize such analyses and strengthen the connection between ecological principles and policies for wetland resources. By establishing standards from reference wetlands chosen for their high level of sustainable functioning, gains and losses of functions can be quantified for wetlands used in compensatory mitigation. Advantages of a reference wetland approach include (1) making explicit the goals of compensatory mitigation through iden- tification of reference standards from data that typify sustainable conditions in a region, (2) providing templates to which restored and created wetlands can be designed, and (3) establishing a framework whereby a decline in functions resulting from adverse impacts or a recovery of functions following restoration can be estimated both for a single project and over a larger area accumulated over time. To establish reference standards, conditions inherent to highly functioning sites must be identified for classes of wetlands that share similar geomorphic settings. Ecological functions are then identified, and variables used to model the functions are employed in developing reference standards. Variables range from the highest levels of sustainable functioning to the complete absence of functions when a wetland ecosystem is displaced. An example given for wet pine flats in the North Carolina coastal plain illustrates how to determine the loss of a given function for an impacted wetland, how to calculate recovery (gains) in function through compensatory mitigation, and how to use the relationships between the two (loss vs. gain in function) to set minimum replacement ratios of restored to impacted area. In all cases, data from reference wetlands provide the benchmarks for making these estimates and for directing restoration or creation of wetlands toward the standards established for the wetland class. Programs to implement the use of reference wetlands require regional efforts that build upon the knowledge base of existing wetlands and their functioning.

Applying wetland reference data to functional assessment, mitigation, and restoration

This study demonstrates an approach for rapidly collecting quantitative field data on reference wetland sites and using those data to assess functions (ecological processes) in wetlands. We demonstrate the hydrogeomorphic (HGM) assessment procedure by identifying ecological functions performed by mineral soil wet flats, obtaining quantitative field data from 19 wet flats (reference sites) in southeastern North Carolina, and modeling wetland functions using variables derived from those field data. We chose a subset of the 19 reference sites to demonstrate how HGM assessment can be used to measure ecosystem functions before and after a project site is altered and the degree to which ecosystem restoration can compensate for a reduction in functions caused by a project’s impact.We also illustrate how HGM assessment can be used to determine the minimum area over which restoration should be applied to achieve a no-net-loss in function objective. This minimum area can be determine by dividing the degree to which a function is reduced through project alteration by the degree to which a function is increased through restoration. The ratio of wetland area restored to wetland area altered by a project impact (compensatory mitigation ratio) varies among functions and is influenced by (1) the magnitude to which any given function occurs at a project site both before and after the site is altered, (2) the magnitude to which any given function occurs at a compensatory mitigation site both before and after restoration is applied, and (3) the rate at which any given function is restored.

Proposed Hydrogeomorphic Classification for Wetlands of the Mid-Atlantic Region, USA

We propose a regional classification for wetlands of the Mid-Atlantic region, USA. It combines functional characteristics recognized by the hydrogeomorphic (HGM) approach with the established classification of the National Wetland Inventory (NWI). The HGM approach supplements the NWI classification by recognizing the importance of geomorphic setting, water sources, and flow dynamics that are key to functioning wetlands. Both NWI and HGM share at their highest levels the Marine, Estuarine, and Lacustrine classes. This classification departs from the NWI system by subdividing the Palustrine system into HGM classes of Slope, Depression, and Flat. Further, the Riverine class expands to include associated Palustrine wetlands, thus recognizing the interdependency between channel and floodplain. Deepwater habitats of NWI are not included because they differ functionally. Mid-Atlantic regional subclasses recognize two subclasses each for Flat, Slope, and Marine Tidal Fringe; three subclasses for Depression; four subclasses for Lacustrine Fringe and Estuarine Tidal Fringe, and five subclasses for Riverine. Taking a similar approach in other geographic regions will better characterize wetlands for assessment and restoration. This approach was applied successfully during a regional wetlands condition assessment. We encourage additional testing by others to confirm its utility in the region.

A Stream-Wetland-Riparian (SWR) index for assessing condition of aquatic ecosystems in small watersheds along the Atlantic slope of the eastern U.S.

As part of a regional study by the Atlantic Slope Consortium (ASC) to develop ecological and socioeconomic indicators of aquatic ecosystem condition, we developed and tested a protocol for rapidly assessing condition of the stream, wetland, and riparian components of freshwater aquatic ecosystems. Aspects of hydrology, vegetation, in-stream and wetland characteristics, and on-site stressors were measured in the field. The resulting metrics were used to develop an index of overall condition, termed the Stream–Wetland–Riparian (SWR) Index. Values of this Index were compared to existing biotic indices and chemical measures, and to a Landscape Index created using satellite-based land cover data and a geographic information system (GIS). Comparisons were made at several levels of spatial aggregation and resolution, from site to small watershed. The SWR Index and associated Landscape Indices were shown to correlate highly with biological indicators of stream condition at the site level and for small contributing areas. The landscape patterns prevalent throughout the entire watershed do not necessarily match the patterns found adjacent to the stream network. We suggest a top-down approach that managers can use to sequentially apply these methods, to first prioritize watersheds based on a relative condition measure provided by the Landscape Index, and then assess condition and diagnose stressors of aquatic resources at the subwatershed and site level.

Forested wetlands of low order streams in the inner coastal plain of North Carolina, USA

We quantified geomorphic and vegetational characteristics of 22 first-to-fourth order riverine forests located in the inner coastal plain of North Carolina. We used Detrended Correspondence Analysis to compare canopy composition among sites and relate measured environmental parameters to distribution of canopy trees and stream order. Both geomorphic and vegetational attributes could be used to functionally divide first-to-fourth order riverine systems into 17 headwater (first and second order streams) and 5 midreach systems (third and fourth order streams). As expected, stream order was found to be positively correlated (P<0.003) with drainage basin size, floodplain width, and channel width. The canopy of headwater reaches was dominated by various combinations ofLiquidambar styraciflua, Nyssa biflora, andAcer rubrum, while midreach systems were typically dominated byTaxodium distichum and/orNyssa aquatica. Canopy composition was similar to other low order stream floodplains in the southeastern USA from Alabama to Maryland. However, the canopy composition of bottomlands differed in that the generaFraxinus, Quercus, andUlmus were generally less important in the North Carolina bottomlands than elsewhere in the Southeast. Metrics obtained from these relatively unaltered ecosystems could be used to develop standards for assessing of wetland condition and provide appropriate criteria for designing restoration of altered low order riverine ecosystems.

RELATIONSHIP BETWEEN HYDROLOGY AND ZONATION OF FRESHWATER SWALE WETLANDS ON LOWER HATTERAS ISLAND, NORTH CAROLINA, USA

Only a few relatively wide barrier islands support shallow freshwater aquifers. Rare, swale wetlands occur on islands where fresh water inundates, at least seasonally, low-lying troughs between interior dunes. Swale wetlands are dominated by emergent vegetation and submerged aquatic vegetation in the deepest areas and by woody shrubs in more shallow areas. On southern Hatteras Island, wetland shrubs have progressively invaded open water areas over the past 40 years, suggesting a change in hydrologic regime. To determine the relationship between vegetation cover type and length of saturation, water-level fluctuations over time were analyzed to tie boundaries of six wetland cover types to the duration of soil saturation at 20-cm depth. We found that areas dominated by herbaceous vegetation had significantly longer flooding regimes than areas dominated by shrubs (85–95% vs. 12–69% of the growing season, respectively). Only 22–25 cm elevation differences were found to separate emergent marsh from the various shrub cover types, suggesting that lowering the mean water level via drainage has likely been responsible for shrub swamps replacing emergent marsh in swales. Although succession from open water to shrub swamp probably occurs naturally in the absence of drainage through the accumulation of organic matter, natural disturbances such as wildfire and storm-driven surges of saline water would have periodically re-set succession. Therefore, managing for long-term maintenance of freshwater swale wetlands on barrier island should include (1) eliminating or controlling drainage through constructed ditches, (2) eliminating man-made barriers that prevent the transport of saline water into ponds during hurricanes and nor’easters, and (3) initiating a prescribed burning program to mimic the historic, natural fire regime.

A REFERENCE-BASED FRAMEWORK FOR EVALUATING THE ECOLOGICAL CONDITION OF STREAM NETWORKS IN SMALL WATERSHEDS

Nine field indicators were identified for evaluating the hydrologic, biogeochemical, and/or habitat functioning of stream channels, riparian zones, or both. We ranked condition from “relatively unaltered” to “severely altered” for each of the identified indicators based on the range of conditions actually encountered among reference sites in the Coastal Plain of North Carolina, USA. The rankings provided a framework for developing a narrative used for scoring condition of the indicators at the scale of a reach (100-m-long × 60-m-wide segment). Reach condition was then derived by aggregating indicator scores, which were weighted by the number of functions with which each indicator was affiliated. Watershed-scale assessments were conducted by sampling randomly chosen first- to fourth-order reaches within stream networks at the density of approximately one 100-m reach per 1.0 km2 of watershed drainage area. We used the association between indicators and hydrologic, biogeochemical, and habitat functions to obtain aggregated, weighted scores for channel and riparian zone condition. We used both aggregated network scores and mean indicator scores to compare condition among stream networks. At a reach scale, scores of indicators suggest strategies for restoration. At the watershed scale, aggregate scores showed differences among stream networks that could be used to prioritize restoration efforts and monitor change over time.

Hydrogeomorphic (HGM) assessment - a test of user consistency

We describe the first test conducted to determine user consistency in the application of hydrogeomorphic (HGM) functional assessment models. Over a three-week period, two teams of individuals trained in the HGM methodology assessed 44 riverine wetlands on the Coastal Plain of Delaware, Maryland, and Virginia, USA. Results demonstrated a high degree of agreement between the two assessment teams for both Variable Subindices and Functional Capacity Index Scores, indicating that the assessment models were robust and results were repeatable. Analyses of the data demonstrated the importance of only using variables whose measurements are repeatable. When variable measurements are not repeatable, HGM functional capacity scores are detrimentally affected, especially functions that are modeled by only a few variables.

Canopy and woody subcanopy composition of wet hardwood flats in eastern North Carolina and southeastern Virginia1

canopy species per stand and tended to be dominated by either Liquidambar styraciflua (sweetgum), Acer rubrum (red maple), and/or Nyssa biflora (swamp blackgum) or by a mixture of these species and one to five Quercus (oak) species. Although oaks occurred in almost all sites, oak dominated stands (total oak IV > 25) constituted only about one third of the sites. The woody subcanopy stratum was less rich than the canopy and varied widely in composition: some taxa tended to be common among sites (e.g., flex opaca, Carpinus caroliniana, Vaccinium spp., Persea borbonia) but were rarely very important (determined by relative density), while other taxa were relatively uncommon (Clethra alnifolia, Asimina triloba) but were important where present. Neither textures of soil strata in the top 50 cm nor soil drainage class (poorly vs. very poorly drained) appeared to be associated with composition, suggesting that wetness may be controlled more by geomorphology than soil drainage capacity.

Carbon storage of headwater riparian zones in an agricultural landscape

BackgroundIn agricultural regions, streamside forests have been reduced in age and extent, or removed entirely to maximize arable cropland. Restoring and reforesting such riparian zones to mature forest, particularly along headwater streams (which constitute 90% of stream network length) would both increase carbon storage and improve water quality. Age and management-related cover/condition classes of headwater stream networks can be used to rapidly inventory carbon storage and sequestration potential if carbon storage capacity of conditions classes and their relative distribution on the landscape are known.ResultsBased on the distribution of riparian zone cover/condition classes in sampled headwater reaches, current and potential carbon storage was extrapolated to the remainder of the North Carolina Coastal Plain stream network. Carbon stored in headwater riparian reaches is only about 40% of its potential capacity, based on 242 MgC/ha stored in sampled mature riparian forest (forest > 50 y old). The carbon deficit along 57,700 km headwater Coastal Plain streams is equivalent to about 25TgC in 30-m-wide riparian buffer zones and 50 TgC in 60-m-wide buffer zones.ConclusionsEstimating carbon storage in recognizable age-and cover-related condition classes provides a rapid way to better inventory current carbon storage, estimate storage capacity, and calculate the potential for additional storage. In light of the particular importance of buffer zones in headwater reaches in agricultural landscapes in ameliorating nutrient and sediment input to streams, encouraging the restoration of riparian zones to mature forest along headwater reaches worldwide has the potential to not only improve water quality, but also simultaneously reduce atmospheric CO2.