The Role of Reference Wetlands in Functional Assessment ... · The Role of Reference Wetlands in Functional Assessment and Mitigation ... THE ROLE OF REFERENCE WETLANDS IN FUNCTIONAL

The Role of Reference Wetlands in Functional Assessment and MitigationAuthor(s): Mark M. Brinson and Richard RheinhardtSource: Ecological Applications, Vol. 6, No. 1 (Feb., 1996), pp. 69-76Published by: Ecological Society of AmericaStable URL: http://www.jstor.org/stable/2269553 .Accessed: 01/06/2011 11:48

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Ecological Applications, 6(1), 1996, pp. 69-76 ? 1996 by the Ecological Society of America

THE ROLE OF REFERENCE WETLANDS IN FUNCTIONAL ASSESSMENT AND MITIGATION1' 2

MARK M. BRINSON AND RICHARD RHEINHARDT Biology Department, East Carolina University, Greenville, North Carolina 27858 USA

Abstract. 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.

Key words: assessment; compensatory mitigation; creation; functioning; hydrogeomorphic classification of wetlands; North Carolina wet pine flats; reference standards; reference wetlands; restoration; wetland.

INTRODUCTION

Design and performance standards are used in en- gineering and manufacturing to ensure that products meet specified levels of quality. With increasing em- phasis on ecosystem sustainability, restoration, and adaptive management (Holling 1978, Lubchenco et al. 1991), the development of "performance" standards for ecosystems could provide the basis for judging the quality of restoration efforts and their effectiveness in offsetting unavoidable impacts. Restoration and creation of wetland ecosystems have been the focus of sym- posia and reviews to address progress and to discuss techniques for replacing lost area and functions (Jos- selyn 1982, Kusler and Kentula 1990, National Re- search Council 1992). We argue that reference wetlands should be central to the development of standards

against which impacts to wetlands and restoration efforts are evaluated. Before we describe how reference wetlands can be used, we must first identify goals of mitigation, explain how assessment of wetland ecosystem functioning is conducted, and provide guidance on how reference wetlands are chosen.

Goals of mitigation

Regulatory programs that deal with mitigation of damages to or losses of wetlands operate within a com- bination of federal, state, and local regulatory programs. The most prevalent of these programs is ?404 of the Federal Clean Water Act (CWA)3 which has the goal of maintaining and improving the chemical, phys- ical, and biological integrity of the Nation's waters (40 CFR Part 230.1).4 Before a wetland can be filled, drained, or otherwise degraded, the landowner or pro-

I Manuscript received 20 March 1994; revised 18 April 1995; accepted 21 April 1995; final version received 30 May 1995.

2 For reprints of this group of papers on wetland mitigation, see footnote 2, page 33.

I United States Code: Title 33. 4 CFR = Code of Federal Regulations, the official com-

pilation of all agency regulations that implement U.S. (federal) law.

69

70 MARK M. BRINSON AND RICHARD RHEINHARDT Ecological Applications Vol. 6, No. 1

ject proponent must obtain a permit from the U.S. Army Corps of Engineers (Corps), the lead federal agency for the CWA ?404 program. The permit review se- quence is outlined in the Corps Regulatory Program Regulations (33 CFR Parts 320-330) (see footnote 4), Environmental Protection Agency (EPA) ?404(b)(1) Guidelines (40 CFR Part 230), and several Memoranda of Agreement between the Corps and EPA.

If a permit is issued, project approval may be con- tingent on restoring, enhancing, or creating wetlands to compensate for any unavoidable loss in wetland area and function. In theory, compensatory mitigation is dealt with only after an application to fill wetlands has passed an "alternatives test" that demonstrates that the project is water dependent, that there are no practicable alternatives, and that efforts have been taken to avoid and minimize impacts to wetlands. Reference wetlands can be used as standards to compare impacts among proposed alternatives, and to determine whether the level or severity of the preferred alternative is signif- icant, whether a proposed mitigation would effectively compensate for the impact, and whether the mitigation, once in place, has actually compensated for the impact. In addition to the more general goals of the Clean Water Act, policy goals specific to wetlands, such as "no net loss," have been articulated (National Wetlands Policy Forum 1988). While the Forum's intent was no loss in both surface area and ecosystem function, debate con- tinues on how the goal might be achieved.

Replacement of unavoidable loss of wetland ecosystem area and functioning may be achieved by restoring and creating wetlands. Restoration of wetlands is the return of a site to its pre-existing wetland condition. Creation of wetlands is the conversion of non- wetland (upland) to jurisdictional wetland status, usually by excavating a depression that intercepts the sur- ficial groundwater table or by connecting the site hy- draulically to a source of surface water.

Results from evaluations of compensatory mitigation have itemized causes of failure to achieve functional replacement (National Research Council 1992). Certain classes of wetlands are particularly difficult to create. River floodplain wetlands cannot be constructed without either creating a river to support them or increasing the flood storage capacity and width of an existing floodplain. The former is virtually impossible while the latter has the undesirable consequence of impacting toe-slope seeps and adjacent riparian and upland vegetation. Peat-based wetlands cannot be created without either removing peat from existing wetlands or waiting the decades to millennia for sufficient peat to accumulate. Of the creation projects that have been evaluated, many have not been successful (Kusler and Ken- tula 1990).

Functions and functional assessment

Ecosystem functions can be defined as the activities or processes that characterize an ecosystem. Four gen-

eral categories have been used for wetlands: hydrologic, biogeochemical, plant community maintenance, and animal community maintenance (Brinson et al. 1994). Other functions, such as maintaining site water balance, facilitating energy flow, supporting nutrient cycling, and maintaining species diversity could be used, de- pending on the goals of functional assessment. Func- tions, then, are useful in compensatory mitigation because they allow expression of the multifaceted nature of ecosystems and provide perspectives around which performance standards can be designed.

Assessment methods to determine levels of functioning may use absolute measurements (e.g., rates of nutrient cycling, etc.) or measurements relative to some reference standard (e.g., 75% of expected species rich- ness). The most widely used method currently is the Wetland Evaluation Technique (WET) which assesses 11 functions (Adamus et al. 1991). The Habitat Eval- uation Procedure of the U.S. Fish and Wildlife Service is another method that evaluates the quality of habitat for a single species or groups of species that share habitat requirements. In each of these cases, a given function or habitat is compared to some reference or ideal condition, and usually is expressed as an index relative to the high or established maximum. Aggregate indices, such as the index of biotic integrity (IBI) for streams (Karr 1991), have been used as standards for ecosystem condition. Such indices rely on the assump- tion that the relative abundance of several key species populations (i.e., fish for the IBI) are reflective of an ecosystem that is relatively intact and functioning at an optimal level. Consequently, the presence and relative abundance of key species populations become the performance standard rather than the ecosystem itself.

As applied here, functions are expressed as indices relative to some reference condition that is the performance standard of a wetland class. Just as the relative abundance of fish species characterizes a stream's IBI, so the relative levels of functioning characterize a wetland ecosystem's condition.

Choosing reference wetlands

Reference wetlands are defined as sites within a specified geographic region that are chosen, for the pur- poses of functional assessment, to encompass the known variation of a group or class of wetlands, in- cluding both natural and disturbance-mediated varia- tions. Examples of classes include bottomland hard- woods of the southeastern U.S., prairie potholes of the northern plains states, and steep riparian wetlands of the northern Rocky Mountains. Properties of reference wetlands, ranging from those with highest levels of functioning to those that are highly disturbed, correspond to the ranges within a defined class at which various functions occur. Reference standards would represent the conditions exhibited by the subset of reference wetlands that correspond to the highest level of

February 1996 REFERENCE WETLANDS 71

functioning of the ecosystem across a suite of functions.

Reference wetlands and functional assessment are used hand-in-hand. To understand how they are related, it is first necessary to consider how reference wetlands can be established and their functioning identified. The first step is to classify wetlands according to similarities in their landscape settings, water sources, and hydrodynamics (Brinson 1993a), following the general hydrogeomorphic approach used for mangroves by Lugo and Snedaker (1974). A number of other classification systems based on similar properties have been described (Gosselink and Turner 1978, Novitzki 1979, Hollands 1987, Gilvear et al. 1989). Experience in stream restoration and assessment also provides some guidelines that could be applied in the choice of reference wetlands (Hughes et al. 1986).

The hydrogeomorphic classification step is critical because it provides the resolution to distinguish between a wetland that has lost a certain function through alteration or degradation and a wetland that would nev- er support the function due to properties inherent to its class. For example, wetland classes that excel in improving water quality, such as headwater riparian zones (Brinson 1993b), would not provide as favorable a habitat for waterfowl as shallow ponds in coastal marshes (Smith et al. 1989). Judgement must be used in deciding on the balance between subclassifying, thereby reduc- ing the variation in reference standards while improving resolution of differences, and using broader clas- sifications, thereby losing resolution due to more widely varying reference standards.

After specifying the wetland class for determining reference standards, functions must be identified for the next class. Next, information on ecosystem structure would be obtained for a number of wetlands within the class. A reference data set for a wetland class should include sites ranging in ecological integrity from highly degraded to highly functioning, and from early suc- cessional to mature. Reference standards are then determined using information obtained on the least degraded members of the class. The presumption is that the least degraded wetlands function optimally, i.e., they support a multiplicity of functions inherent to their class. This is in contrast to other wetlands in the class that may be deficient in one or more functions or wetlands that maximize the level of one function either in an unsustainable manner or at the expense of others.

For forested wetlands, reference standards may include flooding to a certain depth on an annual basis, a range of tree basal areas and densities, and a species composition within a specified similarity to reference standards. Natural disturbances such as the effects of fire, storms, and droughts must be recognized as natural variation within reference standards. The choice of sites that represents acceptable variation requires the application of "best professional judgement" by ex- perienced practitioners familiar with the ecosystem

class and knowledge of pertinent scientific literature. The validity of this judgement depends upon two as- sumptions: (1) that structure (biomass, species composition, soil/sediment type, etc.) reliably indicates corresponding functions and (2) that the structure of highly functioning wetlands corresponds to a suite of functions that are sustainable within the constraints of the existing landscape or watershed condition. Stated another way, each function should be assessed relative to conditions determined in fully functioning, self-sus- taining ecosystems, rather than some maximum level of functioning.

How ARE REFERENCE WETLANDS USED IN

MITIGATION?

We provide a brief treatment of wet pine flats on the coastal plain of North Carolina to illustrate how impact assessment can apply reference standards to measure loss in functions, to measure gain toward established mitigation goals, and to determine replacement ratios based on losses and gains in functioning (Tables 1 and 2). Wet pine flats occur on predominantly mineral soils and are dependent on frequent fire to maintain their characteristic species composition (Fig. 1). In contrast to other wetlands that receive groundwater or overland flow as water sources, wet pine flats receive only pre- cipitation. Consequently, wet conditions result from their poor drainage rather than from supplementary water sources. As such, wet pine flats do not perform all of the same functions that riverine wetlands perform, such as energy dissipation of floodwaters or moderation of groundwater discharge.

Once fully functioning conditions were identified for wet pine flats, functional models were developed for each of eight functions, which roughly followed the format of Brinson et al. (1995) for riverine wetlands. One function, Recycles Nutrients and Other Elements, consists of combinations of eight variables, represented by estimates of living biomass (canopy and subcanopy tree biomass, forb and graminoid biomass, and woody seedling biomass) and various measures of non-living biomass (litter, standing dead stems, and coarse woody debris) (Table 1). We reasoned that nutrient cycling could be inferred from the nutrient pools presumed to exist in living and non-living biomass. The first column of data shows directly measured data for wet pine flats in a highly functioning condition that we designate as "reference standards." Each of the functions, and the variables that logically or empirically have been shown to contribute to each function, were then scaled from 1.0 (the level achieved in fully functioning conditions of reference wetlands, or reference standards) to zero (absence of the function when the ecosystem is totally displaced).

The sequences for assessing the project site before and after impact and the mitigation site before and after restoration are illustrated in Fig. 2. The figure also shows the central role of reference standards as a basis


TABLE 1. Models and indices of ecological functioning attributed to the function "Recycles Nutrients and Other Elements" * for wet pine flats in eastern North Carolina. See How are reference wetlands used in mitigation? for explanation.

Project site

Reference standards Before impact After impact

Variablest Data Index Data Index Data Index

Vtree 15.7 1.0 14.6 0.93 0.0 0.0 VSUbC 12 225 1.0 13 314t 1.00 0.0 0.0 Vforb 33.4 1.0 4.7 0.14 0.0 0.0 Vgram 19.6 1.0 0.0 0.00 0.0 0.0 Vsdlg 55.8 1.0 43.6 0.78 0.0 0.0 Vittr 6.0 1.0 5.9 0.98 0.0 0.0

snag 21.2 1.0 0.0 0.00 0.0 0.0 VCWD 1.03 1.0 0.0 0.00 0.0 0.0 Computation of index: 1.0 0.51 0.0

* Definition: Biotic and abiotic processes that convert elements from one form to another; primarily recycling processes. t Vtree = Canopy biomass, measured as absolute basal area of trees (m2/ha); Vsubc = Subcanopy biomass, measured as

absolute density of subcanopy shrubs and understory trees (no./ha); Vforb = Forb biomass, measured as percentage cover; Vgram = Graminoid biomass, measured as percentage cover; VsdIg = Seedling biomass, measured as percentage woody seedling cover; Vittr = Litter biomass, measured as thickness of litter layer (cm); Vsnag = Biomass of standing dead stems, density (no.! ha) of snags >10 cm dbh; and VCWD = Biomass (in m3/ha) of downed coarse woody debris (CWD) >10 cm diameter and >lm in length.

Computation of function:

{ [(Vtree + VSUbC + [(Vforb + Vgram)/2] + VsdIg)/4] + [(Vittr + Vsnag + VCWD)/3] }/2.

: Even if a datum for the project site exceeds that of reference standards (e.g., VsUbc), the index cannot exceed 1.0 because the condition would not be considered sustainable.

for comparison at each step in the assessment process. Consequently, the differences determined from the project and mitigation sites use the same standard of comparison, that of reference standards for pine flats.

When the Project Site Before Impact (Data column) is displaced by a project, all variables become zero (Project Site After Impact, Data column) (Table 1). Then the data for the project site before and after the impact are normalized to the reference standards.. For Vtree, the index is set as 1.0 for the reference standard, is calculated for the project site before impact as 0.93 (14.6 divided by 15.7), and is set at 0.0 at the project site after impact indicating complete loss of the vari- able. This last calculation is repeated for the remaining seven variables. Finally, the index for the function is computed from the equation for Recycles Nutrients and Other Elements. The score is set at 1.0 for reference

standards, calculated as 0.51 for the project site before the impact, and calculated at 0.0 after the impact.

An additional example is given for the function, Sup- ports Characteristic Vegetation (Table 2). Similarity coefficients for composition or cover of the four vegetation strata are used to compare reference standards with sites being assessed. In this case, the function score of the Project Site Before Impact is only 0.28 because of a low similarity in plant species composition between the proposed project site (a loblolly-pine plan- tation) and forest stands used to develop reference standards (i.e., fire-maintained savanna dominated by longleaf and pond pine).

Function scores are similarly calculated for a site slated for restoration and chosen to compensate for the impacts calculated above. This is done to determine how much area of degraded wetland (degraded relative

TABLE 2. Models and indices of ecological functioning attributed to the function "Supports Characteristics Vegetation"* for wet pine flats in eastern North Carolina. See How are reference wetlands used in mitigation? for explanation.

Project site

Reference standards Before impact After impact

Variablest Data Index Data Index Data Index

VcompC 100.0 1.0 29.0 0.29 0.0 0.0 Vcomps 100.0 1.0 48.9 0.49 0.0 0.0 VcompV 100.0 1.0 0.0 0.00 0.0 0.0 Vgram 33.3 1.0 4.7 0.14 0.0 0.0 Computation of index: 1.0 0.28 0.0

* Definition: Capacity to support floral and compositional attributes of characteristic natural plant community (all strata). t Potential variables (as similarity coefficients between assessment site and the highly functioning condition): VcOmpC =

Canopy composition; Vcomps = Subcanopy composition; Vcompv = Vine composition; and Vgram = Graminoid cover. Computation of function:

{ VcompC + [Vcomps + (Vcompv + Vgram)/2]/2 }/2.


ALL FLATS

Sea-Level- Atmosphere- Controlled Flats Controlled Flats

Mineral Soil Flats Organic Soil Flats

Fire-Maintained Flats Fire-SuppressedFlats (longleaf/pond (hardwood pine dominated) dominated)

FIG. 1. Classes of wet pine flats on the North Carolina coastal plain showing principal features of separation that include dominant water source, soil properties, and presence of fire. Models for indices of functioning were developed only for fire-maintained flats (Table 1).

to a wetland meeting reference standards) would be needed, through restoration, to recover individual functions. For the Recycles Nutrients and Other Elements function in the hypothetical project, the 0.51 amount of loss expected to be caused by the project could be recovered by restoring a degraded wetland that has a score for this function of < 1.0. For a degraded wetland with an index score of 0.25 for Recycles Nutrients and Other Elements, an equivalent area of wetland would have to be restored to a level of 0.76 to replace the expected loss in function (0.51 plus 0.25). The ratio between the amount lost (0.51) and the amount recovered (0.51) is equivalent to a 1:1 replacement ratio. However, if a wetland chosen for restoration was less degraded and began with a score of 0.75, only 0.25 credits would be possible because the function score cannot exceed 1.0 for restoration to reference standards. If this were done, two units of land would be

needed for restoration to offset one unit of function lost in the hypothetical project (a 2:1 replacement ratio).

In this simple example, we have not accounted for functions lost between the time the project site was impacted and the time at which the mitigation site would achieve the reference standards (1.0). Further, compensatory mitigation that restores a function to a level of 0.76 raises two other issues. First, it would be difficult to restore Recycles Nutrients and Other Ele- ments to a level of function at exactly 0.76. Second, even if this were possible, it is inadvisable to restore a site to less than a fully functioning condition. Res- toration should proceed with the goal that all functions can eventually achieve reference standards if the potential exists on the site.

As it turns out, not all functions are affected equally by impacts (unless they all decline from 1.0 to 0.0), nor are all functions recoverable at the same rate and to the same degree over similar periods of time (Golet 1986, Larson and Neill 1987, PERL 1990). For wet pine flats, we specified eight functions, and although all declined to zero with the hypothetical project, they were initially operating at different levels relative to reference standards. For mitigation wetlands slated for restoration of functions, the function-by-function assessment provides enough detail to determine which variables need to be restored.

Once replacement ratios are determined for individual functions, one still must decide how much area to restore for a given impact. While gains and losses of individual functions can be quantified, it is not obvious how replacement ratios for whole ecosystems should be determined. This is the point at which policy must be developed to provide guidance (Fig. 2). Some log- ical alternatives are to (1) give priority to hydrologic variables when possible because they are fundamental

Assess Project Assess Project Calculated Losses

Wetland Wetland with of Individual

Before Impact Expected Impacts Functions T COMPARE WITH REFERENCE STANDARDS DETERMINE

REPLACEMENT

REFERENCE STANDARDS CHOSEN FROM REFERENCE RATIOS BASED WETLANDS FUNCTIONING AT HIGHEST LEVELS ON LOSSES

?/ ? /\ ?AND GAINS IN

COMPARE WITH REFERENCE STANDARDS FUNCTIONING

. . ~~~A Assess Mitigation Assess Mitigation Calculate Gains

Site Before Site for of Individual

Restoration Expected Gains Functions

Develop Mitigation Plan to Restore USE OF REFERENCE IN POLICY

Toward Reference FUNCTIONAL ASSESSMENT REALM Standards

FIG. 2. The central role of reference standards in assessing project wetlands before and after impact, and assessing compensatory mitigation sites before and after restoration. Levels of functioning at each point in the process (in- dicated by "?") are compared to reference standards for calculating gains and losses. Func- tional assessment procedures are described in Smith et al. (1995).


to overall wetland functioning, (2) base ratios on functions most critical to a region, and (3) use the individual function with the highest ratio to establish overall replacement ratios. The latter alternative may often exceed a 1:1 functional replacement overall. However, this could be applied toward deficits in functions that develop between the time of the impact and the achievement of a fully functioning condition in the compensation wetland.

COMPARISON OF COMMON MITIGATION OPTIONS AND THE REFERENCE

APPROACH

Three broad options dominate the execution of compensatory mitigation for damages to wetlands (Kruc- zynski 1990). One is in-kind vs. out-of-kind compensation in which the former restores or creates the same class of wetland (and corresponding functions) that are lost by impacts, and the latter restores or creates alternative wetland classes. In some cases, out-of-kind compensation is the only option. In other quite rare cases, compensation is extended to restoring or protecting upland sites valued as exceptionally rare (or as habitat for an endangered species) rather than creating a wetland class that may be already common in the geographic region. For example, a permit was approved to fill a wetland swale on a barrier island with the condition that protection be provided for rapidly dis- appearing upland maritime forest (Clean Water Act ?401 Certification Files, 1991, North Carolina Division of Environmental Management, Raleigh, North Caro- lina, USA). While such decisions may have long-last- ing socioeconomic benefits, a reference wetland approach to functional assessment cannot compare functions across wetland classes because each class has different reference standards.

A second option is on-site vs. off-site compensation, the former requiring that compensation activities occur adjacent to the permitted project, or at least located within the same embayment, stream reach, or local watershed (Kruczynski 1990). Off-site compensation im- plies that restoration or creation can take place in a different and perhaps distant watershed. If a reference wetland approach is used, on-site restoration within the same local watershed seems justified, especially for wetlands that are physically or functionally connected to one another in the landscape.

A third option is to vary the credit given to restored vs. created wetlands by adjusting replacement ratios (restored or created area divided by area impacted by a project). High replacement ratios usually are required in cases (1) where the risk of failure is perceived as being high (e.g., creation might require more area than restoration of a degraded system), (2) when a long time is required for a fully functioning wetland to develop (e.g., creation of forested wetlands), or (3) when mitigation consists of preservation alone (Kruczynski 1990).

While these three options have not exclusively dominated past mitigation practices in the U.S., there is a prevailing philosophy among the wetland management community that in-kind and on-site restoration is more desirable than its alternatives (i.e., the functioning of damaged wetlands is more likely to be replaced). As such, we contend that this philosophy is reacting to a higher risk of failure that accompanies creation and off-site alternatives (as described by Kruczynski 1990) than an explicit determination of whether or not functions are being replaced. This clearly differs from a philosophy based on assessments of ecological functions standardized to fully functioning reference wetlands and restoration designed to attain the least degraded condition for the wetland class. While the two approaches may arrive at the same requirements for a particular project (e.g., 2 ha of marsh creation required for 1 ha of marsh filled), the reference wetland approach evaluates the potential functional effectiveness of the proposed mitigation wetland relative to losses in the project wetland. The same procedure may be continued to evaluate progress with post-project mon- itoring. The point of this comparison is not to suggest that the three mitigation options mentioned above en- tirely disregard function, but rather to illustrate that the use of reference forces one to constantly revisit and calibrate on reference standards. In so doing, reference standards and reference wetlands take the lead in guid- ing mitigation, and all parties involved in mitigation are fully informed of the process. This potentially leads to greater consistency and efficiency in achieving goals of Clean Water Act ?404.

Additional policy guidelines would still be needed for the framework of reference wetlands. Thresholds for permit compliance would have to be established or negotiated for forested wetlands that require many decades to reach reference standards indicative of a relatively mature condition. Further, an endpoint of highly degraded conditions for the mitigation wetland should not be acceptable even if the project wetland was orig- inally degraded. If this were acceptable, degraded wetlands would continue to accumulate through mitigation practices, a situation inconsistent with restoration goals of the Clean Water Act and the National Wetlands Pol- icy Forum. Additional policy guidance would be needed on what to do if appropriate landscape conditions are lacking for the creation of fully functioning wetlands (e.g., urbanizing landscapes), if opportunities for restoration are lacking, and if there are conditions where creation is favored over restoration. Such policies may be needed if consistent progress is to be made toward a no-net-loss goal.

POTENTIAL MISUSES OF THE APPROACH

While it appears that reference wetlands could im- prove consistency and accuracy in measuring changes in the functioning of wetlands, they are also vulnerable to misuse in the following ways:


The choice of reference wetlands for determining reference standards.-Because highly functioning conditions of reference wetlands would serve as both design templates for restoration and standards to evaluate compensatory mitigation projects, the choice of reference wetlands is critical, as illustrated in Fig. 2. The most obvious misuse would be to accept reference standards from sites that are degraded. This would be coun- terproductive to goals of restoring and creating wetlands that will function at high levels.

Setting reference standards higher than can be sustained in the landscape.-It would be unrealistic to expect that similar levels of sustained functioning be met for wetlands in highly urbanized settings as in forested, rural watersheds. For example, highest levels of functioning would be low for wetlands in some lo- cations in New Jersey where urbanization has affected plant species composition and water quality (Ehrenfeld and Schneider 1993). Just as stream quality is tightly linked to land use activities in watersheds (Hughes et al. 1986), so are many wetlands constrained by activities in their surrounding uplands.

Giving primacy to individualfunctions at the expense of other functions or the wetland ecosystem.-While functional assessments allow one to determine changes in individual functions, the fundamental unit of assessment is the wetland ecosystem. This principle has two consequences. First, functions can be compared only between wetlands of the same class because they have the same reference standards. If functions were the only basis for compensatory mitigation, the logic could lead to the substitution with wetlands of a different class or with non-wetlands that have overlapping functions. For example, surface water storage is a wetland function, but it is maximized by storm detention basins that are normally dry and generally lack attributes of wetlands. Second, individual functions should not be altered to maximize one function at the expense of others, a process known as enhancement (Kruczyn- ski 1990).

The concept of reference wetlands and its application to each step in the mitigation process (Fig. 2) helps to avoid these pitfalls. Functional assessment and mitigation policies that rely on wetland ecosystems as fundamental units of the landscape can be made consistent and stable. For areas in which reference standards cannot be achieved because of urbanization or other land use changes, adaptive management may require alternative pathways for restoration. The use of reference wetlands, however, clarifies the threshold at which scientific information must yield to policy guidelines on mitigation.

CONCLUSIONS

The use of reference wetlands offers a way to standardize components of compensatory mitigation that is consistent with broad policy goals. Advantages of a reference-wetland approach include (1) making explicit

the goals of compensatory mitigation through identi- fication 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. While individual wetland scientists, regulators, and consultants may have their own perceptions of what constitutes fully functioning wetlands, the proper use of reference wetlands removes potential bias and provides the foundation for more objective functional-assessment procedures. Projects for restoration and creation should be designed around existing ecosystems and data that characterize them, and not around lists of design standards and development goals with criteria that do not specify a particular existing class of wetland. If consistently applied, the use of reference wetlands can facilitate the development of realistic goals for mitigation projects and provide a mechanism for evaluating progress toward meeting those goals.

ACKNOWLEDGMENTS

This paper benefitted from extensive comments by L. C. Lee, gentle persuasion by Joy Zedler, many points and coun- terpoints by Frank Golet, and a critical review by Martha Craig Rheinhardt. We thank the following colleagues for extended discussions on the concept of reference wetlands: W. B. Ainslie, R. L. Beschta, F R. Hauer, G. G. Hollands, L. C. Lee, D. Magee, R. P. Novitzki, W. L. Nutter, R. D. Smith, and D. F Whigham. Ronald Ferrell offered the example of trading maritime forest for wetland swales. Manuscript prep- aration was supported in part by the following: Waterways Experiment Station, Army Corps of Engineers contract DACW39-93-K-0023, U.S. Environmental Protection Agen- cy grant 3-BO978NTEX, and a National Science Foundation grant BRS87-02333-04 to the University of Virginia Long Term Ecological Research Program.

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