&EPA Issue Paper
Advances in Restoration Science
United States
Environmental Protection
Agency
Number 1:
Research Needs in Riparian Buffer Restoration
Eric E. Jorgensen,Timothy J. Canfield and Paul M. Mayer
[ADVANCES in
RESTORATION SCIENCE
Riparian buffer restorations are used as management tools to produce favorable water quality impacts;
moreover, the basis for riparian buffers as an instrument of water quality restoration rests on a relatively
firm foundation. However, the extent to which buffers can restore riparian ecosystems, their
functionality and species composition, are essentially unknown. In light of the foregoing, two broad
areas of research are indicated. First, data are needed to document the relative effectiveness of
riparian buffers that differ according to width, length, and plant species composition. These questions,
of managing buffer dimension and species composition for functionality, are of central importance even
when attenuation of nutrient and sediment loads alone are considered. Second, where ecosystem
restoration is the goal, effects to in-stream and terrestrial riparian biota need to be considered.
Relatedly, the effects of the restoration on the landscape need to be considered. Particularly, at what
rate do the effects of the riparian buffer on in-stream water quality, biota, and habitat diminish
downstream from restored sites? Answers to these important questions are needed to further the
advance of riparian restoration from art form to science and to maximize the societal value of future
restorations.
For further information contact Eric Jorgensen (580) 436-8545, or Timothy Canfield (580) 436-8535,
or Paul Mayer (580) 436-8647 at the Subsurface Protection and Remediation Division of the National
Risk Management Research Laboratory, Office of Research and Development, U. S. Environmental
Protection Agency, Ada, Oklahoma.
Riparian Restoration through Buffer Management
Aquatic ecosystems have been altered through the combined effects of municipal, industrial, and
agricultural activities (National Research Council, 1992). Municipalities spend large amounts of money
to remedy non-point pollution problems in their drinking water (Schultz et al., 1995). Nonpoint pollution
is the major remaining cause of surface water pollution (U.S. EPA, 1989; Baker, 1992). Sedimentation
and excess nutrients are the two leading nonpoint pollution problems in rivers and estuaries (Baker,
1992). Nitrogen and phosphorus contribute to eutrophication of surface waters and nitrogen is
frequently limiting in estuaries (Smith et al., 1987), thereby altering biotic interactions and competitive
regimes. Estuaries are the ultimate sinks for most riverine discharge in the eastern United States.
Buffer strips, areas of planted or preserved vegetation between developed land and surface water, are
effective at reducing sediment and nutrient loads (Lowranceetal., 1985; Groffmanetal., 1990; Castelle
et al., 1994; see also reviews in Castelle et al., 1992). Buffer strips reduce the severity of impacts to
riparian areas caused by storm events (Bertulli, 1981) that can incise channels and adversely affect
riparian ecosystem function (Henshaw and Booth, 2000). A relatively sound scientific foundation exists
to support the use of riparian buffer strips to manage nutrients and sediment and to support an
expectation of their success as an ecosystem management tool. (Bren, 1993; National Association of
Conservation Districts, 1994; NRC, 1992; USDA, 1996). Further, it is also believed that even greater
benefits will be realized through incorporation of wetlands as components of buffers (Zak and Grigal,
1991; Niswander and Mitsch, 1995); that is, while either buffers or wetlands alone are expected to
provide benefit, integration of these together will likely provide unexpectedly high rates of success
(Lowrance et al., 1995).
The role of denitrification, a microbially meditated process, in processing nutrient inputs to riparian
zones is an area of primary importance (Jones and Holmes, 1996; Boulton etal., 1998). Denitrification
requires anaerobic conditions and is strongly influenced by ground-water dynamics (Burt et al.,1999;
Devito et al., 2000), typically occurring in hyporheic zones (Dahm et al., 1998; Baker et al., 1999) where
stream geomorphology interacts with ground water and affects sediment deposition, nutrient flux, and
stream temperature (Naiman, 1988; Naiman et al., 1994; Fennessy and Cronk, 1997). Interaction of
these factors can produce high quality habitat for biota including fish (Boulton et al., 1998).
In the eastern United States, agriculture, forestry, and other economic developments are frequently
conducted in areas that closely border rivers, streams, and reservoirs. Restoration of these border
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areas, in the form of buffer strips, should have a measurable
impact on water quality (Rodgers and Dunn, 1992). This may be
especially true in lower order streams where nutrient processing
can be especially efficient (Alexander et al., 2000; Peterson et al.,
2001). Nonetheless, substantial questions remain: how do buffers
function in various landscapes and precisely which ecosystem
services can buffer strips restore (Osborne and Kovacic, 1993;
Schultzetal., 1995)?
Design criteria have been identified as an area of information
deficiency (van der Valk and Jolly, 1992). In the ecosystem
context, design criteria refer not only to functional relationships
sufficient to produce a desired water quality result, but also refer
to functional relationships relative to wildlife, biodiversity,
sustainability, and aesthetics. Although riparian buffer areas have
been proposed and are being evaluated as a nutrient management
method nationwide, the exact design requirements of these riparian
areas (including size of buffer zone, species composition,
interaction between surface and ground water, stream flow
regulation, provision of in-stream habitat, function for terrestrial
wildlife, sediment transport reduction, and maintenance of biotic
integrity) are incompletely understood and must be evaluated
along with nutrient management.
Research Environment
We will now describe the scope and content of research projects
that are needed to help elucidate, define, and quantify the function
of riparian buffers as instruments for riparian restoration.
Frequently, function and design criteria for reducing nutrient
inputs are of central interest to buffer designers. Therefore,
function and design criteria for riparian buffers for reducing nutrient
inputs are of central interest. However, ecosystem and/or
watershed restoration are by definition concerned with more than
water quality. The ecological functions and landscape
characteristics of riparian buffers as expressed by soil, flora, and
fauna are also of primary interest and do interact with nutrients to
a large degree.
The efficacy of riparian buffers as tools of ecosystem restoration
can be established by measuring and comparing effects among
three types of riparian buffers: TYPE 1 areas are stream reaches
that have received plantings and undergo associated management
with the purpose of restoring the riparian buffer; TYPE 2 areas are
stream reaches that are initially similar to those in TYPE 1 but that
do not receive plantings or undergo associated management to
restore the riparian buffer; and, TYPE 3 areas are stream reaches
that are effectively undisturbed or mature, and that represent
stream reaches that TYPE 1 areas are supposed to resemble 30+
years after initiation of the restoration. Comparing these three
basic types of stream reach will allow researchers to quantify
restoration endpoints (TYPE 3 areas), and measure success
(TYPE 2 areas contrasted with TYPE 1 and TYPE 3 areas).
These measures of endpoints and success are critical components
of restoration and are frequently the subject of administration,
management, landowner, and researcher initiated questions for
which there is too often little or no data. This is a void that research
can fill.
Research Questions
There are five general questions driving riparian buffer research:
1) What are the characteristics of existing riparian buffer zones
and surrounding landscape areas?
2) How do riparian areas function?
3) Where should riparian buffers be placed on the landscape?
4) What aspects of riparian ecosystems can be restored with
riparian buffers?
5) How effective is the restoration?
These general questions guide development of focused research
needs. In the enumeration of tasks that follows, it will be apparent
that some tasks are closely related and may be accomplished
together. The tasks are not meant to describe discrete research
efforts; rather, they are meant to describe discrete data needs
that, although in some cases are related, stand on their own as
being important areas where information relative to determining
the ecological function of riparian buffers is needed, including
restoration of water quality, in-stream and terrestrial riparian
habitat for flora and fauna, and landscape function relative to
adjacent land use, upstream activities, and downstream benefits.
Water Quality Effects
Task 1: In-Stream Water Quality as a Function of Riparian
Buffer Condition.
Where waterquality, especially regarding reduction of nutrients
and suspended sediments, is an important motivation for
developing restored riparian buffers, data concerning the
ability of restorations to provide these services, compared to
unrestored and mature communities, are needed.
Task 2: In-Stream Water Quality as a Function of Riparian
Buffer Width.
The width of riparian buffer can affect water quality at the
stream. It is unclear whether these improvements to water
quality accumulate linearly with increasing buffer width, or
whether the rate of improvement declines with increasing
buffer width.
Task 3: In-Stream Water Quality as a Function of Riparian
Buffer Length.
It is reasonable to expect that the length (of stream reach) of
restored riparian buffer will affect water quality to at least the
extent of buffer width. The effect of buffer length needs to be
quantified.
Task 4: In-Stream Water Quality as a Function of Riparian
Buffer Plant Community Composition.
It is hypothesized thatthe composition of the plant communities
that make up restored riparian buffers will have an impact on
the resulting in-stream water quality (i.e., nutrients and
sediment). The effects of plant community composition and
interactions with buffer condition, width, and length need to
be determined.
Task 5: Measure the Relative Contributions of First, Second,
and Third Order Streams to Watershed Loads of Nutrients
and Sediment.
Although there is substantial speculation, the relative
contributions of first, second, and third order streams to
watershed loads of nutrients and sediment need to be more
clearly quantified. This information can be used to target
restoration activities to locations where the greatest amounts
of nutrients and sediment are entering the system.
Task 6: Measure the Relative Contributions of Mixed Riparian
and Wetland Complexes to Watershed Loads of Nutrients
and Sediment.
There is substantial reason to expectthatwetland restorations,
integrated and designed as part of riparian restoration, can
enhance the ability of these riparian areas to remove and
reduce nutrients and sediment from non-point source runoff.
It is thought that the dynamic nature of the wetland system,
coupled with the long-term stability of the riparian area, will
have a synergistic effect and function better than either type
system (riparian or wetland) alone. The effect of integrating
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wetland and riparian restoration needsto be betterunderstood
and evaluated.
In-Stream Flora and Fauna Effects
Task 1: In-Stream Flora Response to Riparian Buffer
Restoration.
Ultimately, a primary reason for restoring riparian buffers is
restoration of riverine flora. The re-establishment and growth
of native aquatic plants is an important water quality and
habitat result that is expected to follow riparian buffer
restoration. The effects of restoration condition and buffer
width and length on the ability of riverine vegetation to re-
establish and prosper needs to be documented.
Task 2: In-Stream Fauna Response to Riparian Buffer
Restoration.
Ultimately, a primary reason for restoring riparian buffers is
restoration of riverine fauna. The re-establishment of native
aquatic fauna is an important water quality and habitat output
that is expected to follow riparian buffer restoration. The
effects of restoration condition and buffer width and length on
the ability of riverine fauna to re-establish and prosper needs
to be documented.
Task 3: Document the Characteristics of Plant and Animal
Communities Associated with In-Stream Riparian Habitat.
Determine population and community characteristics and
habitat associations of extant in-stream flora and fauna.
What changes could be expected if buffer restorations are
implemented?
Terrestrial Riparian Flora and Fauna Effects
Task 1: Riparian Buffer Terrestrial Flora Response to Riparian
Buffer Restoration.
An important reason for restoring riparian buffers is restoration
of associated terrestrial riparian-habitat dependent flora. The
re-establishment and growth of native terrestrial plants is an
important restoration and conservation resultthat is expected
to follow riparian buffer restoration. The effects of restoration
condition and buffer width and length on the ability of terrestrial
vegetation to re-establish and prosper needs to be
documented.
Task 2: Riparian Buffer Terrestrial Fauna Response to Riparian
Buffer Restoration.
An important reason for restoring riparian buffers is restoration
of associated terrestrial riparian-habitat dependent fauna.
The re-establishment of native terrestrial fauna is an important
restoration and conservation resultthat is expected to follow
riparian buffer restoration. The effects of restoration condition
and buffer width and length on the ability of terrestrial fauna
to re-establish and prosper needs to be documented.
Task 3: Riparian Buffer Terrestrial Microbe Response to
Riparian Buffer Restoration.
Microbial communities play a substantial role in the nitrogen
cycle and are especially important relative to the production
and retention of nitrate. The role of microbial denitrifying
activity relative to buffer condition and buffer width needs to
be documented. It is not expected that buffer length will affect
microbial denitrifying activity to any great extent.
Task 4: Document the Characteristics of Plant and Animal
Communities Associated with Terrestrial Riparian Habitat.
Determine population and community characteristics and
habitat associations of extant terrestrial - riparian habitat
dependent - flora and fauna. How do these conditions differ
from the desired state and what changes are expected if
buffers are restored?
Task 5: Measure the Benefits to Terrestrial Diversity from the
Placement of Riparian Buffers, and Document Interactions
between Such Diversity and Buffer Function, Sustainability,
and Succession.
Buffer placement on the landscape can be expected to play
a substantial role in determining its final functionality. This is
especially true regarding effects on diversity, where the size
and shape of the restored buffers will interact with proximity
to other restorations and undisturbed native sites to determine
the ultimate benefit. The interaction of these various conditions
relative to use of riparian buffers as tools to conserve
biodiversity needs to be documented.
Task 6: Determine the Extent to Which Riparian Buffers
Function as Habitat for Terrestrial Species and the Extent to
Which They Function as Corridors and Refugia.
Riparian buffers have the capacity to act as dispersal corridors,
refugia, orhabitatformanyterrestrialfauna. Thesetendencies
need to be documented and management prescriptions
regarding buffer size and placement need to be formulated to
assure that restored riparian buffers function as intended.
Landscape Scale Considerations
Task 1: Mapping of Stream Reaches and Watersheds that are
Highly Suitable for Riparian Buffer Restoration Based Upon
Proximity of Denitrifying Subsurface Strata.
Research indicates that an important factor that limits nitrate
concentration in surface waters is the proximity of denitrifying
strata to the surface. In areas where this denitrifying stratum
is below the root zone of plants, little water quality impact
relative to reduction of nitrate concentration in surface waters
can be expected. It is necessary to map the extent of these
denitrifying strata and determine the miles of stream reach
where water quality restoration relative to nitrate loading is so
limited.
Task 2: Determine Whether Relationships Exist Between
Watershed Size and Proportionate Land Use and the Design
Requirements of Riparian Buffer Restorations.
The type and extent of land uses that are found within
individual watersheds may play an important role in
determining the types and extents of buffer restorations that
are appropriate. In some cases, large extensive restorations
may be necessary to provide desired improvements to water
quality and habitat; in other watersheds, several small
restorations may provide equivalent improvements. The
ecological, social, and economic characteristics of individual
watersheds that may affect the types and extent of necessary
restorations need to be documented.
Task 3: Determine the Landscape Conditions and Spatial
Extent Over Which Substantive Ecosystem Benefits Cannot
be Expected to be Achieved with Restored Riparian Buffers
Alone.
Riparian buffers cannot address all resource management
questions. The subset of questions that buffers can be used
to impact needsto be distinguished from the subset that they
cannot. Particularly, spatial and temporal scale concerns
need to be addressed relative to this question.
Task 4: Measure the Distribution of Riparian Buffer Widths
Associated with Each Stream Order and Document Their
Distribution.
Determine the extant coverage and frequency distributions of
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buffer widths by stream order. Assess how these values
compare among watersheds, particularly contrasting
watersheds with good versus poor water quality.
Task 5: Measure the Effects of Improvements Attributable to
Riparian Buffer Restoration in Parts of the Watershed not
Directly Restored. Particularly, to What Extent are
Downstream Areas Cumulatively Impacted?
Determine the extent to which the water quality and in-stream
biota of areas downstream of riparian buffer restorations can
be positively affected. In particular, is it realistic to expect
water quality and habitat to improve in estuaries because of
riparian buffer restorations?
Task 6: Measure and Document Interactions Among Upstream
Land Use and Downstream Aquatic and Terrestrial Effects.
To an extent, restoration of riparian buffers can only be
expected to produce a limited suite of outputs. The effects of
upstream activities also play a large role in determining the
final function of the buffers on the landscape. The extent to
which upstream activities limit the range of outcomes that can
be seen downstream needs to be documented.
Monitoring and Indicators
Task 1: Determine Which Variables to Monitor to Assess
Restoration Success Over Short, Intermediate, and Long
Time Periods.
Identify indicators that can be used to assess short,
intermediate, and long-term performance and function of
restored riparian buffers. Identifythevariabilityofthe indicators
- determine the conditions under which they can be
appropriately used - and assess error rate.
Task 2: Identify Biotic and Abiotic Indicators, and Sampling
Schedules for Such Indicators, that will Help Determine
Restoration Effectiveness and Measure Senescence - Such
that the Need to Renovate or Otherwise Maintain Restored
Riparian Buffers can be Determined.
Identify indicators and concurrent monitoring schedules that
can track restoration senescence and predict the need to
conduct renovation - including expected renovation benefits
and costs.
Physical Sciences
Task 1: MeasuretheExtentto Which Restored Riparian Buffers
Influence Hydrology.
Riparian buffers are hydrologically connected to streams and
stream processes. Thus, to what extent are water quality and
habitat limited by the characteristics of hydrology that may be
amenable to engineered manipulations? For example,
denitrification, a microbially mediated process in which nitrate
nitrogen is converted to gaseous nitrogen, is strongly
influenced by ground-waterdynamics in riparianzones. When
the watertable is close to the soil surface, nitrate in the ground
water interacts with carbon-rich soils in an anaerobic
environment creating high potential for denitrification. Natural
or anthropogenic factors that lower water tables greatly
reduce riparian denitrification potential and other
biogeochemical processes.
Task 2: Measure the Extentto Which Riparian Buffers Influence
Flood Plain Processes.
Riparian zones represent a distinct component of stream
flood plains. Riparian zones and associated buffers may
influence annual overbank flows and/or in-stream flow
dynamics, consequently affecting biogeochemical processes.
For example, beavers (Castor canadensis) are natural
engineers that manipulate stream flow dynamics through
construction of impoundments. Such impoundments may
have watershed-wide influences on sediment deposition,
nutrient flux, and stream temperature.
Task 3: Measure the Extentto Which Riparian Buffers Influence
Hyporheic Zone Processes.
Riparian buffers may influence the way in which ground water
and surface water interact in stream ecosystems. Such
hyporheic zones are regions of high microbial activity that
greatly influence rates and fluxes of nutrients and oxygen
through assimilation and respiration. Consequently, hyporheic
zones represent productive, high quality habitats for both
invertebrates and vertebrates including fish.
Task 4: Document Differences in Restoration Potential for
Riparian Zones in Urbanized Areas versus Streams in Less
Affected Ecosystems.
In urban areas, high volume diversions of storm water runoff
into stream channels cause stream incision that disconnects
floodplainsfromgroundwaterprocesses. Similarly, infiltration
of runoff is reduced under zones of pavement or other
impervious surfaces, thereby reducing riparian water tables
in urban watersheds. The extent to which effects attributable
to these and related alterations limit stream restoration
potential needs to be well understood.
Socio-Economic Perspectives
Task 1: Measure Economic Impacts of Riparian Buffer
Restorations, Especially Relative to Reduced Crop Yields
Attributable to Buffer Shading and Competition, But Also
Accounting for Economic Gains Attributable to Improved
Nutrient Management.
Where the establishment of restored riparian buffers depends
to a large extent on the cooperation of private landowners,
especially agriculturists, the economic impacts of buffer
placement relative to crop shading and potential competition
need to be documented in concert with potential economic
benefits that may accrue due to improved efficiency in use of
fertilizer and to improved fin- and shell-fish fisheries.
Integrated Research
Task 1: Determine Whether Riparian Buffer Function Varies
Quantitatively or Qualitatively in Relationship to Varying
Types of Adjacent Land Use. This Includes the Functions of
Nutrient and Sediment Attenuation and Provision ofln-Stream
and Terrestrial Riparian Habitat for Plants and Animals.
The questions of whether and how adjacent land use affects
riparian buffer function - for water quality improvement and
for provision of plant and animal habitat - need to be
investigated. For instance, do buffers adjacent to industrial,
residential, and farming operations provide the same quality
and quantity of benefit, or does the adjacent land use limit the
effectiveness of the buffer in some regard?
Task 2: Measure Differences in Riparian Buffer Function,
Especially Relative to Nutrient and Sediment Attenuation -
but also Including Provision of In-Stream and Terrestrial
Riparian Plant and Animal Habitat - Among First, Second,
and Third Order Streams.
Just as adjacent land use can be expected to have an
influence on buffer, stream order may also have an effect.
Wherein a buffer in a first order stream may intercept 70% of
the water that enters the stream and may therefore have a
large effect on water quality and habitat - so may a buffer in
a third order stream only intercept 10% of the water entering
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the stream, and thereby have a small effect on water quality
and habitat.
Task 3: Measure the Rate at Which Benefits Attributed to
Restored Riparian Buffers are Attenuated Downstream,
Especially Relative to Nutrients and Sediment.
Realistically, riparian buffer restorations can only be conducted
along a relatively modest number of stream reaches. It is
thought that these will be mostly first, second, and third order
streams. The ability of supposed water quality improvements
occurring within these stream reaches to propagate
downstream and be maintained is unknown.
Task 4: Determine Types of Engineering Controls that can be
Used to Complement Restored Riparian Buffers to Provide or
Enhance Benefits Relative to Nutrient and Sediment
Attenuation, and Provision of In-Stream and Terrestrial
Riparian Plant and Animal Habitat.
Under what conditions can engineering controls be used to
complement riparian buffer restorations? Which engineered
solutions have the greatest promise of working? What are the
complementary benefits to terrestrial and aquatic riparian
ecosystems that are expected from engineered solutions?
Task 5: Measure the Effects ofAbove-GroundBiomass, Plant
Communities, and Animal Communities on Surface Water
Runoff and Surface Water/Ground Water Interactions,
Especially Relative to Nutrient and Sediment Transport.
Animal communities affect plant communities and plant
communities affect subsurface chemistry, and ultimately
interact with the chemical characteristics (particularly regarding
nutrients)ofin-streamwaterandin-streamhabitat. Therefore,
interactions between terrestrial riparian flora and fauna and
the chemical characteristics of riparian ecosystems need to
be understood to help design buffers that will remove or
otherwise retain nutrients and sediment.
Conclusion
This issue paper identifies subject areas where there are critical
data gaps concerning the function of riparian buffers as ecosystem
and watershed restoration tools. We acknowledge that whereas
there is a reasonable foundation of data to support an expectation
that restored riparian buffers can provide some water improvement
relative to reduced loads of nutrients, there are insufficient data
concerning other benefits to aquatic and terrestrial riparian biota
and habitat to allow the generalization that restored riparian
buffers can function astools forecosystem restoration. Therefore,
we identify research that focuses on water quality measured at the
stream while concurrently measuring other biotic and abiotic
responses.
Restoration of the Nation's waters will require manipulation of
upstream watersheds, including buffer management. The relative
contributions of activities in these watersheds and the ability to
manage the effects of these activities with riparian buffers are
important questions that bear upon research needs described in
this paper. Care must be taken to ensure that the contributions of
all watersheds are considered and statistically represented in any
proposed management solutions that rely upon buffers. The
effects of small headwater, first, and second order watersheds
relative to the larger ones need to be clearly understood.
Notice
The U.S. Environmental Protection Agency through its Office of
Research and Development funded and managed the preparation
of this Issue Paper. It has been subjected to the Agency's peer
and administrative review and has been approved for publication
as an EPA document. The authors acknowledge help provided by
Frederick W. Kutz on an earlier draft of the paper.
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