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Extension > Environment > Agroforestry > Design of riparian forest buffers

Design of riparian forest buffers

Phyllis Bongard, Extension educator in agriculture production/water quality
Gary Wyatt, Extension educator in agroforestry

PDF version (472 K)

fish

Illustration by Robin Freese

Introduction

Minnesota is home to more than 450 miles of Department of Natural Resources designated trout streams. As a cold water species, trout are sensitive to warm stream temperatures. Establishing trees in riparian buffers is widely recognized as a significant tool for stabilizing stream temperatures and improving trout habitat. Riparian forest buffers (RFBs) provide other benefits, as well. They filter sediment, nutrients and pesticides, thus preventing movement of these nonpoint pollution sources downstream. Trees in the buffer zone provide woody debris for the stream, an important component of trout habitat. The woody roots also help stabilize stream banks and help with flood control.

Identifying objectives for the riparian forest buffer (RFB)

Landowner objectives should be identified before the riparian forest buffer is designed. In addition to improving water quality and stream habitat, a landowner may wish to generate income from the buffer or enhance wildlife habitat.

Considerations for income generation

Multi-species, multi-use RFBs offer landowners an opportunity to explore both short- and long-term sources of income from the buffer itself. High value hardwoods, such as walnut, oak or maple, may become a sustainable source of long-term income. Hybrid poplars might be harvested for pulpwood, sawlogs or biomass. On a shorter-term basis, decorative woody florals may be ready to harvest in two years, while income from edible berries and nuts may take 2 to 15 years to produce. In all cases, markets for the specialty forest products should be explored and identified before the RFB plan is finalized. (See Table 5 for a list of possible plantings.)

Government and other agency programs that provide technical and financial assistance to conserve and enhance soil and water resources may also be a source of income. Refer to Financial assistance opportunities for riparian forest buffers for more detailed information about the types of programs available and their requirements for establishing and maintaining riparian forest buffers.

In the longer term, United States Department of Agriculture Ecosystem Services is exploring opportunities to advance markets and payments for practices such as wildlife habitat and diversity, carbon storage and watershed services. While these practices have traditionally been viewed as free benefits to the society, Ecosystem Services is developing policies to stimulate market-based conservation and stewardship.

Considerations for enhancing wildlife habitat

Plant materials can be selected to provide food, shelter or nesting areas for various wildlife species. For example, deer and wild turkeys prefer acorns when they are available and both use woody edges for food and cover. Trees and shrubs with high wildlife merit rankings can be selected for the buffer from Tables 3 and 4. If runoff is not a major concern, a wide variety of native grasses and forbs can be selected for Zone III to provide greater habitat diversity and attract more wildlife species. If creating winter habitat for wildlife is a goal for the buffer, see the recommended conifers listed in Table 4.

Buffer widths need to be increased when a landowner wishes to support wildlife. In general, width guidelines of 100 to 300 feet are considered a minimum for enhancing wildlife habitat and providing a travel corridor (Table 1).

Table 1. Minimum buffer widths for wildlife (not site specific)

Wildlife species Buffer width (ft.)
Bald eagle, cavity nesting ducks, heron rookery, sandhill crane 600
Common loon, pileated woodpecker 450
Beaver, dabbling ducks, mink, and to maximize bird species diversity 300
Deer 200
Lesser scaup, harlequin duck, muskrat 165
Frog, salamander 100
NRCS - Conservation Practice Standard for Riparian Forest Buffer, Code 391f

Vegetation selection

The purpose or goal of the riparian buffer planting will have a direct relationship with the design and vegetation selected for the planting. Trees, shrubs and grasses vary in the kinds of benefits they provide to the buffer. Table 2 shows the relative effectiveness of these different vegetation types in providing a variety of benefits.

Table 2. Relative effectiveness of different vegetation types for providing specific benefits

Vegetation type
Benefits Grass Shrub Tree
Stabilize bank erosion Low High High
Filter sediment High Low Low
Filter nutrients, pesticides, microbes
Sediment-bound High Low Low
Soluble Medium Low Medium
Aquatic habitat Low Medium High
Wildlife habitat
Range/pasture/prairie wildlife High Medium Low
Forest wildlife Low Medium High
Economic products Low Medium High
Flood protection Low Medium High
Iowa State University

Three-zone buffer design

A three-zone riparian forest buffer is widely recognized as the best approach for mitigating agricultural impacts (Figure 1).

three-zone-riparian-forest-buffer

Illustration by Robin Freese.

Figure 1. Example of a three-zone riparian forest buffer that is bordering cropland on the left side of the stream and pasture on the right. A strip of alfalfa may be substituted for the warm-season grasses in Zone III when the buffer borders pasture.

Zone I is the unmanaged woody zone nearest the stream. Trees in this zone are selected for rapid root development and tolerance of wet conditions. Tables 3 and 4 include information on growth rates and flooding tolerance for trees and shrubs that are appropriate for riparian areas in Minnesota. Zone I trees provide perennial root systems to stabilize stream banks, woody debris for aquatic habitat and shade for stream temperature moderation. For shading streams and rivers, the canopy at maturity should have at least 50 percent crown cover with average canopy heights at least equal to the width of the water body (Figure 2). A minimum width of 35 feet is required by Natural Resources Conservation Service programs.

optimal-stream-shading

Illustration by Robin Freese.

Figure 2. For optimal stream shading, the canopy height of Zone I trees should be equal to or greater than the width of the stream.

Zone II is the wider managed woody zone. Trees and shrubs in this zone should be fast-growers that can tolerate periodic flooding (Tables 3 and 4). The primary role of this zone is to absorb and store nutrients, degrade pesticides and slow floodwaters. It can also add diversity for wildlife habitat and be managed for additional income. See Table 5 for a list of riparian trees and shrubs that can produce a marketable crop. Four to five rows of trees and one to two rows of shrubs are recommended for this zone. Recommended widths within and between rows for Zones I and II can be seen in Table 6.

milti-species-three-zone-riparian-forest-buffer

Illustration by Robin Freese.

Figure 3. An example of multi-species, three-zone riparian forest buffer that includes potential income producing trees and shrubs.

Zone III consists of at least a 20 to 24 foot width of warm-season grasses and forbs and is essential in an agricultural setting. Nutrient uptake and sediment filtering are the major roles of this high infiltration zone. Where runoff is an issue, switchgrass is the preferred warm season species due to its dense, stiff stems that slow water flow. In areas where runoff is not a concern, other warm season native grasses, such as indiangrass and big and little bluestem can be used (Table 7). Non-native cool season grasses like smooth bromegrass and reed canarygrass are not appropriate for this zone, since they have less root mass for stabilizing soil than warm season grasses. They tend to lay down under water flow and can be invasive. However, native cool season grasses should be included as part of the seed mixture, since they establish more quickly and provide cover earlier than warm season grasses. As the warm season natives become well established, the cool season grasses will naturally diminish. An example of a design that includes switchgrass, other warm-season grasses and potential income-producing trees and shrubs can be seen in Figure 3.

Buffer width

wildlife-diversity

Illustration by Robin Freese.

Figure 4. A mixed and randomly planted design for wildlife diversity.

Narrow buffers are effective at trapping sediment, but extensive buffers are better at transforming nutrients and pesticides. Buffers narrower than 66 feet generally do not hold water long enough for chemicals to be removed. As stated above, wider buffers are also needed to enhance wildlife habitat.

To design a wider buffer, any of the three zones can be increased. Widths may also vary to address runoff hotspots or to more easily accommodate machinery and straighten the outer buffer edge on a meandering stream. While wider buffers are generally better, the landowner must be sure to follow the minimum and maximum width requirements when the buffer is enrolled in a government program. Landowners may be able to create buffers wider than government program requirements but incentives and cost share would not be allowed on those acres. Other sources of cost share funds may be available through other organizations.

"Naturalized" planting as an option

Although establishment and maintenance of a buffer is more convenient when it is arranged in rows, a mixed planting of trees and shrubs that mimics native woodlands is particularly beneficial for wildlife and may be more aesthetically pleasing (Figure 4).

An economic forest model developed for the Chesapeake Bay region uses a multi-layer, block approach. A cross between planting in rows and a more naturalized approach, the plants are grouped in blocks with understory forest crops in the same area as taller trees. Modeled after a Mayan tropical forest practice, this model produces several non-timber forest products, including fruits, nuts and woody ornamentals.

Repairing problem areas

Riparian forest buffers can have little effect on filtering dissolved nutrients and pesticides if subsurface tiles discharge directly into the stream. If this is the case, creating a small wetland to intercept field drainage tiles will help protect the stream. In addition, tree roots can clog clay or perforated tiles, so these should be replaced with solid PVC tile. If problem tiles cannot be replaced, a 30 to 40 foot strip of cool season grasses should be planted above the drainage line.

Other problem areas, such as gullies and eroded stream banks, should be identified and addressed before the riparian buffer is installed. Additional management practices may be required to repair eroded areas, reduce undercutting of the stream bank or slow water movement. Many different types of stabilization practices are used and help is available from the Minnesota Department of Natural Resources or your local Soil and Water Conservation District.

Table 3. Deciduous woody species recommended for riparian forest buffers in Minnesota1

Common name Scientific name Growth Flooding tolerance Large woody debris Shade value Wildlife merit
Shrubs
American cranberry Viburnum trilobum Slow H-M L L H
Arrowwood Viburnum dentatum Moderate H L L H
Black chokeberry Aronia melanocarpa M L L H
Black elderberry Sambucus nigra Mod/fast H L L H
Dogwood, red osier Cornus sericea Fast H L L M
Dogwood, silky C. amomum H L L M
Dogwood, gray C. racemosa Moderate M L L M
Nannyberry Viburnum lentago Slow M L L H
Ninebark, common Physocarpus opulifolius Moderate L L L H-M
Willow, sandbar Salix exigua Fast H L L H
Winterberry Ilex verticillata Moderate H L L H
Small deciduous
American plum Prunus americana Fast L-M L L H
Chokecherry Prunus virginiana Fast L-M L L H
Crabapple Malus spp. Moderate H
False indigo Amorpha fruticosa H L L L
Hawthorne, cockspur Crataegus crusgalli Slow/mod M L L H
Hazelnut, American Corylus americana Moderate M L L H
Serviceberry (Juneberry) Amelanchhier alnifolia Moderate M-L L L H
Willow, peachleaf Salix verticillata Fast H L L H
Tall deciduous
Ash, green2 Fraxinus, pennsylvanica Fast M M M M
Ash, white F. americana M M M M
Ash, black F. nigra Moderate H-M M M M
Aspen, quaking Populus tremuloides Fast L M M H
Birch, white Betula papyrifera Fast M-H M M H
Birch, river B. nigra Fast M-H M M M
Birch, yellow B. alleganiensis Fast M-H H M H
Basswood Tilia americana Moderate L-M H H L
Cherry, black Prunus serotina Fast M
Cottonwood, eastern Populous deltoids V. fast H-VH H H-VH M
Hackberry, common Celtis occidentalis Fast M M-H H H
Maple, silver Acer saccharinum Fast H H H M
Maple, red A. rubrum Fast H H H M-H
Maple, sugar A. saccharum Slow
Oak, bur Quercus macrocarpa Slow/mod H-M H H H
Oak, northern pin Q. ellipsoidalis M-L H H H
Oak, red Q. rubrum Mod/fast L H H H
Oak, swamp white Q. bicolor Fast H M H H
Poplar, hybrid Populus Fast H
Walnut, black Juglans nigra Fast H M H M
Walnut, white (Butternut) J. cinerea Fast L M M H
Willow, black Salix nigra Fast H M H M
Relative ranking values: VH=Very high; H=High; M=Medium; L=Low
1 Tree list compiled from NRCS Conservation Practice Standard, Riparian Forest Buffer, Code 391; 2009 Minnesota State Forest Nursery Order; Forest Management Guidelines.
2 Planting ash species should be highly scrutinized due to the possible infestation of the invasive species emerald ash borer.

Table 4. Woody conifer species recommended for riparian forest buffers in Minnesota1

Common name Scientific name Growth Flooding tolerance Large woody debris Shade value Wildlife merit
Small conifers
Cedar, red Juniperus virginiana Slow/mod M M H H
Cedar, white (arborvitae) Thuja occidentalis Slow/mod H-M M H H
Tall conifers
Balsam fir Abies balsamea Slow/mod M-H M M H
Pine, jack Pinus banksiana Fast L L M L
Pine, red P. resinosa Mod/fast M H M M
Pine, white P. strobes Fast M H H H
Spruce, black Picea mariana Moderate H-VH M M M-H
Spruce, white P. abies Moderate M-L H M-H H
Tamarack (Larch) Larix laricina Fast H-M H M M
Relative ranking values: VH=Very high; H=High; M=Medium; L=Low
1 Tree list compiled from NRCS Conservation Practice Standard, Riparian Forest Buffer, Code 391; 2009 Minnesota State Forest Nursery Order; Forest Management Guidelines.

Table 5. Woody plant materials for the riparian buffer that can produce marketable crops 1

Common name Product Minnesota regions
Shrubs
American cranberry Fruit preserves, wine all
Elderberry Fruit preserves, wine all
Dogwood, red-osier Woody ornamentals all, but nor as well in southwest
Nannyberry Fruit preserves all
Small deciduous
American plum Fruit preserves, wine all
Chokecherry Fruit preserves, wine all
Crabapple Fruit preserves all
Hawthorne Fruit preserves all
Hazelnut, hybrid Biofuels (oil), biomass, nuts all
Serviceberry (Juneberry) Fruit preserves all
Tall deciduous
Birch, white Woody ornamentals(branches, birch cones) all but Red River valley
Birch, river Woody ornamentals (branches, birch cones) better in south
Maple, sugar Maple syrup all
Poplar, hybrid Fuel biomass, pulp and paper all
Walnut, black Nut, shells, lumber southeast (best), southwest
1 Tree list compiled from Producing Marketable Products from Living Snow Fences, University of Minnesota Center for Integrated Natural Resources and Agricultural Management.

Table 6. Recommended spacing within and between rows of trees and shrubs in the riparian buffer1

Spacing
Spacing within rows (feet)
Shrubs 3-8
Small trees 6-10
Tall trees 10-16
Spacing between rows
Shrubs 10
Shrubs and small trees 12
Small trees 12
Small trees amd tall trees 16
Tall trees 16
Fast growing trees/conifers and conifers 20
1 G. Kopp, personal communication, 2009.

Table 7. Native grasses (not inclusive) for Zone III of the riparian buffer and their tolerance to drought and flooding1

Common name Scientific name Cool/warm season Drought Flooding Wet soil
Canada wild rye Elymus canadensis Cool Fair Good No
Slender wheatgrass* Elymus trachycaulus Cool Good Good No
Big Bluestem* Andropogon gerardii Warm Fair Poor No
Little bluestem* Schizachyrium scoparium Warm Good-Excellent Poor No
Indiangrass* Sorghastrum nutans Warm Fair Poor No
Sideoats grama* Bouteloua curtipendula Warm Fair Poor No
Prairie cordgrass Spartina pectinata Warm Poor Excellent Yes
Switchgrass* Panicum virgatum Warm Good Good Yes
*Varieties adapted for different areas of Minnesota. See NRCS Conservation Practices Standard: Filter Strip.

1 Native grass list compiled from NRCS Conservation Practices Standard: Filter Strip; Producing Marketable Products from Living Snow Fences, University of Minnesota Center for Integrated Natural Resources and Agricultural Management; MN/DOT Seeding Manual: 2007 Edition.


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Reviewed 2010


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