A guide to riparian tree and shrub planting in the Willamette Valley: Steps to success

Planting trees and shrubs to restore streamside areas, enhance fish and wildlife habitat, improve water quality and achieve other environmental benefits is increasingly common in Oregon (figure 1). But the success of riparian plantings varies widely, and some fail outright (see “Survey shows variable success with riparian tree planting").

Ecologists recommend restoring or enhancing important riparian functions (figure 5), not just vegetation conditions. For example, development across the Willamette Valley has led to loss and degradation of many habitats, including once-extensive networks of riparian forests, prairies and savannas. This, in turn, reduced plant and animal populations. One functional goal is to improve habitat by restoring or enhancing conditions that are critical to survival of a species or group of species. See Appendix A for more examples of riparian functions.

Riparian vegetation provides many important functions for aquatic habitats. For example, restoration projects designed to aid salmon populations often focus on establishing tree species that create shade, reduce stream temperatures, and drop leaves and insects into the stream. Plantings will eventually provide large wood, which is important for modifying stream channels and creating in-stream habitats.

Riparian areas are also important terrestrial habitats. A well-developed shrub layer provides foraging and nesting sites for migratory songbirds.

Large trees are needed to provide nesting or foraging sites for large birds, such as herons and pileated woodpeckers. Large trees take time to develop, but you may accelerate their growth by planting fast-growing species, spacing them far apart, and controlling weeds and other competition. You can also provide some functions normally provided by large trees through interim actions, such as installing nesting boxes (for wood ducks and screech owls) or platforms (for osprey).

Consider passive restoration

Consider whether your site will be able to grow and develop as you desire without actively planting it (a passive restoration approach). For example, you may be able to meet tree establishment goals by encouraging natural regeneration of species already present and reproducing on the site (see Appendix B). Perhaps the only action needed is to remove grazing livestock, weed competition or intensive cultivation.

Identify obstacles

It is important to assess operational constraints, identify problems, and resolve conflicts during the planning phase. Think about and discuss the following questions with advisors and organizational partners:

• How much money, time and energy are available for this project?
• Is the budget adequate for the size of the project?
• Do you have a good understanding of demands on resources to accomplish key activities throughout the project (from planning, to planting, to weed control and maintenance)? Can you realistically meet those demands in a timely way, considering your other commitments, health, and access to and skill in using equipment and tools? If not, is there money in the budget to hire help?
• What conflicts might arise with adjacent land uses (e.g., farming or grazing), and how can these be resolved?
• Is there good access to the planting site, and can you move in any needed equipment or supplies?
• Does the site location allow frequent visits for monitoring and maintenance, or will you visit the site only occasionally?
• How will you determine project success? What are the consequences of an unsuccessful planting?

The scale of the project is also an important consideration. Small projects (e.g., dozens to a few hundred trees) allow use of a wider range of techniques, such as hand-cutting competing vegetation, that might be prohibitively expensive on larger projects. As the project’s size and complexity increase, so does the need for cost-effective methods of site preparation, planting and vegetation control. Balance the scale of your project with the budget, time and other resources available.

Design your project

Although it is helpful to look at other plans and projects, be sure to design a riparian planting that is specific to your site, reflects identified goals, and can be accomplished with available resources.

Your design will need to address many features:

• Width and position of the planting
• Species to plant and type of planting materials
• Plant spacing and arrangement
• Access for people and equipment (for maintenance and monitoring)
• Fencing or other protection from livestock and wildlife

You also need to decide how to prepare the site for planting, how to protect seedlings from weed competition, and how to maintain the planting. The following sections provide more information on these topics. See Appendix C for additional project design considerations.

Step 2. Select and obtain plant materials

Species selection

Your plant selections will affect the appearance and function of your riparian planting for decades. Trees and shrubs must be able to survive and prosper when planted and also provide the functions you need in the future. Tables 1 and 2 list characteristics of native trees and shrubs.

Site adaptation

Choose species that are well adapted to site conditions. Moisture — either the lack or excess of — is often the most important factor in both planting success and long-term survival. Consider how the local climate and soils affect moisture, and select species on the basis of moisture needs and flood and drought tolerance. Start by identifying native species growing on similar sites nearby.

Conifers, such as Douglas-fir or western redcedar, are often a priority for riparian plantings in mountainous areas because they provide dense shade and durable, large wood. However, many conifers are not as well adapted to riparian areas in the Willamette Valley that are flood prone or poorly drained and should be selected with caution.

Tolerance to floods and poor drainage

Areas along the stream channel and banks as well as sloughs and swales may be subject to frequent or prolonged flooding. Species selected for these areas must have high flood tolerance. Black cottonwood, bigleaf maple, western redcedar, red alder, white alder and Oregon ash tolerate flooding (figure 6).

A related but often separate problem is soil drainage. Soils on higher terraces in the Willamette Valley often have heavy, fine-textured (clayey) soils with very poor drainage and poor aeration during the rainy season, regardless of whether they flood regularly (table 3). These are essentially wetland soils. Plants on these soils are subjected to saturated conditions for much longer periods than during surface flooding.

Drought tolerance

The Willamette Valley has hot, dry summer weather, and moisture stress is often a limiting factor in seedling survival. Sites close to major rivers can actually be quite dry for new seedlings. These riverbanks and levies often have sandy or gravelly soils with low nutrient-and moisture-holding capacity, and they dry out rapidly each summer once the rains stop.

Although mature black cottonwood trees thrive along the sandy banks of the Willamette River, this species has relatively low drought tolerance and may be difficult to establish on such sites until its roots reach moisture deep in the soil. In some situations, you can offset this issue with weed control and irrigation.

Shade tolerance

Fast-growing riparian tree species, such as willow, cottonwood and alder, quickly colonize disturbed areas after floods. But these trees are intolerant of shade and not suitable for planting in the understory of existing trees. Oregon ash, bigleaf maple, grand fir, western redcedar and many shrub species are tolerant of shade and more suitable for such areas (tables 1 and 2).

Shade tolerance also comes into play as young trees grow and begin to compete with each other for light and other resources. Slower-growing and smaller tree species will tend to fall behind in growth; if these species are not shade tolerant, they will likely die and disappear from the stand. Such competition is part of a natural process but can lead to less diversity and a simpler stand structure than planned for or desired. Address this issue with thoughtful species selection, appropriate spacing and planting arrangements (see Appendix C), and selective thinning.

Local plant materials

Many species you select for your planting likely grow in many locations in Oregon and North America. For instance, white alder is plentiful in southwest Oregon, and ponderosa pine is abundant in central Oregon. However, plants of the same species from different environments can perform very differently on your site. It is important to find seedlings and planting stock grown from Willamette Valley sources. Buying from a local nursery is not necessarily enough; it is where the seed (or other parent material) comes from that counts, not the place where seedlings are raised. See “Seed sources and genetic issues.”

Planting stock

Several types of seedlings and stock types are used in riparian plantings (table 4). Consider these characteristics when selecting a stock type:

• Availability
• Handling sensitivity
• Cost
• Ease of transport and planting
• Survival and growth potential

In general, bigger is better. Bigger stock is less likely to be overtopped by competing vegetation and recovers better when browsed. However, it is also more expensive to buy and plant. Ensure that bigger stock has an adequate root system to support the large shoot system.

The 1-1s tend to be larger than 2-0s and have denser, more fibrous root systems. They also cost more. Plug-1 seedlings have characteristics of container and bare-root seedlings and well-developed, fibrous root systems. They are more expensive than other bare-root seedlings.

Advantages of bare-root seedlings:

• Low cost
• Easy to transport
• Wide availability of many forest tree (and some shrub) species

Disadvantages of bare-root seedlings:

• Narrow planting window (winter dormant season)
• Requires skill during planting
• Greater sensitivity to improper handling and planting (e.g., root drying and other damage)

See Selecting and buying quality seedlings, EC 1196, for more information about stock types.

Ball-and-burlap

Ball-and-burlap (B&B) stock has a large ball of soil around the root system held in place with a burlap wrap. These tend to be much larger, much more expensive trees that are generally inappropriate for most restoration situations but may be used in park settings and other green spaces in or near urban areas.

Advantages of ball-and-burlap stock:

• Provide an “instant tree”
• Good survival and growth

Disadvantages of ball-and-burlap stock:

• High cost
• More difficult to plant than other stock types

Many commercial forest nurseries in the Pacific Northwest grow plants suitable for riparian restoration projects. The popularity of these projects, and the demand for planting stock, has grown enormously in the past decade. Most nurseries grow some stock on speculation (i.e., based on expected demand), which they sell on a first-come, first-served basis. These supplies don’t always meet demand, so order early for the best availability of species, seed source and stock type.

You can also contract with nurseries to grow plants to your specifications. This gives you the greatest control of seed source. A minimum number of seedlings (e.g., 2,000) is required for such contracts, and you will likely need to start this process 2 years before your anticipated planting date.

Many local soil and water conservation districts and local chapters of the Oregon Small Woodlands Association have annual plant sales, which are a good place to purchase smaller numbers of plants. Be sure to ask about seed source. Use the following resources to find nurseries that sell or specialize in native plants:

• Sources of native forest nursery seedlings, Oregon Department of Forestry
• Oregon Association of Nurseries
• Oregon State University Extension forester
• Oregon Department of Forestry stewardship forester

Mechanical control

Mowing

Mowing is commonly used to clear herbaceous plant debris and woody species, such as blackberry, before planting. However, mowing alone is not an effective way to control perennial weeds, which simply regrow after cutting. In some cases, multiple annual mowings over several years can substantially reduce blackberry cover and vigor. Tractor-mounted mowers are preferred where access and slopes permit, although soil compaction might be a concern. Soil compaction has been shown to reduce seedling growth in upland forests. Where machinery use is not feasible, use machetes, loppers or brush cutters. A chainsaw with a hedge-trimmer attachment is also effective.

Manual pulling

Manual pulling is extremely labor intensive but can be effective if done thoroughly (i.e., remove all roots and rhizomes of blackberry). Like tillage and mechanical scalping, pulling can also leave lots of disturbed soil, increasing the risk of erosion and weed invasion.

Grazing

Grazing is not an effective means of site preparation but may be suitable in some cases once seedlings are well established.

Tillage

Tillage (including plowing, harrowing and rototilling) can clear ground and break up sod, allowing easy planting and a clean start. But unless it’s done repeatedly during the season before planting, tillage alone is generally not very effective against perennial weeds because of their ability to resprout from root fragments.

Tillage can also create several unintended consequences. It leaves the ground open and vulnerable to erosion and recolonization by other weeds. A freshly tilled field can be a sticky mess at planting. Rototilling strips (3 to 4 feet wide) can minimize some of these drawbacks.

Subsoiling, or ripping, is a special form of tillage that may be justified in cases of serious soil compaction, such as on pastures used for winter grazing. Combine tillage with mulch mats or other methods to maintain the target weed-free area after planting.

Chemical control

Herbicides are an effective, and versatile, method for controlling perennial weeds before planting. They can be applied with power-driven spray equipment or backpack sprayers to provide control across a range of terrains without soil disturbance. Use herbicides according to your needs; create weed-free patches or strips to ensure good survival, or use a broadcast application to clear the entire site. Timing is critical, and herbicide effectiveness depends on the chemical used and species targeted.

Timing

If applied when weeds are actively growing, herbicides containing glyphosate will kill common perennial grasses (e.g., reed canarygrass, bentgrass and tall fescue), annual grasses and other herbaceous vegetation. Application rates will vary depending on the weeds present. Glyphosate is a foliar-active herbicide, so it will not control weeds that emerge from seeds after treatment. Additional methods will be needed to maintain weed control in the first few seasons after planting; these could include mulch, reapplications of glyphosate or soil-active herbicides.

For blackberry and other woody species, herbicide application is most effective when plants have not been recently cut or mowed before spraying. When this is not practical, mow early in the season, and allow shoots to regrow before spraying.

Formulations

Several formulations of both glyphosate and triclopyr can control blackberry, Scotch broom and other woody species. Chemical formulation and timing greatly affect effectiveness on target weeds and sensitivity of nontarget plants. These chemicals will damage or kill the hardwood trees and shrubs commonly included in riparian plantings, which is why controlling weeds during site preparation, before planting, is so important. Use extreme care when spraying, especially near riparian areas.

Consult the Pacific Northwest weed management handbook, the Oregon Department of Forestry, your local soil and water conservation district and your local OSU Extension office for more information about formulation and timing of herbicide applications. Also see “Important information about using herbicides.”

Step 4. Plant your trees right

Well-established guidelines for tree planting in forestry and horticulture generally are applicable for riparian plantings. See The care and planting of tree seedlings on your woodland, EC 1504, and other resources in “For more information”.

When to plant

Plant bare-root seedlings during the winter dormant season, preferably when soils are above 40° F. Avoid planting during warm, dry or windy weather. Plant container seedlings (ranging from small plugs to ball-and-burlap stock) from fall through early spring. If soils are moist and warm (above 40° F), fall planting may be advantageous since seedlings’ roots may experience significant fall root growth. Plant hardwood cuttings in the dormant season from late fall to early winter.

Seedling care and handling

Once dug or “lifted” at the nursery, bare-root seedlings are vulnerable to damage during packing, transport, storage and planting. Many problems with seedling survival can be traced back to improper care and handling:

• Keep seedlings moist. Roots dry out rapidly when exposed to sun or wind. If roots appear dry, fine roots and root tips are likely damaged or dead already.
• Keep seedlings cool. When seedling temperatures exceed 42° F (and especially when temperatures remain above 50° F for more than a few hours), they begin “growing in the bag,” using energy needed for survival and growth after planting.
• Handle seedlings with care. Physical damage can result from crushing, dropping and excess vibration. Avoid tearing roots when unpacking seedlings.

Table 6 lists recommendations for seedling care and handling, focusing on bare-root stock.

Cuttings

• Depending on cutting size and planting depth, you can push cuttings into moist soil or place them in holes excavated with hand tools or machinery.
• Cuttings are less fragile than bare-root seedlings, but general rules for handling, storage, moisture and temperature apply.
• Soak willow and cottonwood cuttings in water for one to 10 days before planting.
• Plant cuttings right side up (i.e., the buds point up).
• Cuttings need to be planted deeply enough so that emerging roots can access moist soils throughout the summer drought period.

Spacing and arrangement

The planting arrangement helps shape stand development and, ultimately, riparian functions. Plant according to the design you developed during your planning process (see "Step 1"). Use a layout suited to your access needs, maintenance plan and equipment. If planting a mixture of species, be sure to accommodate differing plant growth patterns and competition among species. See “Appendix C" for more information. Remember that planting is just one step in creating a riparian woodland. Other actions, such as weed control and thinning, will likely be required to keep the planting on track and achieve your functional goals.

Step 5. Take care of the planting

Maintenance weed control

The task at this stage is to maintain the weed control achieved before planting. Without continued control, weeds will quickly return and affect seedling survival and growth. This is a common source of failure in riparian plantings.

Once the planted species begin to shade a significant portion of the ground, weed control becomes much easier. The time required to reach this stage varies. With effective weed control, conifer plantings on unirrigated, upland sites in western Oregon commonly reach this point (called “free to grow”) in four to six years. There is less research or experience for riparian plantings to guide us, so plan for ongoing weed control in your planting for at least that long.

An added challenge in postplanting vegetation control is the need to mow, trim or spray around vulnerable seedlings. If site preparation was effective, the main concern will be annual grasses and broadleaf weeds. Try to maintain the same control targets established before planting (e.g., a 3 foot by 3 foot or 4 foot by 4 foot patch or a 3- to 4-foot-wide strip).

Mowing

Mowing is popular in young plantings. With available tools ranging from tractor-mounted mowers to handheld weed trimmers, mowing can keep woody weeds under control across a range of terrains and planting sizes. Mowing alone is not a good way to control herbaceous plants and gives only modest relief from moisture competition.

Mulches

Mulches are effective for suppressing weeds around planted seedlings and retaining moisture into the summer. Various mulch materials are available.

Animal damage control

Animal damage is another common cause of plant mortality in riparian plantings. Hardwood species are particularly attractive to many animals. Carefully assess the need for protection, taking into account the food and shelter available in the area for various animal species. Physical methods of damage prevention (e.g., fences, cages and tubes) are useful, but they are also expensive and require periodic inspection and maintenance. Using large planting stock and controlling weeds to promote rapid seedling growth will help minimize many animal damage problems.

Beaver and nutria

Beaver and nutria can cause extensive damage to young seedlings. Removing these animals may be an option; however, the site will likely be repopulated from adjacent areas. Plantings quickly grow beyond nutria but may remain attractive to beaver for many years.

Protection methods include individual tree protectors (e.g., 2- or 4-inch wire mesh cages), chicken wire or sheet metal loosely wrapped around larger tree trunks, and fencing between the planting and stream.

Depending on your situation, you may be able to meet your objectives using a species not preferred by beaver, such as ninebark. Consult with local experts. Small, isolated plantings that are the only woody plants in the neighborhood are more attractive and more vulnerable and will likely require more protection than large plantings adjacent to other areas of woody vegetation.

Livestock

Livestock (e.g., cows, horses, sheep, goats and llamas) find young seedlings palatable and can heavily damage unprotected seedlings. The best way to keep livestock out of planted areas is standard woven wire or electric fencing.

Irrigation

Although all plants can benefit to some degree from irrigation, it is usually not essential. Selecting appropriate species and planting stock, using proper handling and planting techniques, and effectively controlling weeds will help ensure success in the absence of supplemental water.

Irrigation can help seedlings survive summer drought and improve seedling vigor and growth. Hardwoods are more susceptible to drought than conifers. Irrigation will be a temporary practice, so you must select species that will survive without irrigation after they are established.

Carefully evaluate the need for irrigation. If your planting is on farmland with water rights and irrigation equipment, irrigation may be feasible and effective. On other sites, irrigation can be time consuming and expensive. Irrigation is most appropriate under the following conditions:

• During the first one or two growing seasons, to help seedlings establish
• On very coarse soils (e.g., sandy or gravelly) with minimal water-holding capacity
• When planting water-loving species (e.g., alder, willow and cottonwood)
• When water rights are available

Irrigation methods include sprinklers, drip systems and hand watering. Select a method on the basis of cost and available equipment, labor and water. If you don’t have a water right to irrigate, you may have to carry water in from off site. Contact the Oregon Water Resources Department for more information.

For maximum benefit, begin watering by early summer, before plants develop high levels of moisture stress. Plan to water every 10 to 14 days, tapering off as summer progresses. Drying down at the end of the growing season helps induce dormancy and increase winter hardiness, so avoid frequent irrigation in late summer. Tree seedlings may not expand their root area much in the first growing season. They depend on nearby soil for moisture. Monitor this area. Water deeply and slowly to thoroughly wet the rooting zone.

Step 6. Monitor and learn from results

Monitoring your riparian planting allows you to complete several essential activities:

• Identify immediate maintenance needs to ensure trees survive and grow.
• Determine whether goals for tree survival and growth are being met (implementation monitoring).
• Determine whether goals for riparian functions are being met (effectiveness monitoring).
• Improve future riparian restoration efforts by learning which treatments or techniques worked, which didn’t, and why (adaptive management).
• Document project implementation and results for funding agencies.

Recordkeeping and evaluation

Before implementing the project, develop a monitoring plan that addresses these questions:

• What do you want to monitor (e.g., tree survival, shade and bank stability)?
• How will you measure or evaluate these items?
• When do key monitoring tasks need to be done?
• What is appropriate given your goals, time, skills and budget?

Good documentation is essential:

• Prepare an overall project description including location, site conditions, site preparation and maps and descriptions of planted areas to be monitored.
• Compile planting records, and note what was planted, where and how.
• Record all project activities by date in a project log.
• Note results from periodic visual inspections and other forms of monitoring.

Consider marking the locations of planted trees with surveying ribbon or wire flags. Although time consuming, this will make it easier to locate trees for needed maintenance and monitoring and help ensure that you don’t accidentally damage or cut trees.

Periodic visual inspection

This is the simplest and most important type of monitoring. The main purpose is to see how the planting is doing and to decide what, if any, corrective actions are needed to make sure trees survive and grow adequately to meet project goals. Plan for a visual inspection of the planting site at least once per season:

• Are grasses or other weeds encroaching on or threatening to overtop planted trees?
• Is there evidence of browsing, girdling or clipping of stems?
• What steps need to be taken to correct the problems?
• Are trees vigorous and healthy?
• Are trees of some species or trees in certain areas doing better than others? If so, why?

Note your observations in the project log.

Photo monitoring

A series of photos over time is a powerful tool for documenting starting conditions and changes over time. Mark photo points (reference points) with steel pins or rebar so they can be found in the future. Document the location of photo points in your project log, and indicate the direction (compass heading) the photographer is facing from the photo point. See “For more information" for additional resources on photo monitoring.

Other monitoring techniques

Most riparian monitoring will focus on project implementation: Did the planted trees survive, and are they healthy and vigorous? What problems require immediate attention? These questions can be addressed largely through periodic visual inspections.

More intensive monitoring techniques can provide quantitative information that is objective, repeatable and statistically valid. These approaches are important for evaluating project effectiveness but are expensive and time consuming.

Regardless of the technique used, it’s important to follow a consistent format for recording measurements and taking notes on project conditions.

"Appendix D" lists examples of common monitoring questions, objectives and techniques. See “For more information” for additional resources on monitoring.

Many riparian species also regenerate vegetatively, including resprouting from the root crown, root suckering, layering, and sprouting from stem fragments. This makes natural regeneration a useful but often unreliable source of recovery in years when seed production is low or matches poorly with natural disturbances, or when you want to develop woody cover farther from the active stream channel.

What can be done to enhance natural regeneration?

• Kill or cut back competing vegetation. Riparian plantings often contain more desired woody plants than first meet the eye. Locate and flag these plants before cutting.
• Stimulate root suckering, layering and sprouting by cutting back competing vegetation twice per year. Black cottonwood (Populus trichocarpa) and sandbar willow (Salix exigua) sprout vigorously from underground shoot buds on lateral roots. A temporary reprieve from competition can help these species move rapidly into previously unoccupied areas (figure 22).
• Minimize cover of competing vegetation to stimulate germination and rapid early growth of seedlings of desirable species. A large amount of seed falls into riparian areas, but new plants can regenerate only under favorable conditions. It isn’t important to eliminate competitors entirely, but you must reduce their cover to low levels while desirable species get established.
• Protect plants from livestock or other animal damage with fencing or individual tree protection.

Appendix C. Project design features and considerations

You considered functional goals (e.g., shade, bank stabilization and habitat) when planning the project and selecting species. Optimize those functions with a thoughtful layout. Consult others; staff from soil and water conservation districts, watershed councils and the Natural Resources Conservation Service generally have extensive experience with these practical design issues.

Buffer width

There is no one-size-fits-all buffer width. Wider buffers generally provide more benefits, but beyond a certain size, there may be relatively little increase in benefit for a relatively large increase in buffer width.
Optimal buffer widths vary depending on the desired riparian functions. For bank stability, relatively narrow buffers suffice. For wildlife habitat or travel corridors, wider buffers may be required (see "Appendix A").

Consider a variable-width buffer. Good places for wider buffers are the outside bank of a stream bend (so the entire buffer isn’t lost when the channel migrates) and on low terraces and low-gradient reaches (to enhance bank stability and off-channel habitat were flooding is more frequent).

On forestlands, the Oregon Forest Practice Act dictates minimum buffer widths. On agricultural lands, there are no specific buffer-width requirements; owners must comply with local water-quality management plans established under Oregon Senate Bill 1010. Contact your local soil and water conservation district or watershed council to learn about local water-quality management plans. City or county jurisdictions may have ordinances dictating buffer width.

Location

Some functions are more location-dependent than others. Vegetation on the south and west sides of the stream provides direct shade, which helps moderate water temperatures more than similar vegetation on the north side. Habitat functions (e.g., food and nesting) are not strongly related to position. Bank stability may be a key functional goal in areas subject to scour or stream meanders.

Access requirements

Provide access and adequate space between trees for any machinery to be used. Consider the benefits of rows for maintenance (e.g., mowing and spraying). Regular rows and adequate spacing between rows (5 to 10 feet) are needed for most small tractors and ATVs. Rows can look natural; they do not need to be straight lines. Sweeping curves work, too. Closer spacing and irregular tree distribution are OK if you will use smaller mowers, backpack sprayers or hand tools.

Plant spacing

Appropriate plant spacing depends on several (sometimes competing) factors:

• Tree size, stand structure and species composition needed to meet a project’s functional goals
• Species characteristics (size, growth rate, form and shade tolerance)
• Need for rapid establishment and occupancy by seedlings (increasing their ability to compete with weeds)
• How long and how well you can control competing weeds
• How quickly and to what degree seedlings compete with each other (affecting tree size, stand structure and species composition)
• Your ability to thin in a timely manner

Mixed-species plantings

Mixed-species riparian plantings generally serve two purposes:

• They provide “insurance” to cover uncertainties associated with seedling survival and species adaptation.
• They establish a diverse plant community that will provide key habitat functions in the future.

The most common approach to developing a mixed-species planting is to use a random mix of species. This serves the first purpose but not necessarily the second.

Differing early growth rates affect how species interact and compete. Trees that establish and grow quickly often do so at the expense of neighboring plants, occupying space and consuming resources needed by each. Slower-growing trees fall behind and, depending on degree of crowding and their shade tolerance, may eventually die unless the planting is thinned. In a single-species stand, this competition affects the size and number of trees surviving but not the species present. In a mixed-species planting, competition favors one species over another and tends to shift the planting toward the faster-growing species. Diversity of the planting decreases as slower-growing species die, and any functional goals associated with these species won’t be realized.

Research with a wide range of plants has demonstrated that this interaction is more pronounced the more dissimilar the growth rates among competing species. The interaction also is sensitive to plant spacing (density). The closer the spacing, the faster and more complete the displacement of the slower-growing species (figure 24a).

Strategies to allow slower-growing species (of a given shade tolerance) to survive longer include wider spacing (figure 24b) and clusters of individual species (figure 24c). Plants at the edge of a cluster of slow-growing species will be lost, but those inside are a safe distance from faster-growing competitors and can survive. Clustering is a good strategy to use when you want to plant close enough to cover planting losses and encourage rapid site capture but may be unsure of your future capacity for management (both weed control and thinning).

Consult with Oregon State University Forestry and Natural Resources Extension for more information about competition, stand development and spacing and arrangement strategies for creating diverse riparian forests.

For more information

The following publications are available through the Oregon State University Extension Service Catalog:

• Introduction to conifer release, EC 1388
• Pacific Northwest weed management handbook
• Selecting and buying quality seedlings, EC 1196
• Selecting native plant materials for restoration projects, EM 8885
• Drought-tolerant landscape plants for Western Oregon
• The care and planting of tree seedlings on your woodland, EC 1504
• Tree buffers along streams on Western Oregon farmland, EM 8895

Additional resources

• Locate the watershed council in your area.
• Anderson, M., and G. Graziano. 2002. Statewide survey of OWEB riparian and stream enhancement projects. Salem, OR: Oregon Watershed Enhancement Board.
• Crowder, W., and W. Edelen. 1996. Riparian moisture zones: Planting locations of woody and herbaceous species. Technical Note 31. Pullman, WA: USDA-NRCS Pullman Plant Materials Center.
• Falk, D.A., M.A. Palmer, and J.B. Zelder (eds.). 2006. Foundations of Restoration Ecology. Washington, D.C.: Island Press.
• Fisher, R.A. 2004. Using soil amendments to improve riparian plant survival in arid and semi-arid landscapes. Vicksburg, MS: USAE Research and Development Center, Environmental Laboratory.
• Hall, F.C. 2002. Photo point monitoring handbook, part A — Field procedures. General Technical Report PNW-GTR-526. Portland, OR: USDA Forest Service, Pacific Northwest Research Station.
• Oregon Department of Forestry. Sources of native forest nursery seedlings (annual catalog with seed zone maps).
• Oregon Watershed Enhancement Board. 2004. Oregon riparian assessment framework.
• Prichard, D. 1995. Process for assessing proper functioning condition. Technical Reference 1737-9. Denver, CO: U.S. Department of the Interior, Bureau of Land Management.
• Watershed Professionals Network. 1999. Oregon watershed assessment manual. (Developed for the Governor’s Watershed Enhancement Board).
• Winward, A.H. 2000. Monitoring the vegetation resources in riparian areas. General Technical Report RMRS-GTR-47. Fort Collins, CO: USDA Forest Service, Rocky Mountain Research Station.