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Riparian Restoration

CHAPTER 5: RESTORATION TECHNIQUES

This chapter provides practical approaches for workers involved in restoration.

Soil

Soil condition is a critical concern in restoration. Soil health is particularly important for success because soil compaction and soil characteristics affect soil permeability and plant vigor. Compaction is site specific. Certain soils, such as sand, might show shallow compaction, while others, such as clay and silt, could compact to 2 feet. Workers should test soil composition and make borings to check for a clay lens (hardpan), for instance. Because soil types vary across a site, workers should take more than one test.

To test easily for soil compaction, workers can use a regular shovel with a rounded edge. Workers should be able to dig easily in moist soil. If, when stepping on the shovel, it slides into the soil, this generally indicates that the soil is not compacted. If the shovel bounces off the soil or the effort requires repeated jumping on the shovel that just chips away at the surface, the soil is compacted. It is important to do this at multiple site locations.

Workers can determine where the compaction stops and what the soil horizons look like by digging a hole at least 18 inches deep. A sharpshooter spade (18 inches long and 4 to 6 inches wide) will penetrate the soil well; another option is using mechanized equipment. After digging the hole, a worker would lie on the ground, reach into the hole, and try to push a knife blade into the dirt down the side of the hole. Eventually, the knife should slide in with little or no effort, indicating where the compaction stops (Mueller 2000). Soil horizons should be visible. Workers should observe layers of deposition and should consult a soil scientist to determine whether the topsoil, or horizon A, is intact and compacted or absent, and whether subsoils B or C are exposed.

Workers should check the soil for the presence of living organisms, which show that the soil is not sterile (Soil and Water 2000). Affected soils may have high salinity, which may reduce riparian plant vigor. Irrigation-caused leaching, agricultural drainage, and dams can cause salt accumulation due to changes in flow patterns. Workers should test the site’s soil pH to determine whether it will support the site’s native riparian plants. They should remove contaminated soil.

In complex projects, the team should invest in a site-specific soil survey. The soil survey will characterize the soil and give the chemical properties, the amount of productive elements that might be lacking for plant growth, the physical properties for engineering use in the field, and the water-holding capacities. Forest Service soil scientists can do this work.

Soil compaction can be reversed by providing a way for water and air to saturate the soil. A soil scientist can help decide which of the following decompacting techniques are appropriate to the site:

Shovel. If an area is small and not very compacted, a shovel is effective. In general, if the topsoil is deep enough (within the same soil horizon), the soil can be turned over. If the topsoil layer is thin, the soil should not be turned over. In Yosemite National Park, shovels were used to decompact social trails and areas with well-established vegetation (trees and large shrubs). Shovels were driven into the ground and moved back and forth to loosen and decompact the soil without disturbing vegetation (roots) (Cunningham-Summerfield 2000). The soil was not turned over, keeping the soil profile intact and leaving existing vegetation undisturbed.

Long bar. A 5- to 6-foot-long metal bar with a footrest punches holes in the ground to increase water infiltration through piping. (It also can create holes for plant cuttings.)

Auger. An auger is used in spring or fall to drill 6- to 12-inch-deep holes, depending on the depth of the compaction, at 1-foot intervals. The holes allow water to pipe or drain into the soil. For example, meadows in the Great Basin have been aerated using a ¾ inch power ship auger with a total bit length of 15 inches (Chambers 1999).

Excavator and backhoe. The teeth on the bucket of an excavator and backhoe can rip soil 4- to 6-inches deep, or deeper. Unlike other excavators, the Gradall excavator has a bucket that can be rotated in a wrist-like motion and a telescoping boom that can rotate 360 degrees.

Dozer with ripper. A dozer or tractor with a ripper attachment treats large areas. The tines are 12 to 18 inches long. Work from the tree dripline away from the tree and rip approximately 50 percent of the ground surface (Mueller 2000). The attachment is mounted on a three-point lift hitch or a hydraulic system that can be connected into it. A Bobcat also can pull tines.

A subsoiler, a ripper with wing-like blades, can be attached to the dozer to fracture soil strata at depths up to 36 inches. The subsoiler lifts the soil, breaking through the hardpan, “without burying the forest floor or topsoil or bringing unfavorable subsoils to the surface” (Gov. of British Columbia 1997). Using the subsoiler allows moisture to infiltrate to a deeper depth. The subsoiler should be used in the dry season when there is less plant growth. This instrument is appropriate for pastures, some roads, and other wide-open spaces. Rippers or subsoilers may be used in rocky soils, depending on the size and characteristics of the rocks; it does not work with boulders, but does work with large round rocks. See figure 72.

Drawing of a subsoiler.
Figure 72—Subsoiler. (Courtesy of Bigham Brothers, Inc., TX)

The ripper or subsoiler requires multiple passes to rip an open area such as a former parking lot or open untreed compacted areas. According to Mueller, Natural Resources Conservation Service (NRCS), four passes will yield better results. Approach the ripping area from a different direction each time, with the last pass being across the slope to create rough horizontal rows to catch runoff and precipitation (Mueller 2000). It is important to decompact areas that may have been compacted during the recontouring process.

When decompaction is not enough to revive the soil to sustain healthy plants, workers should till in soil amendments or green waste. They should not amend the soils to change the soil type, but rather to rebuild nutrients and the microorganisms that help sustain healthy soils.

Workers can bring in topsoil, but it is expensive. A Forest Service soil scientist, NRCS specialist, or a local nursery employee can provide information. In areas that are totally denuded, grasses or certain other soil-building plants can rebuild the soil. This is a long-term process.

Hydrology

It is extremely important for workers to reconnect the site as much as possible to its natural hydrology at ground and subsurface levels. Soil temperature and water availability for plants are notably affected by depth of the water table and by growing season conditions such as variable annual rainfall and air temperature. Regrade the site to restore the topography and reconnect contours so natural drainage occurs. The soil should be left rough and not bladed smooth; the roughness slows runoff and enables nooks and crannies (microcatchments) to foster water infiltration and seed germination.

Riparian Vegetation Recovery

The following are three approaches to riparian vegetation recovery:

  1. Remove the persistent degrading agent, humans, goats, sheep, cattle, llamas, geese, and exotic plants, and allow the area to recover on its own. Riparian plants evolved with frequent natural disturbances; this resilience allows them to recover when imposed uses are curtailed. A passive restoration technique is probably most effective in climates and soil conditions that have an inherent resiliency; for example, when the soil is relatively healthy and in wetter climates. In arid climates, recovery will take longer.

  2. Plant a nurse crop of an early successional collection of herbaceous plants and subshrubs (Rieger 2000).

  3. Plant a significant amount of the species found in the transects. See figure 73. Plant seeds, cuttings, and rooted stock of the same species; if the seed does not sprout, the plants and cuttings may grow or vice versa. Seeds and rooted stock/cuttings used together provide stability and permanence. If species are dioecious, use a ratio of one male to five female plants/seeds (this may vary with the species). [Both genders are planted to achieve cross-pollination, and more females are planted for increased seed generation.] (Evans, as seen in Griggs and Stanley 2000). Also, a mixture of seeds and rooted stock is less expensive than rooted stock alone. Note that moisture availability, soil health, and a microclimate specific for certain plants can be limiting factors to success (Zabinski and Cole 2000).

Photo of a densely vegetated riparian ecosystem.
Figure 73—A restored riparian ecosystem in San Diego.

Evans suggests that the “life strategy” of a plant (or its place in ecosystem development) helps determine what form to plant it in; for example, plant an annual from seed. His plant categories are as follows:

  1. Soil Builders—Early successional (mostly fast annuals from seed)

  2. Opportunistic pioneers—Early successional (mostly herbaceous perennials and semiwoody subshrubs). Plant annuals from seed; plant perennials from seed and from containers.

  3. Climax community—Late successional (mostly permanent woody shrubs and trees). Plant rooted stock or cuttings. (Griggs and Stanley 2000).

The combination of seeds with cuttings and rooted stock is good for flower and seed production, wildlife habitat, and microclimate development. Rooted stock and cuttings quickly provide shade, shelter, moisture retention, and leaf mulch (a microclimate) for seeds and smaller plants that might need these conditions to survive. Plants that require shade should be planted at a later date, if necessary, although they may come in on their own once site conditions are favorable. Seeds that germinate, whether they mature or not, will provide soil-building nutrients.

Workers should always pay attention to how the plant variety and density varied in the reference site. Over time, soil variability and natural mortality will create gaps (Rieger 2000).

Stimulate the Growth of Native Plants

In order to grow, all plants need “live” soil; it is as important as water. Fungi, bacteria, arthropods, protozoa, and nematodes form a soil food web. Many of these organisms are missing from severely disturbed soils. Mycorrhizal fungi, for example, form associations with plant roots that remarkably enhance the plant’s uptake of nutrients. Different plants like different fungi (Soil and Water 2000). Plants can be inoculated with the fungi before they are planted. Although products are available for such inoculations, this field of study is relatively young.

One way to amend soils and give plants a good start is to use leaf litter, duff, or soil from an adjacent healthy riparian area or the reference site. When gathering media, workers should be as specific as possible by taking leaf litter, duff, and soil from the same plant-community type that they are trying to reestablish. They should add the medium to the hole that is dug for a plant or till it into the soil.

It is important not to fertilize. In addition to stifling growth of beneficial nematodes, fertilizers add a “blast” of nitrogen that stimulates weed growth more than native plant growth. To achieve a slow release of nutrients, workers should use green waste (compost created from yard waste within a local municipality) or native duff.

Weed Control

Generally, weeds outcompete plants by using the soil moisture (Lardner 1999). One of the objectives in weeding is to shift the balance to more native plants and fewer weeds, allowing the native plants a chance to grow and take over. Mulching is a good way to suppress weeds without resorting to herbicides, although some weed growth always will occur while the plants are becoming established. Such weed growth can hide the new plants from browsing animals.

Workers should rid the soil of weeds before work begins on the site. If there are only a few weeds, they should dig them out; otherwise, they should use the herbicide Rodeo® where appropriate. Rodeo® breaks down faster than other herbicides and has been approved by the U.S. Environmental Protection Agency (EPA) for use near water; Roundup® has not. Weeding should begin (when no crust is present on the soil) as soon as weeds appear. It is important to find the source, if possible, and eradicate it. Workers should know when weeds will drop their seeds and avoid disturbing them during this period. Disturbance may cause seeds to disperse farther than usual. Before the seedpods have opened, workers should cut off seed heads and place them in a plastic bag. The State weed abatement office can advise on destroying weed seeds.

Soils that have a crust layer are also important for weed control. If a crust exists, it is important not to pull the weeds out; this will break up the crust and make it easier for weed seeds to sprout. The weeds can be killed by cutting them off at the ground or by applying an herbicide (where permitted). The Bradley Method of eliminating exotic plants advocates pulling errant weeds and those on the edges of a mixture of native and exotic plants (Bradley 1971). It is labor intensive. This proven technique is outlined in appendix D.

When, What, and How To Plant

This section addresses climate considerations and approaches to planting seeds, transplants, cuttings, and rooted stock.

Local weather conditions dictate when to plant. Consult a botanist, a soil scientist, a native plant horticulturalist or local horticulturalist (nurseryman), a landscape architect, or a NRCS employee for the best times to plant. Geographic location and soils will dictate planting season and whether or not irrigation is needed. In some places, cold weather is a limitation. If planting is done in early spring, roots have time to become established before winter. On the other hand, drought or dry summers and mild winters may necessitate fall planting when there is a better chance for precipitation. Manci (1989), for example, reports, “Winter is the best time to plant in desert riparian areas due to low evaporation rates and thus greater saturation of soil from surface to water table.” Ask for local advice about when to plant.

To preserve genetic integrity workers should gather seed and plant material from an adjacent site, or at least from the same watershed. If seeds or plants are purchased from a nursery, the nursery should maintain records that show where the materials came from to ensure a genetic match. If a nursery is hired to do the collection, workers must tell nursery personnel where to collect what species.

Restoration companies will gather seeds and cuttings and grow plants for projects and/or help set up a nursery to provide an ongoing stock of the genetically correct plant materials. Where plant material is in short supply, avoid repeatedly taking cuttings because it will negatively affect the structure and function of an area. If workers need an ongoing supply of cuttings, they can establish a nursery or orchard of specific species for harvesting purposes. Specifics for harvesting, storing, and planting cuttings are in appendix B.

Spacing for rooted stock, cuttings, and seed broadcast amounts vary with species and planting densities desired. Placing cuttings and rooted stock too close together can cause too much competition, which leads to plant mortality. When possible, workers should plant trees instead of shrubs and vines because they generally form the dominant element of the riparian ecosystem (Stanley 2000).

Workers can use a waterjet stinger or power auger to bore a hole to the dry-season water table before inserting the plant cuttings. The waterjet stinger does more than create a hole for the cutting. It saturates the soil the length of the hole, and “liquefied soil settles around the cutting eliminating air pockets around the root zone that prevent root growth” (Hoag and others 2001). Details about the waterjet stinger and its uses are in appendix C.

Planting Considerations

Seed

Seeds have a higher degree of failure than do rooted stock. Workers should—

Image of a rocky area that is densely populated with young plants and tall grasses.
Figure 74—Seeds were used to begin restoring riparian
vegetation on this flood plain.

Drawing of three seed pods on a stem that has been stuck into the ground.
Figure 75—Seed pods on a stem.

Transplanting

Workers should—

Cuttings

Workers should—

Photo of a small tree stump with a red ribbon tied around it.
Figure 76—Live cutting.

Cuttings can be inserted horizontally into channel and lake banks for stability.

Rooted stock

Rooted stock has a higher degree of success than do seeds. Nurseries may have rooted stock from the project area. They or the workers can collect native cuttings and/or seeds and grow the stock. See figure 77. It may take 2 years or more for plants to produce enough woody growth to survive in the wild. Workers should—

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