Soil Rehabilitation Guidebook
[Soil Rehabilitation Guidebook Table of Contents]
Revegetation strategies to restore ecosystem function
Revegetation is a fundamental part of all rehabilitation projects. The type of revegetation techniques used depend largely on the rehabilitation objectives, which may include:
- controlling surface erosion
- increasing slope stability through the restoration of a root mat
- creating, restoring or improving soil structure
- restoring biological properties affecting soil nutrient cycling
- reducing recompaction after tillage operations
- changing water relations on site
- conserving or adding nutrients
- preventing the establishment of noxious weeds
- maintaining or achieving aesthetics
- producing a commercial forest
- restoring and providing habitat or forage for wildlife.
Determining desired characteristics of vegetative cover
Revegetation of some kind is an essential part of every rehabilitation process. Bare soil will erode, recompact (if it has been recently tilled), lose soil structure, lose nutrients and undergo invasion by weed species. A wide variety of approaches to revegetation is possible. Traditional approaches have usually involved seeding a mixture of agronomic grasses and legumes with erosion control as the primary objective. Modern rehabilitation projects have more demanding objectives, however, such as re-establishing a commercial forest. Sometimes, techniques required for one objective (such as developing complete ground cover to prevent erosion) may conflict with others (establishing a free growing crop of trees).
The guidelines below, and the discussion of various revegetation strategies that follow, show how different techniques might be used to achieve a variety of rehabilitation objectives.
- To control surface erosion, use grass and legume seed mixes as your first choice, then shrubs and hardwood species, then conifers. However, keep in mind that grass, particularly sod-forming species, may interfere with conifer establishment on some sites.
- Use vegetation with ecological characteristics that are compatible with your long-term objectives. Gain familiarity with the potential of native grasses and legumes by experimenting on sites with low erosion potential. Keep in mind that experience with native grasses and legumes is limited, and they may be more risky than agronomic seed mixes where immediate erosion control is required. Consult with an agrologist in your area for help in selecting native plants for use in rehabilitation prescriptions.
- Use grasses and legumes to restore and maintain soil structure, especially in medium- and fine-textured soils.
- To enhance development of soil with a distinct forest floor, consider using native shrubs and hardwood species. They may be successfully interplanted with conifers. Keep in mind, however, that native shrubs and hardwood trees are less effective than grasses and legumes for short-term erosion control and rapid improvement in soil structure.
- Develop or have an expert develop site-specific requirements for seeding rates, planting densities and species mixes. More intensive approaches are often required in rehabilitation work than in agriculture.
- Consider using bioengineering techniques in situations of high sensitivity or risk.
- When developing strategies for revegetation, consider other uses of the site and their possible effects on your prescription. For example, where deer populations are large, plan to control browsing by creating barriers to deer access, delaying planting until other food sources are available, or by using other techniques. Also, in areas with cattle grazing, plan and implement measures to prevent cattle from congregating on the rehabilitated area, minimize grazing use until the tree seedlings become established, or try other techniques such as obstacle planting to protect seedlings. Local experience will often be the best source of innovative solutions to problems associated with shared use of rehabilitated areas by wildlife or cattle.
- Remain flexible. There is an unlimited number of possible approaches. Be guided by the ecological considerations of the site, an analysis of the risk and your ultimate objectives.
Grass and legume seeding
Native plants, domesticated native plants and introduced agronomic species are all useful for rehabilitation.
Seeding of agronomic grass and legume species is an established technique for erosion control in British Columbia (Carr 1980, 1985). There is a very large selection of species to choose from, and seed mixes can be easily tailored to achieve particular effects.
Grass and legume seeding can also be used to meet many other revegetation objectives such as restoring soil structure, enhancing site nutrient status, hastening green-up and producing forage.
Native and domesticated grasses and legumes can be used in the same situations and applications as agronomic varieties, and in many cases, they will be better suited for use on a particular area. Many native plants may be well adapted to conditions of low nutrient status, and they likely reduce potential adverse effects on biodiversity that may arise from seeding introduced plant cultivars in forest and range ecosystems.
Choosing the proper species for a particular situation requires that the characteristics of the species be matched with the site conditions and rehabilitation objectives. Attributes such as root form, reproductive system, growth form, timing, and adaptability all affect the suitability of plants for a particular site and objective.
- For help in formulating seed mixes, consult experts (including range agrologists working for the B.C. Ministry of Forests) who are familiar with your area.
- See Appendix 2 for more information on developing grass and legume seed mixes.
Advantages of seeding grass and legumes for rehabilitation
- Seeding is inexpensive, fast, easy.
- Many different varieties are available for specific conditions or to meet a variety of objectives.
- Seeding promptly after disturbance provides an almost continuous ground cover.
- Dense root mats formed by some species are very favorable for stabilizing soil and developing soil structure.
- Deep-rooted species are highly suitable for reducing soil moisture levels where slope stability or site wetness is a problem.
- Some species establish rapidly and yet are short-lived, thereby reducing competition over the longer term.
- Many species of grasses and legumes are widely adapted.
- Some native species can be used in many situations and others (such as less palatable or low-growing species) can be developed for special uses.
Disadvantages of seeding grasses and legumes for rehabilitation
- Seeded species may affect tree growth by competing for moisture and nutrients.
- Seeded species may increase the risk of frost damage and snow press damage to tree seedlings in some situations.
- Many species currently used are not native to British Columbia, or are used outside their native range, and may have adverse effects on biodiversity when used in forest and range ecosystems.
- Where emergency revegetation is required, the effect of uncontrollable factors such as weather or seed predation on the prescription need to be considered.
- Few nitrogen-fixing legumes can be grown on acid soil (common in forests) or at high elevation.
Revegetation with native shrubs can be a valuable rehabilitation tool, particularly in highly sensitive areas such as recreation areas, alpine tundra and grasslands. Native shrubs have not received a great deal of attention for rehabilitation of forest lands in the past, but the infrastructure necessary to allow routine use of shrubs has been rapidly developing in recent years. Commercial seed-pickers are available to collect material from anywhere in British Columbia and several nurseries currently grow native species in a variety of container stock types.
Various methods can be used to establish native shrubs. Container stock can be hardy and, although it may be browsed in some areas, it can be planted in areas where seed retention and survival are problems. Many species such as alder, rose, soopolallie and juniper may be planted by direct seeding if a source of seed can be found. As well, several species can be established as rooted cuttings – and willow can be established as unrooted cuttings of stems – but mortality rates are high. Species such as saskatoon and wolf willow can be easily established as cuttings from roots.
- For help in developing rehabilitation plans involving the use of shrubs, consult with ecology and soil specialists in your area.
- See Appendix 2 for more specific information on the use of shrubs and hardwoods.
Advantages of planting shrubs for rehabilitation
- Shrubs have deep, woody root systems that give mechanical support to slopes. When planted with grass, they can help to prevent sloughing of the shallow sod layer. The woody top growth also helps to stabilize rehabilitated areas by reducing surface wind velocity.
- Shrubs establish more quickly and easily than trees, and often grow on sites not suitable for conifers.
- Available shrub species tend to be indigenous and better adapted than introduced grasses and legumes.
- Some species will fix nitrogen even in areas where legume success is unlikely.
- Shrubs may serve as nurse trees to a conifer crop, providing a source of browse to draw animals away from seedlings and protecting seedlings from frost.
- Shrubs improve soil and forest floors by drying them out, adding organic matter, and some fix nitrogen. Compared to grasses and legumes, these objectives may be achieved with fewer negative effects on conifers.
- Shrubs provide a good source of food and protective cover for wildlife.
- In forested areas, shrubs may improve visual quality by screening disturbed areas from view, and by softening the harsh lines of disturbed areas.
Disadvantages of planting shrubs for rehabilitation
- Ground cover is discontinuous and it may take many years to develop a forest floor and continuous root mat. Planting shrubs is not the best choice for short-term erosion control.
- There is no direct return on the cost of planting, though there may be indirect returns.
- Shrubs will compete with conifers, but in well-designed plantings there may actually be a net growth benefit to conifers.
- Some shrub species may be severely checked by heavy browsing.
Planting hardwood trees
Much of the previous discussion on native shrubs applies to hardwood trees as well. Hardwood planting is gaining more acceptance in British Columbia as markets expand for the common species. Investigation into mixed planting has indicated that, in certain situations, these techniques may confer some growth and performance advantages to the conifer component of a mixed planting.
Advantages of planting hardwood trees for rehabilitation
- Hardwood species are often natural pioneers of disturbed sites. They can establish easily, produce large quantities of leaf litter which helps rebuild the forest floor, stimulate plant and microbial activity and re-establish soil ecosystem functions. Their roots help improve soil physical and chemical properties. Red alder has the additional benefit of being able to fix nitrogen.
- Hardwood trees grow fast, which helps "green up" disturbed areas and enhance visual quality.
- A hardwood cover on rehabilitated areas within a cutblock may contribute to the achievement of biodiversity and wildlife habitat objectives.
- Depending on the stocking levels, hardwoods may act as a nurse crop and improve conifer growth by moderating temperatures and protecting softwood regeneration from browsing and windthrow.
- Shorter rotations for most hardwood species allow more flexibility in the timing of rehabilitation. For example, on blocks requiring access for some time after planting, rehabilitation of roads and trails could be delayed without losing the opportunity to produce a merchantable hardwood crop.
Disadvantages of planting hardwood trees for rehabilitation
- Returns on planting cost investment may be lower than for other species, though that may change. There continues to be psychological resistance to planting hardwoods.
- Planting material for hardwoods is not as readily available as for softwoods.
- At high densities in mixed planting, hardwood trees will compete with conifers.
- Hardwood trees may be affected by herbicides used on other parts of the block.
- Birch is often browsed by moose, which may reduce stocking success.
Planting conifers for rehabilitation requires more flexibility and perhaps more stringent planting practices than would be used in planting most cutover areas. For example, consider planting at higher than maximum stocking densities where soil conservation or risk of increased seedling mortality are issues; in rutted areas, take more care to select good microsites for planting; consider larger than normal stock where the need for more robust seedlings is indicated, and species requirements may change as a result of disturbance.
Advantages of planting conifers for rehabilitation
- Planting conifers offers the most direct route to a forest of anticipated commercial value.
- Stock and seed are readily available.
- Seed-rich debris may serve as both seed source and mulch.
- Rapid growth of conifers mitigates visual impacts associated with soil disturbances.
- Conifers have greater root strength for slope stabilization than grasses or legumes.
Disadvantages of planting conifers for rehabilitation
- Above- and below-ground growth is slow compared to that of some other types of vegetation.
- Forest floors and site-nutrient pools are restored more slowly than with other vegetation types.
- Root systems are coarse compared to those of some other types of vegetation and are therefore not the best for controlling erosion or restoring soil structure.
- Commercial planting densities are often too low to provide optimum rehabilitation effects such as erosion control or nutrient capture.
Bioengineering in rehabilitation refers to the use of living plants to create structures, usually to control erosion or stabilize slopes. Bioengineering techniques involve the very intensive use of relatively large pieces of living material. The living material is used in such quantity that it helps provide slope stability even before it begins to grow. As the living material grows, the benefits of revegetation and slope stabilization are achieved in one action.
Bioengineering can be used to stabilize existing slopes or to help reshape slopes to more stable forms. Small terraces, for example, can be created to trap sediments and dissipate the energy of running water. Bioengineering techniques may be useful where:
- slopes are very active and seed loss is likely or damage is imminent
- there is high risk of damage and significant public concern.
The major categories of bioengineering techniques include (from Beese et al. 1994):
- Live staking: Inserting, driving or burying individual cuttings in random, grid or linear patterns to immediately stabilize eroding or slumping slopes
- Wattles (sticks interwoven into fences) and fascines (bundles of sticks): Staking or burying fences or bundles of interwoven live branches in rows or shallow trenches, either parallel or diagonal to the slope contours, to create relatively large structures to trap sediments, slow water movement and ultimately revegetate slopes
- Cordons, hedges and brush layers: Constructing terraces or trenches, either parallel or diagonal to the slope contours, for hedge-like plantings of live cuttings or rooted trees or shrubs, to stabilize loose slopes.
For help in applying these techniques, consult an expert in bioengineering.
Advantages of bioengineering techniques for rehabilitation
- Bioengineering techniques provide immediate results for erosion control and slope stabilization. This is important for sensitive or risky situations.
- The approach is very flexible. Many effects can be achieved by varying techniques and design.
- The techniques can be successful where less intensive approaches are likely to fail, such as on steep slopes where surficial materials are unstable.
Disadvantages of bioengineering techniques for rehabilitation
- The techniques are very labor intensive and expensive, and are best used only in critical areas.
- The techniques can only be implemented where there is an available source of suitable material.
- Specialized knowledge is required to implement the techniques. Relatively few people are familiar with them, compared to other revegetation techniques.
Cattle grazing and rehabilitation
In some situations, cattle grazing may interfere with the prescribed goals of establishing conifer seedlings and maintaining vigorous vegetation cover. Agronomic seed mixes are often very palatable and may attract cattle to the rehabilitated area, resulting in trampling damage to planted tree seedlings. Vigorous vegetation cover is required to rebuild soil structure. High rates of vegetation removal by cattle may seriously reduce the effectiveness of this treatment. It is therefore important on blocks with grazing to consider how best to manage cattle.
Cattle management is carried out within the broad strategies established under a range use plan for the range agreement holder, usually a local rancher. Changes in cattle management and the range use plan require the cooperation of the rancher and consultation with local agrologists. Rehabilitation plans must include provisions to ensure that the accumulated effect of cattle damage does not reduce conifer stocking to less than acceptable levels.
To prevent cattle from congregating on rehabilitated areas, consider such practices as:
- deferring use until late in the season
- allowing for only short periods of grazing on blocks with rehabilitated areas
- locating salt blocks to encourage cattle to congregate away from rehabilitated areas
- leaving an irregular ground surface, pulling logging debris onto the roads or landings, or otherwise making it difficult for cattle to move around on the rehabilitated areas
- obstacle planting
- portable electric fencing
- using seed mixes or native plants that are low-growing or unpalatable to cattle
- regular herding of the cattle (using horses or other means) to improve grazing distribution
- seed distribution along roads, landings and cutblocks to disperse cattle into other areas
- avoiding configurations at the layout stage that concentrate cattle on landings and skid trails. For example, avoid coalescing disc-trenches, windrows, or debris piles that direct cattle onto the rehabilitated area.