Revegetating with Native Plants at Drastically Disturbed Sites
Revegetating with native species requires functional understanding of how these plants grow, which is generally different from exotic species. Native topsoil retention is usually one of the fundamental keys to being successful with native seeding efforts. At drastically disturbed sites, the potential for success is generally reduced given the relative absence of topsoil, unless a carefully balanced program is followed.
Baseline Information
Analysis of accurate baseline information helps guide decisions concerning constraints and opportunities for naturalizing a revegetation site. The end result of this data collection process is the delineation of naturalized landscapes to be created, restored, or enhanced. A site survey should be conducted that includes the biota and an analysis of the light, moisture, soils, slope, altitude, wind, and, if applicable, microclimate. This information is then used to select appropriate ecological communities as landscape models. The plant and animal data are especially useful to help identify the potential of existing communities for preservation, restoration, or enhancement.
Baseline information forms the framework of the naturalization plan. It is usually presented on a base map, which provides a visual record of the plan. Some types of baseline information to be included on the base map are such features as property lines, existing trees, shrub borders, meadows, water bodies, and other natural features; paved and unpaved roads; and permanent structures, with utility and service corridors noted. Symbols may be used to identify orientation, seasonal wind patterns, slope, aspect, and solar aspect coverage. Information on existing local regulations concerning vegetation, setbacks, and other restrictions should be included. A series of GIS computer overlays can be used to easily record physical, biotic, and visual survey data.
Creating a Site-Specific Revegetation Design
Each site is unique, and understanding the fundamentals of the subject area and the necessary requirements for successful revegetation is the first step. Until a satisfactory and realistic plan is prepared, the revegetation effort has only a random chance of success. Working closely with a botanist or knowledgeable plant ecologist is generally advisable to prepare a site-specific revegetation plan. The major components of such a design usually include the following:
Site-description predisturbance. Identify indigenous vegetation if possible. Include percentage tree, shrub, forb, perennial, and annual grasses. Pay special attention to early native seral stage species that naturally occupy disturbed sites. Identify concentrations of noxious weeds present in surrounding areas.
Characterize soil. Obtain preconstruction soil samples for analysis to establish topsoil baseline information. Be prepared to collect postdisturbance soil samples for analysis to identify the actual growth medium, especially if topsoil will not be returned to the site.
Catalog organic layer. Include depth, texture, color, consistency, and vegetative litter.
Microorganism. Evaluate for presence of microphytes, mycorrhizae, and other soil organisms in soil predisturbance.
Exposure. Classify exposure: full sun, partial sun, full shade. Identify type of canopy cover: closed, broken or open, coniferous or deciduous, height.
Climate. Identify climate, temperature, average, highs, lows, and corresponding dates. Include annual percentage sunshine, estimated annual average precipitation, and rainfall periods.
Aspect. Distinguish aspect–i.e., north, south, east, west-and subcombinations.
Slope. Categorize postconstruction slope, percent low, high, average, and aspect. Ascertain hydrologic potential of drainage areas above slopes that might provide the source for water erosion.
Primary objective. Select purpose of revegetation: aesthetic, habitat restoration, screening, erosion control, filter strip, soil stabilization, etc.
Secondary objective. Define subpurpose areas and categorize.
Attributes. Establish optimal plant characteristics and attributes: mixed rooting depth to control erosion and help dispense with water, nonpalatability to wildlife, screening, filter sediment, color and texture as design/aesthetic elements, rhizomatous, tolerant of inundation, drought tolerance, etc.
Criteria. Establish general design considerations: i.e., priority areas, wetland areas, wildlife mitigation sub areas, watercourse, and riparian plantings.
Hydrology. Identify function and type of hydrology to ascertain wetland characteristics.
The organic-matter content of a soil is generally considered to be one of the most important factors relating to a productive soil. Organic matter performs two major functions in the soil. One is to improve soil structure for better soil aeration and moisture movement, and the other is as a source of microorganic and plant nutrients. The organic content of any soil is governed primarily by the amount of living material added and the rate that it decays. Regardless of different climatic areas, other factors-such as drainage, soil texture, and kind of organic matter added-influence the amount of soil organic matter. Under poor drainage or fine-textured soils we should expect higher soil-organic matter content than where good drainage or coarse-textured soils predominate.
Soils at some disturbed sites have below-average soil organic-matter levels. Alternatives are to add high carbon-lignin material, such as livestock manure composts. These compost products also have stable levels of soil nutrients required for plant establishment, and the nutrient is in a chemically and biologically complex form beneficial to microorganisms required for native species growth.
Legumes should be used primarily for immediate improvement of soil structure for soil aeration and water movement. Many legumes will naturally vanish from a planting site following completion of this function. Generally, nitrogen fixing and modification of soil structure can be achieved using legumes, and regionally native species are preferable to introduced varieties. Some introduced legumes, such as alfalfa, might persist in an ecosystem long after the goal for its introduction has been achieved. Thus, if at all practical, use of indigenous species is encouraged.
Mycorrhizal Associations in Soils
Mycorrhizal fungi that allow plants to survive the stresses of extreme temperatures, drought, and soil infertility colonize soils in significant functional ways. Soils in regions with low rainfall tend to be low in organic matter, low in available phosphorous and nitrogen. Mycorrhizal associations (fungal colonies) are present in the root systems of most indigenous plant species on arid and semiarid lands, as well as higher rainfall areas with coniferous forests. The role of mycorrhizae in this habitat appears related to their capacity to acquire nutrient resources for plants. Beneficial mycorrhizae solubilize mineral elements such as phosphorous for uptake by plant roots. The plant roots utilize the fungus that surrounds them. The root systems of these plants grow in a sheath of mycorrhizae (fungus) in symbiosis between plant and fungus. That is, energy in the form of organic carbon compounds moves primarily from fungus to plant, and inorganic resources (primarily phosphate) move from mineral to the fungus.
Nutrient cycling is, in large part, controlled by bacteria and the relative growth rates of the active fractions of the bacterial biomass, including root algae. Loss of significant portions of bacterial biomass, or loss of certain nitrogen fixers or nitrifying bacteria, severely limit the productivity of a site. The thin lens of topsoil common to the earth’s crust is in fact rich in bacterial biomass and mycorrhizae. Loss of this thin lens of topsoil during road construction or other disturbance makes it difficult to reestablish vegetation on the exposed subsoil. For this reason, cut-and-fill slopes exposed during construction will generally not fully revegetate and thus remain sparsely populated with vegetation. Rebuilding the lens of biomass necessary for successful vegetation to establish can take many decades. Though the airborne spores of indigenous mycorrhizae may be present, absence of hospitable conditions for reestablishing biomass makes it difficult for conditions to improve.
Revegetation of Road Cuts to Minimize Erosion
Reestablishing vegetative cover on the cut-and-fill slopes along roadsides mitigates for some of the soil particulate eroding from transportation corridors. Accomplishing this objective presents notable challenges given the conditions of the growth medium, the steep angle of hillside slopes, the absence of biomass, and the lack of suitable quantities of soil mycorrhizae and soil microorganisms. Notwithstanding, successful revegetation experiences at roadsides with disturbed soils offer instructive evidence of the potential revegetation success obtainable with a well-orchestrated plan. Seeding early seral-stage plants with a demonstrated propensity toward building soils aids in the establishment of soil microorganisms and mycorrhizae. Most importantly, this group of plant species does grow and establish where others might not succeed.
Advances in soils research and technology have led to development of organic soil amendments that stimulate the development of microflora in soil, most notably mycorrhizae. Use of such additives in revegetation efforts greatly speeds the establishment of indigenous vegetation at difficult sites and can literally make the difference between success and failure.
On areas subject to impacts of snowplowing and/or sheetflow erosion potential, it is recommended that an organically based semipermeable polymer soil-stabilizing compound be applied to prevent the mulch and seed from eroding. This semipermeable membrane should be flexible and water-insoluble and form a porous membrane in the topmost layer of soil that is permeable to rain and oxygen and won’t impair vegetative growth. This product should provide surface stabilization in various soil classifications without inhibiting water infiltration.
Soil Ecology: Microbial Activity
In general, the organic content of a soil is an indicator of its fertility, ability to support microbial populations, retention of elements, and water retention. This is an important issue, especially given the symbiotic relationship of species native to mycorrhizal fungus and other soil organisms.
Soil organisms perform many processes, often with varying degrees of redundancy. In healthy soil, there are usually several organisms that perform any particular process, and in highly disturbed soils these organisms might be lacking or sufficiently impacted to dramatically limit the success of revegetation. Unless topsoil is returned to the disturbed site, compost or organic mulches typically must be used so that early seral-stage plant life can be reestablished.Disturbed soils, especially soils that have been mined, generally have low diversity and, depending on the site, might be relatively depauperate and poorly colonized with indigenous microbial and fungal populations. To mitigate for this, one must initiate the soil-building process by importing organic compounds. One of the questions that land managers face is what commercially available products function in this capacity on badly degraded sites. Products that supply microorganisms and enzymes to activate soil microbial activity and improve soil fungal development are of necessary importance. Humic acid activates soil microbes, improves its physical properties, and aids in water retention. In addition, cytokinin, an organic growth hormone, serves to stimulate root growth and the development of essential microorganisms.
Mycorrhizae and Land Disturbance
One of the most successful land management methods for disturbed sites has been the retention of topsoil to reestablish mycorrhizae. Concurrent reclamation techniques respread stockpiled topsoil to reestablish mycorrhizae, followed by reseeding the desired early seral-stage species.
It should be noted that use of inorganic fertilizers, especially those containing superphosphate, should be discouraged since they can drastically inhibit mycorrhizae formation. This important point regarding fertilizer can not be overemphasized. Use of fertilizer with native plantings will not further establishment objectives; rather, it will result in weeds and non-native species occupying the site in questions.
By contrast, even the addition of relatively small amounts of topsoil (1-2 in. deep) to a site results in improved mycorrhizal infection and subsequent establishment of indigenous grass, shrub, and forb species-species superior for erosion control and stabilization.
In the absence of topsoil reclamation on road and bridge construction projects, the use of soil amendments that facilitate initial plant establishment and mycorrhizal development appears to be necessary. Mycorrhizal inoculation products, roots dips, and similar applications have proven consistently unsuccessful in the American West for reasons having to do with the site-specific nature of indigenous soil fungi in this region. Soil microorganisms in the West are not generic in nature; they are adapted to the specifics of given soils chemistry, plant-cover type, aspect, elevation, and climate.
Transplanting native and introduced species as container stock at roadside construction and other construction projects is common, and often met with failure. Frequently such transplants struggle and do not flourish for various reasons. A common reason is dysfunctional root systems missing necessary indigenous bacteria, mycorrhizae, and associated microorganisms to obtain adequate moisture and nutrients. Nutrient cycling is in large part controlled by bacteria and the relative growth rates of the active fractions of the bacterial biomass, including root algae. Loss of significant portions of bacterial biomass, or loss of certain nitrogen fixers or nitrifying bacteria, severely limit the productivity of a site.
Other problems include construction damage, reduced root volume, soil compaction, low organic matter and fertility, adverse soil pH, and competition from grasses. In these cases, colonization of plant roots by mycorrhizal fungi and the introduction of beneficial bacteria to the soil would greatly reduce plant mortality. Adding elemental fertilizer will not achieve these objectives. Use of appropriate, nutrient-rich organic compost and biostimulants will, by contrast, greatly assist in the establishment of container stock planted at disturbed sites. Avoid using wood-based compost products that functionally tie up nitrogen and nitrogen-related microorganisms.
Reestablishment of native vegetation is greatly expedited through direct seeding. When it is possible to minimize the size of disturbance, advantage can be taken of the seed dispersal from vegetation at the edge of the disturbance. Topsoil should be salvaged and replaced as quickly as possible, thus utilizing a source of plant and seed materials. These techniques by themselves are generally insufficient to rapidly establish a vegetation cover; therefore, augmentation by direct seeding is usually required.
Considerable progress has been made in direct seeding in the past decade. Much emphasis has been placed on introduced grasses and legumes, however. Seeds of these life forms generally germinate readily, have limited scarification or germination requirements, and are composed of relatively clean seed; thus, they are easier to meter and to seed. Seed of native species has tremendous variability in size and shape and is often difficult to use with standard seeding equipment. Varieties of introduced grasses and legumes have also been selected for aggressiveness, particularly in the seedling stage. These varieties successfully compete with other seeded species and even native vegetation causing competitive exclusion of some desirable species.
The seeding process is perhaps the single most important component of a revegetation plan to establish native plants on roadway slopes following disturbance.
Seeding methods can be divided into three general categories: drill seeding, broadcast seeding, and hydroseeding. Selection of the appropriate seeding method depends on site accessibility and terrain, seedbed characteristics, time of seeding, and species characteristics and variability within a mixture.
Drill seeding is limited to slopes of 3:1 or flatter and areas that are not extremely rocky. The “rangeland drill” is often the most effective machines for reclamation drill seeding if the soil is rocky or contains other large debris. The rangeland drill is a heavy-duty drill with a high-clearance, reinforced frame and disk furrow openers that are independently suspended. The furrows are covered with drag chains or pipe drags. The disks can be equipped with different-size depth bands to control furrow depth. Multiple seed boxes can be used for metering different-size seeds, planting at differing depths. The drill should be capable of handling fluffy or trashy seed. This is usually accomplished by mechanical “pickers” (revolving teeth) that clean the delivery path within the bottom of the seed box.
Drill seeding improves seed coverage, allows reduced seeding rates, provides accurate seed metering and calibration, and can be used to seed into difficult and uneven, rocky terrain. Rows might be aesthetically unappealing, however, and can result in increased competition from the concentration of seeds in the row. Drill seeding, if not handled by an experienced operator, can also result in some extremely small seed being planted too deep. Optimum seeding depth sometimes varies widely among native plant species because larger-seeded species often require a deeper planting depth than small seeds. A properly adjusted rangeland drill can accommodate the seeding requirements of a variety of differing species types.
Broadcast seeding is accomplished with a spinner plate or propeller device attached under a seed container that scatters or throws seeds in all directions as a vehicle passes over the area to be seeded. Broadcast seeding is generally required on slopes steeper than 3:1, on extremely rocky sites, or on remote or inaccessible sites where seed is small in size or trashy and in areas where the appearance of drill rows are undesirable. Broadcast seeding requires raking, chaining, or harrowing to ensure seed coverage where possible. Broadcasting normally requires higher seeding rates (1.5-2 times) and results in less efficient use of the seed. It is difficult to precisely calibrate seeding rates. Germination and seedling establishment is somewhat slower, although diversity is generally higher than in drill-seeded areas. Broadcasting into a rough seedbed, followed by harrowing, can result in a variable range of seed placement depths and may allow better establishment of small-seeded species. It is common to have a tractor with a mounted broadcast seeder tow a “furst” or “pasture” harrow to cover seed, provided debris is not present to clog harrow.
The benefits of broadcast and drill seeding can be obtained through a combination of the two methods using a seeder/cultipacker. This basically consists of a seedbox mounted above and between tandem cultipackers consisting of metal wheels with a shallow groove around the perimeter. The first roller firms the seedbed and makes shallow grooves. The second roller splits the ridges left by the front roller, covers the seed, and presses soil around it. This type of seeder can be used instead of the rangeland drill, but only on well-prepared areas where the surface is relatively free of rocks. It usually results in superior control of seed depth and avoids or lessens the appearance of distinct drill rows.
Hydroseeding is a modified form of broadcast seeding that involves using a pressurized spray of water containing seeds and other materials such as mulch and tackifier. Some seeds might become damaged from the mechanical action of the hydroseeding machine. Hydroseeding is somewhat more expensive and is dependent on local water supplies. It might be the only alternative for steep, inaccessible slopes. Generally, the freeze-thaw cycle helps incorporate surface applied seed into the soil. Hydroseeding is a suitable technique for areas with adequate and dependable moisture during the germination period. When used with appropriate tackifier and soil bonding products, it can provide unequaled seeding capability at difficult sites.
Seeding of disturbed areas should begin during the first available seeding “window.” A seeding window is that period of time most suitable for seeding and offers the best potential for seeding success. The factors that contribute to the determination of the seeding window can include:
- seeding prior to a period of adequate moisture for seed germination,
- seeding prior to an extended period of adequate moisture for early seedling growth and establishment,
- seeding when soil temperatures are adequate for seed growth,
- seeding prior to a period that could meet the stratification requirements of the species. (A common requirement of many native species for breaking of seed dormancy is a cold, wet stratification.)
The seeding window is greatly influenced by temperature and precipitation. Note that seeding is generally recommended just prior to periods when temperatures are low (above freezing) and precipitation is high. Fall and spring seeding windows exist in most areas of the West. Since many of these windows are brief because of precipitous weather, site preparation should be completed shortly before the window begins, thus allowing the maximum amount of time possible for seeding. Areas to be seeded during the spring should be prepared during the fall if possible. Contracts should be awarded and seed materials ordered well in advance in order to ensure prompt start-up of seeding operations.
Generally, as much area as possible should be planned for seeding during the fall. Fall seeding allows utilization of soil moisture recharge during the winter. By the time conditions are dry enough in the spring to allow access for seeding, part of the soil moisture buildup has been lost. Site preparation during the spring will also dry out sites quicker. Some shrub species require winter stratification for germination. Many native legumes have extremely hard seed coats and seem to establish more readily from fall seeding. The timing of seeding can be used to some degree to manipulate the species composition of the revegetation. Grass seeds generally germinate more quickly and seedlings establish more readily than shrubs and trees. The competition from grass seedlings might limit or prohibit establishment of shrub seedlings. On flatter areas where erosion control is not a major problem, slower growing forbs or woody natives could be sown one growing season prior to more aggressive grasses to enable them to become better established prior to any competition from grasses.
It is important to revegetate disturbances as soon as possible during the first available seeding window following site preparation. The longer the time period between seedbed preparation and seeding, the more susceptible the area is to surface crusting, erosion, and weed infestation. For this reason, it is also important to revegetate correctly the first time.
All seeds furnished to the operator should be those specified in the project plan and should be measured by pure live seed (PLS) weight. The advantage of using seed on a PLS basis is that trash and empty seeds do not confuse seeding-rate calculations. All seed should be tested by a certified seed analyst in an accredited seed testing laboratory within 18 months for grass seed; nine months for forb, shrubs, and tree seed; and six months for all seed crossing state lines. Consult the state seed laboratory in the respective area where the project is located to obtain current information regarding laws governing location-specific native seed. These regulations are rapidly changing and vary from state to state. Each species should be furnished with a tag, and what is on the tag should be what is in the bag.
All legume seed should be treated with a commercial rhizobium inoculate at the time of planting to enhance the development of nitrogen-fixing root nodules.
The seed mix should not contain prohibited noxious weed seed, as listed by state or federal law. Note that no noxious weed seed may cross state boundaries by federal law. Wet, moldy, or otherwise damaged seed should not be accepted. If a specified seed variety is not available, the contractor should be required to consult with the operator prior to any substitutions. It is best to use seed relatively soon after purchase, and do not store it for any length of time unless proper facilities, such as those that are cool and dry, are available. Seed should be kept at room temperature or below but above freezing prior to use.Direct seeding rates for individual species and species mixtures should always be derived on a PLS basis. Testing to evaluate purity and germination should have been performed recently to ensure an accurate determination of PLS values.
Seeding rates vary by revegetation zone and species composition. It is important to focus on the number of seeds per square meter much more so than the number of kilograms per hectare. The kg/ha rate should be derived from the seeds/m². A reliable rule of thumb for broadcast and hydroseeding is 480 PLS seeds/m²; for drill seeding, use 320 PLS seeds/m². The rate for broadcast seeded areas should be 150% (1.5 times) the normal drill-seeding rate. This allows greater numbers of seed per unit area to allow for poor seed germination and seedling mortality. Do not assume that more is better. Having too many seeds per square meter is undesirable, creating undue competition among the germinating plants for moisture and nutrient. The cost of native seed is a considerable component of the revegetation program; it is not diminutive by any means. A general rule of thumb is that seeding rate should fall somewhere between 7 and 20 lb./ac., depending on selected species.
Selection and use of native species for disturbed land revegetation depends on an integrated consideration of six basic factors: (1) adaptation capability-be sure to use early seral stage, not climax species; (2) ecotype limitations-elevation, moisture, nutrient requirements, shade tolerance; (3) germination and establishment requirements; (4) functional utility-aboveground portion of plant; (5) function-below ground, rooting depth, association, and mineral fixing capability; and (6) seed availability and cost.
Given that most roadside revegetation projects involve use of federal funds, it is recommended that the project designer(s) work directly with botanists from the district of the agency near the subject property. It is important that regional experts be consulted when developing plans for revegetation of disturbances. Revegetation specialists generally know many of the tolerances and requirements of native species and can provide valuable information about their proper use in seeding and planting of disturbed areas.
Roadside revegetation projects have unique requirements different from other more generic seeding projects. Typically, they must avoid species that might be palatable to wildlife, avoid especially flammable species, avoid species combinations not germane to the existing landscape, and endeavor to blend into what is in the surrounding natural environment.
One factors that often limits the use of native species in revegetation is the general lack of knowledge concerning their proper use and adaptability. The literature on the use of natives is expanding rapidly and much more is known today than 10 years ago, although it is still somewhat limited. As the use of native species increases, even more will be discovered about their basic growth requirements and adaptation.
Identify which native plants grow in the subject area that are suitable for revegetation and base your species selection on data obtained from the soil analysis. Use the right plants for the job at hand. Start with early seral-stage colonizer species. Do not plan on putting higher seral-stage or climax-stage species into immature soils at disturbed sites; it will not work.
Early seral-stage plants, by contrast, are capable of germinating and establishing in primitive soils, and they build microbial associations required by higher seral-stage plants. Being cognizant of ecospecific adaptations in local plant species is important. Such adaptations may include the ability to accommodate higher levels of salt, boron, and other minerals or shorter growing season at higher elevations. Pay attention to which native plants voluntarily colonize disturbed sites in the project area; these might be among the most promising early seral-stage plants for inclusion in the revegetation species list for a given for difficult site.
Given the ecospecific adaptations common to native plants, local collection is highly advisable. This need not cost more if planned for well in advance of acquisition. Correlating conditions as closely as possible at the revegetation site with those of the seed-collection site will yield greater probability of success. Wetland species are highly adaptive to soil conditions and elevation, making local collection the optimum choice when success is imperative. Plan ahead. It is advisable to require the contractor obtain seed for revegetation 60 days from issuance of contract. This will greatly alleviate problems of availability and resultant attempts at substitution.
A major factor hindering the use of many native species is their comparative lack of availability. With increasing interest in the use of natives since the early 1970s, however, a definite native plant and seed industry has developed. Numerous companies are now selling both seed and plant material of native species. A list compiled by the Natural Resource Conservation Service, the Bureau of Land Management, and the Forest Service is available for review. These lists might be incomplete, and local directories should be consulted when purchasing seed.
The increase in the availability of native seed shows that with increased demand for natives, many species have become commercially available. With adequate planning, most species could be available for revegetation along roadways and other native revegetation sites. Many of the more dominant native species are available commercially now, and as their use increases, more species will be made available and research and testing will continue to identify and improve selections of natives much the same as was done with many of the introduced species.
