A Guide to the Revegetation and Environmental Restoration of Closed Landfills

Chapter 6: Aspects of California’s Vegetation

Plant Assemblage Profiles

Just as plants develop associations based on climate, moisture, and soil type, the plant community can establish itself into a simple to complex interrelationship as a layered or stratified structure. As the natural succession of a plant community develops through time, the larger vegetation supersedes the previous pioneer plants. Pioneer weeds and grasses begin the succession, preparing the soil for the succeeding plants. The pioneer weeds and grasses are eventually shaded out by larger shrubs. These shrubs are displaced or dominated by the larger trees. This system of layering provides environmental levels for wildlife and plants alike
(Figure 4).

The main plant layers include the understory, intermediate, and overstory layers.

Understory

This includes the smallest vegetation such as mosses, ferns, grasses, small wildflowers, and low ground covering varieties of herbaceous or woody plants.

Intermediate

This layer will include smaller and larger shrubs and smaller species of trees or young saplings of larger overstory tree species. These plants may be adapted to softer light and cooler temperatures created by the shading effect of the overstory canopy. Woody perennial plants dominate the intermediate story.

Overstory

This vegetative layer consists of the larger species of trees in the natural assemblage. This layer can create a canopy that influences the overall light availability and average temperatures at the lower levels. These trees can be sparsely distributed or closely growing together to create a tight canopy. Destruction of the canopy trees can adversely affect the understory environment, or provide a point of opportunity for saplings to fill in. Not providing these trees in a poorly planned restoration project may jeopardize the success of understory plant species growth and the project.

A landfill revegetation or restoration project would shorten some of this successional process, compressing the sequence into roughly one step, with grasses, shrubs, and trees planted at the same time. Invasive plants, including pest weeds, would impose on this plan if a maintenance program to remove these invaders were not exercised.

Figure 4 Plant Assemblage Profiles Jacques Graber 1999 (81 KB)

Within each of the four vegetation zones, plants have established themselves into assemblages or communities. Each community, when viewed as a whole, is an integrated system adapted to that particular environment. Similar plant communities may be found in the Coastal and the Mountain Zones, the Interior Zone as well as the other zones. Though superficially resembling each other in function, two similar looking plant communities will have entirely different species assuming similar ecological functions. Species aside, these vegetative communities follow several basic patterns, such as grasslands, wetlands, woodlands or forests, etc. Within these major patterns, though, is a whole spectrum of variation. Some of the major plant assemblages most found in California include⁴:

This vegetation community consists primarily of annual or perennial grasses with, now, predominantly introduced annual grass species from Europe (Festuca and others), and annual or perennial wildflowers. Grassland or prairie generally lacks major trees and shrubs. Though species such as oaks may be dispersed throughout this community, they would generally not constitute a "forest."

Major streams or river channels that traverse grasslands or prairies may support dense stands of hardwood trees and shrubs as Riparian lands, being restricted to available water from the stream. Grasslands and the remnants of a once expansive riparian environment that existed along the major rivers and streams dominate the Central Valley (Figure. 5a or b).

Similar to prairie. Open temperate hill grasslands or glades, or bald hills on the west slopes of the outer and middle coast ranges in Mendocino and Trinity counties, north, and scattered to counties southward to San Francisco County. In the past, native bunch grasses and flowering herbs dominated coastal prairies. Because of overgrazing, these native bunch grasses have been displaced by annual grasses and by intrusion by non-native grasses.

This vegetation community consists frequently of an understory grass soil cover with wildflowers within which are distributed, in varying density, different species of shrubs and oaks or juniper as intermediate and overstory plants. Chaparral is predominantly a dry climate plant community. Many of the trees naturally found in this environment possess thick corky bark (cork cambium) and are adapted to the wildfires that raged through prior to human involvement. These trees may include Interior Live Oak, Blue and White Oak, Manzanita and chamise. This plant assemblage occupies areas along Central Valley and foothills regions of California from Redding, along the western Sierras. Chaparral can be found in the southern California counties and along the eastern flanks of the Coast Ranges from Redding to the San Bernardino Mountains, as well as distributions south to San Diego County (Figure 5b).

These vegetation communities consist of a complex understory and intermediate plant relationship, with high diversity in these layers in natural mature forests. Woodlands’ overstory trees are represented by two dominant tree types, conifers and deciduous, with a mixture of the two as environmental conditions may dictate. The dominant overstory trees will be made up of either species of conifers in the higher altitudes or deciduous trees at lower elevations.
(Figure 5c).

Forest communities create the most impressive and oldest plant communities in their natural mature state as exemplified by the redwood and old growth forests of the northwest state, the Big Trees National Forest in the central Sierra Nevada and the Bristlecone Pines in the high Sierras. Conifers or deciduous (broadleaf) trees can wholly dominate the overstory canopy to the exclusion of the other, or they can share this niche in varying proportions depending upon climate, elevation and soil conditions. Forest or Woodlands command the Coastal and Mountain Zones as well as the riparian environments in the Central Valley. Several divisions of the forest community are listed here. Forest communities are characterized by the dominant conifer found in each of them:

• Closed Cone Pine Forest. This community is found at intermittent locations along the California coast from Mendocino to Santa Barbara counties.
  
• Redwood Forest. Located along the west slopes of the Coast Range from Del Norte to Santa Cruz counties. Some small areas are found in Monterey County.
  
• Douglas Fir Forest. This community is found in the north Coast Ranges from Mendocino County southward, with scattered remnants to Sonoma and Marin Counties, easterly of the redwood forest regions.
  
• Yellow Pine Forest. Found in the North Coast regions, to Southern California.
  
• Red Fir Forest. Found in the North Coast ranges to Southern California.
  
• Lodgepole Forest. Found in northernmost California to the central Sierra.
  
• Northern Juniper Woodland. A variant of woodland community in which the dominant tree species is Juniper (Juniperus occidentalis). This community can be found in central Siskiyou County, easterly to Modoc County and south to Mono County.

This plant assemblage will be found in those areas of California where moisture and temperature are at their extremes. Soil conditions are harsh and difficult for plants to grow in. The dominant plants, known as Xerophytes are deep-rooted, and slow-growing. Many plants in the desert community are defensive; producing terpines that will keep invasive plants away from their roots; blocking competition for valuable water. Leaves are thick and physical defenses such as spines, thorns, and foul-tasting resins keep herbivorous animals from destroying their slow-growing foliage. Knowledge of these traits can help in planning desert restoration projects, so as not to put these highly competitive plant species too close to each other. Alkali sink vegetation is found in poorly drained alkali flats and playas on the floor of the Central Valley and arid regions on the east slopes of the Sierra Nevada. (Figure 6a).

These three aquatic plant communities constitute the vegetation assemblages found along natural water bodies throughout California. Some are extremely small and fragile, such as vernal pools, their combined statewide total areas barely covering several acres.

Plants in these communities are highly moisture-dependent, yet they can be adapted to intermittent dry cycles, going into dormancy. Many of these species are microscopic and may be sensitive and vulnerable to minor changes in their conditions. They can also serve as effective bioremediation mechanisms, filtering out certain effluent components in sewage treatment projects (Figure 6b, c).

• Wetland or Estuarine. These plant communities include Coastal Salt Marsh and Freshwater Marsh. They can be found generally as bordering lands along bays (large or small) salt, and fresh water bodies, lakes and rivers. The largest single wetland environment in California can be found at the north shore of Suisun Bay northeast of San Francisco. This plant community can consist of vast expanses of rushes and grasses adapted to live in water-saturated soils and conditions of brackish to fresh water. Floating plants, including an introduced species of water hyacinth, can be found in areas of the Sacramento and San Joaquin River Delta. Sloughs, lagoons, or river channels are generally associated with the wetland or estuarine community. Many estuarine or wetland systems can be influenced by tidal conditions (Figure 6b, c).

• Riparian. The riparian environment is an ecologic community including plants growing along an established stream or river channel and the floodplain associated with it. Riparian vegetation consists of rapidly growing, moisture dependent species such as poplars or willows, assorted thick-growing shrubs, vines and understory grasses or other smaller plants. The riparian environment can form a complex multi-layered vegetation community. Dense forests of deciduous trees, understory shrubs and grasses can occupy areas embracing the stream channel while small wetland environments may be interspersed along the river or stream.

Early in California’s history, the riparian environment dominated the Central Valley where flooding along the Sacramento River floodplain was uncontrolled and frequent. Since western man’s immigration to California began, approximately 98 percent of this vegetation community had been destroyed by flood control programs, and by agriculture and other development.

• Vernal Pools. Endemic to California, the vernal pool is one of the smallest environments, and one of the most sensitive to damaging impacts. The vernal pool plays an elusive role in project development and restoration issues. The discussion of what constitutes a "vernal pool" has caused some debate and legal consternation among developers and environmentalists. The ultimate indicator of what constitutes a vernal pool is the presence of certain plants that are restricted locally or entirely by this type of habitat.⁵

The vernal pool vegetation community can be very small in scale. A vernal pool is a water body often only several dozens of square feet in area and only inches deep. Vernal pools are seasonally created when water collects in a natural surface depression that is rendered water retentive by hardpan, claypan, or other low porosity soils in the basin. The vernal pool is intermittent, evaporating several days or weeks after it reaches its fullest level. This characteristic makes defining it even more confusing. The vegetal assemblage can consist of very small species of grasses, mosses, and some associated trees and/or shrubs.

Some rare or endangered species of vernal pool life such as fairy shrimp are found on the macroscopic and the microscopic level. (Figure 6c). Many vernal pool inhabitants often hibernate during the dry seasonal cycles. Vernal pools create issue for their often being located on prime lands for development and for being temporary or "insignificant" causing debate over their importance both in their destruction as well as their mitigative significance. On the other hand, vernal pool "construction" can be a mitigative opportunity for a project proponent, as vernal pools are small yet often of environmental significance.

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When a closure project features a golf course, business park, or recreational park, the operator may select nursery rather than native plants for the landscaping. Selection of plant communities for such projects would be based upon the dominant geographic, soils, and climatic characteristics in which the project is located. These conditions would then determine the dominant plant types used. The main difference is that the plants that are selected would not be California native species, but they would still be compatible with the environmental characteristics of the site to ensure survival.

This type of planting program may require a more intensive maintenance regimen to control invasive plant species, pests, irrigation, and nutrient provision. Using plant volunteerism by neighboring native species to stock the site would not be employed to avoid competition with the introduced plants. If an aggressive plant control program is not followed, invasive (volunteer) plants could still establish themselves. Using the natural overstory-understory distribution concept could be applied to cultivated non-native planting.

Figures 5a, 5b, and 5c Grassland or Prairie, Chaparral, and Mature Woodland (47 KB)

Figures 6a, 6b, and 6c Desert or Arid, Wetlands or Estuarine, and Vernal Pool (64 KB)

Figure 7 General Plant Succession from Bare Soil to Mature Hardwood Forest (23 KB)

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Chapter 7: Landfill Vegetative Design

As a landfill approaches its waning years of active use, the operator should consider the details of the final closure and postclosure land use planning for the facility. At this time, the operator must consider if the closure will include an integrated planned postclosure use such as creating undeveloped land, a park, a playing field or preserve, golf course, business park, or industrial park. These "uses" will make a difference in the planned final cover design and possible slope design or contouring design for the decks and side slopes, irrigation, and drainage control. The more complex or structured the final land use, the more complex will be the design requirements for the final cover and revegetation planning. An undeveloped, open grassland will demand less from the final cover design than a planned recreational park or a business park. The final cover planning will be dictated by other conditions such as the final cover and moisture layer components, physical aspects of the landfill site, and the surrounding environments.

Costs and availability of materials such as soil and plant stocks will dictate the proposed design characteristics. The elements to consider in the restoration or other final closure plan will include landfill design and additional uses for vegetative cover.

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Landfill Design Considerations

If there is an actual planned postclosure use or idea in mind for the site, this aspect will have to be analyzed prior to any actual design planning of the final cover. Different uses will impact the demands placed on the final cover, including its thickness, slope profiles, and, in considering revegetation or restoration projects, the quality and quantity of the vegetative layer soil that will be utilized.

Drainage and supplemental irrigation issues will have to be addressed. If the final plan is to install a simple grass cover on the landfill, a thin vegetative layer that is in compliance with the regulations (Title 27 or Subtitle D) may be all that is necessary. If more complex plantings are proposed, berms, supplemental soils on benches and other such enhancements to the vegetative layer may be required to support deeper-rooted plants and to protect the underlying moisture barrier from root penetration.

Some final closure plans are proposing the use of a monolithic cover instead of the current multi-layer design in use on most landfills. The monolithic cover proposals use a thicker, single soil layer that would provide sufficient depths to include deep-rooted vegetative plantings. One proposed plan incorporates moisture control elements into the monolithic cover by using poplars or other similar vegetation and a groundcover plant such as clover to wick off soil moisture by evapotranspiration.⁶

Whether planning for root depths to determine the vegetative cover thickness or planning root depths to best work with the planned soil thickness, either technique requires forethought in designing the cover and vegetative systems as a unit.

Development of contours of the final cover can be impacted by the final postclosure use. An operator who intends to create a restoration project may create more naturally compatible slopes and contours to the adjacent landscape. Or, the operator may require basic contours in compliance with current regulations and the engineering needs of the final project.

• Natural Parks, Preserves, and Mitigation Sites. These uses require "permanent cover" of native plants that will not be displaced for a future land use such as a business park or other structures.
  
• Recreational parks and golf courses. These uses will entail permanent or long-term post-closure use cover elements. These sites can be developed with "disposable" cover. The vegetation may usually be nursery plantings, though natural plant species are optional. A potential for their removal exists, allowing re-use of the site for a business park or other structured development at a later time.

This will dictate what kind of planting and vegetative layer conditions will be placed on the landfill final cover. Steep slopes on final cover will require more aggressively rooted vegetative types than shallow slopes. Benches can provide planting areas for deep-rooted plants. These benches must be sufficiently wide to accommodate maintenance vehicles in addition to the proposed plantings such as trees or large shrubs. A thin vegetative cover will restrict plant options to shallow-rooted varieties that will not penetrate moisture barrier layers. Irrigation from natural rainfall will dictate the types of vegetation available for use in dry or moist conditions. This may require supplemental irrigation to support the desired vegetative cover, at least until the plants are strongly established. Slopes and naturalized contours can provide alternative planning options for revegetation projects with areas to enhance opportunities for vegetation planting.

• Benches. Benches can provide deep soil zones for trees if they are wide enough to provide space for the vegetation and maintenance activities, usually with the vegetation at the outside edge of the bench, where soil is deepest. Grouping trees instead of lining them up creates more natural "groves."
  
• Decks. If top layers of vegetative soil are sufficiently supplied beyond the minimum 12-inch requirement (preferably 48 inches or more), larger plants with deeper roots can be supported.
  
• Berms. Berms add small areas of thicker vegetative soils as hills or other raised land features (48 inches or more); the added soil can provide sufficient depths for trees or large shrubs to enhance the natural vegetative appearances of the final cover.

Advanced planning of the landfill, taking the postclosure land use into account at the very outset, can help the operator plan for adequate soil and topsoil supplies for the final cover. By planning the final elevations and contours of the cover layer in advance, the underlying waste volume can be graded to within more precise tolerances to conform to the projected design.

By designing the waste contours to closely match the finished design specifications, less financial resource is expended on poor grade foundation soil built to grade and, instead, can be used more efficiently in providing greater volumes of adequate quality topsoil in the vegetative layer. This could result in additional capital for other closure project costs or savings to the operator overall. (Figure 7 a and b).

Figure 7a, and 7b Landfill with Waste Configured to Basic Elevations, and Landfill with Waste Configured to Postclosure Project Plan (10 KB)

Figure 7a. A landfill with wastes that are not closely configured to the planned final contours will require additional foundation soil to achieve design grades. Less funding will be available for the acquisition of useful fertile topsoil for the vegetation layer.

Figure 7b. A landfill with wastes more closely configured to planned final contours will require less foundation layer soil to achieve grade. This allows for more funding to be allocated toward better quality, thicker topsoil for the vegetative layer. A thicker topsoil layer will allow for more vegetation design options and improve chances for plant survival.

Where the landfill is located will influence the type of vegetation that can be used on it. Landfills in hot, dry regions will support appropriately selected vegetation for those climates. Irrigation can provide added options for vegetation selection, but it is more expensive to include as a planned element in vegetation selection and closure maintenance plans. In addition, atmospheric conditions can affect plant choices. Revegetation in urban areas may pose a challenge because airborne pollutants can adversely affect some vegetation. Conifers in the Los Angeles basin and grape plants in the Central Valley have demonstrated this sensitivity by loss of foliage and higher mortality.

General wind conditions at site should be considered when designing a vegetation cover. Pollutant gases can collect in windward-facing valleys or pockets, creating adverse atmospheric conditions injurious to plants.

Excessively tall trees may prove vulnerable to blow-down ("wind-throw") if left unprotected. This can cause damage to the final cover, should the roots peel the soil layer up with the toppled tree. Natural, established tree groves in areas with a prevailing wind tend to develop an airfoil-like profile, due to natural pruning. Smaller trees of the expanding grove tend to grow at the perimeter of the grove with larger, mature trees in the center area of the grove. This dome-like form encourages airflow gradually over and around the tree grove (Figure 8a). Single-line hedgerow-like plantings or isolated individuals, especially at the edges of top decks and maintenance roads or benches, place adult trees in a vulnerable position to strong winds, encouraging wind-throw (Figure 8b). Planting shorter trees at the perimeter of a grove around taller varieties or adult trees can provide a windbreak by slowing wind velocities and directing airflow over or around the taller canopy layer.

Figure 8a and 8b Airflow over Natural Tree Stand and Airflow over Uncontoured Tree Stand (95 KB)

When a vegetative cover is installed, extensive planning must be exercised in laying out the details of the vegetative layer and the final plantings. This can be most complex when all vegetation is planned at the initial planting. A strategy for interim maintenance must still be instituted when the natural population process of a vegetative cover is attempted. A planned vegetative community can be designed and installed in a variety of ways, with three suggestions as follows:

• Developing and providing all of the major elements of the plant community, such as grasses, shrubs and trees, at the very outset of planting.

This procedure will require the most advanced planning but it should provide the greatest element of control in the overall plant community design and final outcome. The final plant community would be established and maturing early in the revegetation, and postclosure maintenance program. Some invasive volunteerism by outside plants could occur if the operator does not exercise aggressive control efforts by keeping them out. The initial hydroseeding of annual grasses, or hand planting of native perennial grasses can be introduced for slope stabilization with larger plants installed at later dates.

• Providing the proper environment and soil conditions to encourage plant growth and allowing natural invasion (volunteering) by native plants adjacent to the site.

This procedure provides the lowest element of control on the types of plants that may be introduced to the site. This process is the most dependent upon the unpredictable phenomenon of natural plant establishment and succession that may take longer than the immediate planting procedure. Some sort of initial soil stabilization planting with a rapid growing annual and/or perennial grass or ground cover will still be required to prevent erosion of the soil cap. The plant succession process occurs as the selected area matures.

Naturally, the pioneer plants, most adapted to the harsh conditions of bare, usually poor quality soils, begin the process. This community usually consists of low growing or prostrate weeds and grasses with deep taproots. This initial plant association begins the soil nutrient construction and softening of the soil that provides the conditions more conducive to the later succeeding plants to establish themselves.

As the soil is broken up and softened, taller grasses gain a foothold and establish themselves. In time, legumes, herbaceous perennials and woody perennials can begin the larger plant occupation as soil quality and nutrient content improves. Eventually, shrubs and the larger trees assume the mature level on the location.

A landfill preferably will have an annual and perennial native grass planting in its earliest vegetation phase, which may skip the pioneer phase of the succession. Some stronger invasive weeds may still try to occupy the site.

Shrubs may not be allowed on the site to avoid root penetration, but after the postclosure maintenance program is complete, shrubs and trees may complete the progression anyway (Figure 9).

• Combining planned planting with volunteering by adjacent native species to create the final vegetation cover.

This technique can allow some control in the selection and establishment of the larger plants with other plant selections and distributions left to chance. Efforts may still be required to control undesired pest plant intrusion, especially plants with deep reaching taproots that could damage the moisture barrier.

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In addition to the obvious uses of vegetative cover that include regulatory compliance, soil stabilization and aesthetic contribution to the landfill site, vegetative cover has some specialized functions that can be used to advantage.

Vegetative cover can be used in certain situations to attenuate concentrations of certain chemicals, salts, trace metals, and other toxic materials such as boron and selenium present in soils. Certain grasses have the capacity of surviving in higher concentrations of these compounds than other vegetation candidates. In addition these plants tend to store these compounds in their leaf and stem tissues while removing them from the soil. This process can be used to advantage to prepare contaminated soils at problem sites for future population with less tolerant plants. By planting with these salt-tolerant species and mowing them at maturity, the contaminants can be removed. The contaminated cuttings are disposed at a different, appropriate disposal site as fill. After repeated cycles, the salts are removed from the soil; the less tolerant vegetation selections can be planted.

A pilot project at Mountain View Sanitary district wastewater treatment facility employed conifer trees as a moisture exchanging evapotranspiration mechanism to process water. The trees transpire the water into the atmosphere, and create a harvestable revenue crop at maturity. This activity can provide a year-round operation as a serviceable alternative to conventional irrigation disposal systems that shut down during the winter months. This technique could be employed at landfill sludge or septage ponds or possibly leachate ponds, upon closure.

Cattails and similar estuarine plants can absorb certain materials in solution, utilizing them as nutrients. Where high concentrations of these substances in water from sludge or leachate could pose pollution problems such as in leachate and certain liquid waste ponds, cattails and related water plants can mitigate these situations. With naturalized leachate ponds and cattails or similar rushes, natural looking artificial "wetlands" could be created. The cleaned water from these ponds can be recycled as irrigation water after additional treatment. The City of Arcata, in Humboldt County, is employing this technique at the final stages of its wastewater treatment process.

Remediation of landfill gas and detection of landfill gas can be accomplished using surface vegetation. Some landfill gas in low concentrations in soil can be absorbed and attenuated by the nitrogen fixing properties of certain legumes and other plant species that possess such bacteria or fungi in their roots. If gas concentrations do become excessive, plants are sensitive to these gases and their abnormal appearance, loss of leaves, usually will alert the operator to a potential gas problem that will need attention. This condition must be responded to quickly before permanent damage is caused to the impacted vegetation, or the gas migrates and spreads further from the initial site.

Extreme exposures of vegetation to high gas concentrations can lead to stunting of growth or defoliation in some instances, or plant death, requiring subsequent removal and replanting.

Leachates that develop at a landfill and accumulate in areas shallow enough to impact plant roots can be detected by plants. Loss of leaves and die-off of vegetation on or in close proximity to the landfill site may indicate signs of a possible leachate problem. With the right conditions and leachate composition, bioremediation (see above) can be employed to reduce leachate impacts.

Certain plant groups, the legumes particularly, have a natural ability, through nitrogen-fixing fungal symbiotes, to fix nitrogen in the soil. This nitrogen-fixing ability aids in improving the nutrient quality of the soil that will encourage other plants to grow.

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Last updated: April 18, 2008