3.11 Developing a Vegetation Management Strategy during Project Design
During the planning phase, it is important to consider how vegetation will be maintained after the road project is completed. The effects of road surface and roadside vegetation management can have unexpected and often, unwanted effects on the long-term outcome of the revegetation project. Developing a written maintenance strategy can assure that there will be a rational documented approach to the management of roadside vegetation long after the project is completed and can be used to gain acceptance of the strategy from the maintenance department.
The intent of a maintenance strategy is to consider how the results of a revegetation project will affect the management and maintenance of the roadside and to incorporate this understanding into the revegetation plan. The strategy also anticipates and mitigates for those biotic and abiotic factors that may affect the development of native plant communities and pollinator habitats. Ideally, the planning team or designer meets with local roadside maintenance personnel to discuss their current and future anticipated maintenance procedures in order to learn what problems can be expected in reestablishing roadsides with native plants and how these problems could be addressed. This is a time when maintenance personnel can raise concerns and then work collaboratively with the design team to develop solutions. The planning phase is a good opportunity to develop a plan that maintenance personnel will understand and support.
The intended audience for this section is the designer and design team because they will integrate road maintenance and operations into how to achieve the long-term revegetation objectives during the planning process. Chapter 7 also covers vegetation management but from a maintenance perspective and the intended audience for that chapter is maintenance and operations staff. Chapter 7 focuses on approaches to road surface and roadside maintenance that will meet revegetation objectives after the revegetation project is completed.
A vegetation maintenance strategy may cover some, or all of the following:
- Protection of areas currently free of invasive species
- An outline of an integrated vegetation management approach to weed control
- How existing weeds may be controlled prior and during construction
- How a weed-resistant road environment will be created
- How roadside vegetation maintenance objectives will be achieved after construction
- How the vegetation maintenance strategy is handed off to maintenance personnel
- How roadside disturbances will be treated during the maintenance phase
3.11.2 Integrating Road Maintenance Objectives into the Revegetation Plan
For a successful project, it is important that road surface and roadside maintenance objectives align with treatments and species outlined in the revegetation plan. Many states have developed IRVM (Integrated Roadside Vegetation Management) Plans that outline roadside maintenance objectives. When available, it is strongly recommended that the designer refer to these individual plans while developing the revegetation plan. The IRVM plan is "an approach to right-of-way maintenance that combines an array of management techniques with sound ecological principles to establish and maintain safe, healthy and functional roadsides" (Brant and others 2015). It applies many of the Integrated Pest Management concepts developed for agriculture, horticulture, and forestry to roadside vegetation management. The IRVM elements include prevention, monitoring, action thresholds, pest treatments, and evaluation (NRVMA 1997). Most of these plans are available on the internet or by contacting the state Department of Transportation (DOT) or the local maintenance agency.
A vegetation management strategy considers how road maintenance objectives are integrated into plant species selection, planting patterns, vegetation control, invasive species control, and site treatments. Typical road objectives are stated below, along with their possible effects on roadside vegetation. In the development of the vegetation management strategy, consider how each objective will affect vegetation and, in turn, how vegetation treatments and design will affect roadside objectives.
- Maintain line of sight—It is important to not plant masses of tall grasses, shrubs or trees in areas where line of sight is important. If they are existing to remain on a site that is being revegetated, they may need to be maintained or removed and replaced with appropriate smaller plant material so as not to reduce line of sight. Designing for safety is further discussed in Section 3.11.7.
- Maintain clear zones—The FHWA (2017) defines the clear zone as "an unobstructed, traversable roadside area designed to enable driver to stop safely or regain control of a vehicle that has accidently left the roadway ... and an effective strategy for prevention and mitigation of roadway departure crashes." The publication further states that "trees are the single most commonly struck objects in serious roadside collisions. Therefore, it is important to integrate the selection of appropriate plant species with safety objectives in mind." The integration of habitat features such as brush piles or dead wood should be placed well beyond the clear zone.
- Maintain road surface—Encroaching vegetation can reduce the longevity of road surfaces. Selecting plant species that are least likely to do this can preserve the longevity of roads.
- Reduce fire hazards—One of the reasons for maintaining low growing roadside vegetation is to reduce the starting of fires by motorists and the spread of wildfires. In planning, choosing plants that are more fire resistant and encouraging plants that stay green in the summer (e.g. native perennials) by creating good optimum growing environment (e.g. good soil conditions). Mowing, herbicides, grazing, and fire are also measures that may affect pollinator habitat and need to be planned accordingly (Section 7.5).
- Maintain stable roadside—Cuts, fills, and drainage facilities need to be maintained so that they are stable. When they are not, they create road hazards and degrade water quality. Unstable material will need to be removed and replaced. The removal area and replacement material will need to be reviewed and revegetated. Areas that fail will also need to be repaired and revegetated. It is important that the maintenance provider have the native-sourced seeds or plants for the waste or disturbed areas (Section 7.2.2).
- Maintain or increase water quality—Maintaining water flow off pavements, down ditches, and into natural waterways is important for road maintenance but may affect water quality or plant growth. Cleaning out ditches affects plants and cut slopes. Poor flow of water off pavements can cause rill and gullies on fill slopes affecting plants. Poorly designed culvert placement can cause downcutting of natural drainage channels.
- Reduce or eliminate invasive species—Controlling invasive species may also negatively affect non-target plants and pollinator species and need to be considered when developing treatments. (Section 7.5).
- Maintain safe road surfaces during winter months—Winter roadway surface treatments vary throughout the country. Gravels used in mountainous, heavy snowpack areas, coarse sand used in smaller urban areas, and the more prevalent deicing chemicals can be the most devastating treatments on roadside vegetation. Sweeping gravels off pavement during melt periods buries roadside plants. Reusing gravels from along roadsides can spread weeds. Blowing snows can create drifts that melt later in the year and may call for different plant species in those areas to withstand that condition. Sand accumulations in swales and drainage structures can create clogs, ponding, and spreading sand bars which can smother groundcover over time. Deicing with salts and other chemicals can kill or reduce growth on certain plant species. This is further discussed in Section 3.11.9 (see Deicing for Winter Safety.
- Reduce impacts of wildlife to motor safety—The selection of plant species and where they are planted along roadsides can affect how wildlife moves through a road corridor and their potential hazards to motorists. Planning for wildlife is an important consideration when selecting plant species and planting patterns. Providing desirable browse or a refuge of thick vegetation on both sides of an identified animal crossing can draw wildlife to the safe crossing location. Existing vegetation near known unsafe animal crossing locations may be controlled through removal, thinning, and mowing to reduce cover and forage opportunities, and to increase visibility in an attempt to reduce animal-vehicle collisions. Vegetation types, patterns, and maintenance can encourage the movement of wildlife to crossings. Wildlife fencing that direct animals to grade separated crossings are very successful in reducing wildlife-vehicle collisions (WVC). The fencing should be planned with wildlife biologists who are familiar with wildlife passage patterns.
- Protecting utilities—Encroaching vegetation can overgrow and hide utility structures, which may lead to accidental damage during maintenance procedures, may affect maintenance access, and may cause maintenance review to be missed. Utility service may also be damaged and disrupted by accumulation of heavy vines, wind-blown branches, and by falling trees or branches. It is important to that species that may interfere with utility access and protection are not planted near these structures.
The vegetation management strategy will ideally span the entire length of the project, into the operation and maintenance phase (Chapter 7). In the early stages of planning, weed sources are identified and treatments to control or eliminate often begin prior to construction (Section 3.11.6). Areas of functioning plant communities, relative free of invasive species are identified and plans for protection are developed (Section 3.11.3). During planning, road maintenance objectives are integrated into the revegetation treatments. As the project moves into construction, treatments to create a weed-resistant environment (Section 3.11.4) and measures to reduce the introduction of weed sources are implemented (Section 3.11.5). Once construction is completed, the project is handed off to the agency responsible for road maintenance. At this point, the responsible agency implements their vegetation maintenance plan which dovetails with the revegetation plan objectives (Chapter 7).
3.11.3 Protecting Healthy Plant Communities
Mature forests, prairies, and wetlands are often high quality, healthy, and diverse landscapes supportive of a wide range of native plant, pollinator, and wildlife communities. These healthy plant communities are typically more resistant to invasive plant and animal species, and contain flora and fauna capable of supporting high level predators, particularly in larger areas. For the purposes of this report, landscapes of healthy plant communities capable of providing quality habitat for large animals, are referred to as â€˜refugia.'
Many of the larger blocks of refugia habitat have been preserved by local, State, and National parks, but remnants remain on undeveloped public and private land across the country that are essential for the preservation of native flora and fauna. Migrating animals and those that require large ranges depend on refugia and safe connections between refugia areas. Revegetated roadsides can offer safe connections for wildlife to under and over-crossings and to other areas of refugia.
Despite best efforts to prevent impacts to environmental assets during the planning and construction of roadway expansions, development of roadway systems often causes disturbances to refugia. Introduction of road rights-of-way near or through these natural areas often results in a slow degradation of the landscape and wildlife population through the removal of habitat, altering or cutting off of wildlife corridors, and introduction of construction and traffic noise, pollution, deicing chemicals, and invasive plant and animal species.
Roadside revegetation treatments can be designed to provide a measure of protection for uninfested areas through buffering of on-going pollutions and disturbances and by preventing encroachment of invasive plants and annual weeds. Further protections of uninfested areas can be achieved through the processes listed below and described in more detail in following subsections:
- 3.11.4 Creating a Weed-Resistant Roadside Environment
- 3.11.5 Keeping Weed Sources from Entering the Project
- 3.11.6 Controlling Unwanted Vegetation
- 3.11.7 Designing for Safety and Utility Protection
- 3.11.8 Designing to Isolate Wildlife from Vehicles
- 3.11.9 Designing for Disturbances
3.11.4 Creating a Weed-Resistant Roadside Environment
The foundation of long-term weed control is to create a growing environment that encourages the development of a healthy native plant community resistant to the introduction and spread of weeds. For most weeds to become established, there must first be an opening, or space, in the native plant community for plants to grow. These conditions occur in disturbed areas associated with recent construction, landslides, bladed ditches, gullies, waste areas and other areas where native plants and soil have been removed or disrupted. Secondly, the disturbed environment must be favorable for weeds to take hold and thrive. This requires that the soils and climate be optimal for the environmental needs of the weed species. Third, the weed species must have an advantage over native species. If a weed is more resilient or robust in its growth habits in both adjacent lands and disturbed sites, then it is more likely to become established.
Some mitigating measures that can be taken to limit the environment where weeds can gain a foothold are:
- Minimize ground disturbance—Most weed seeds need bare soil to germinate and become established. Reducing the footprint of the project results in less area being disturbed and exposed to possible weed invasion. Avoid ill-timed blading, mowing, dredging, and other ground-disturbing activities (Harper-Lore and others 2013).
- Develop optimum environments for quick native plant recovery—Establishing a healthy native plant community after construction greatly reduces the possibility for weed invasion. Optimum native plant establishment is dependent on the site's soils and climate. Site factors restricting the downward root growth of native perennial plant roots (e.g., compacted soil) give the advantage to weed species. For example, a site with loose, reapplied topsoil is much more likely to support a native plant community than a compacted site lacking topsoil and organic matter. Unlike perennial plants, the roots of most annual weed species occur in the surface layer (Jackson and others 1988; Claassen and others 1995) and do not require deeper soils to survive and grow. Annual species therefore have an advantage over perennials on compacted or shallow soils. Developing site treatments that create an optimum environment for native perennial plant recovery will limit the establishment and spread of weeds (Sheley 2005).
- Develop a weed-resistant seed mix—Where appropriate, apply a seed mix that resists weed establishment. Using a seed mix composed of a diversity of species can reduce the potential for weed establishment by creating a plant community that fills niches that otherwise would have been occupied by weeds. Sowing native seeds at high rates can also flood bare soil with the seeds of desirable species, reducing the germination sites where weed species can become established. The objective of this strategy is to fill the openings first with desirable species to crowd out unwanted species (Section 5.4.1).
- Establish desirable non-native plants if establishment of natives is not feasible—Due to surrounding site limitations, project parameters, timelines, or other circumstances, using locally adapted native plant materials may not always be possible. For example, if the project is surrounded largely by contiguous populations of cultivars, using locally adapted plant materials may not be worth the effort. If the surrounding area is primarily native species, careful selection of cultivars is important to consider. Many cultivars, both native and introduced, have the potential to cross with their locally adapted native equivalent genera (Section 3.13.2).
- Avoid applying nitrogen fertilizers in the first year—Many annual weeds out-compete native plants on soils that are high in nitrogen because they are more adapted to utilizing nitrogen during early plant establishment. High rates of nitrogen fertilizers can encourage the growth of weedy annuals at the expense of the establishment and growth of native perennials (McLendon and Redente 1992; Claassen and Marler 1998), whereas perennial grasses have a competitive advantage at lower nitrogen levels (Welker and others 1991). Application rates of up to 108 lb/ac N (121 kg/ha N) have been shown to promote the establishment of introduced grasses over less competitive native grasses (DePuit and Coenenberg 1979). High rates of nitrogen fertilizers can also affect revegetation efforts by decreasing species richness and increasing the presence of non-native species (Munshower 1994; Wedin and Tilman 1996). When nitrogen fertilizer is applied at seeding, root systems of native perennials establish in the upper portion of the soil. The decrease in deep roots gives the annual weed species a competitive growth advantage (Claassen and others 1995). Limiting the amount of nitrogen fertilizer used the first year (during vegetation establishment) will help force the roots of perennials deeper into the soil where there is more moisture. The deeper root system increases the competitive advantage of the native perennials over the annual weeds. This strategy, however, does not preclude the need for fertilization. One option is not to apply fertilizer until a year after sowing when the native plants have become established (Section 5.2.1), though this may not be possible due to construction schedules and budget constraints.
- Apply mulch—Mulch can be applied to the surface of a disturbed soil to create a poor germination environment for weed seeds (Section 5.2.3). The mulch should have high void spaces (typical of long-fiber mulch materials) and low water-holding capacity, which is an unfavorable environment for seed germination. The deeper the mulch, the less weeds will become established. Depending on the weed species, 2 to 4 inches of mulch are required to effectively control the establishment of many weed species (Pellett and Heleba 1995; Ozores-Hampton 1998; Ozores-Hampton and others 2002; Penny and Neal 2003).
- Apply high C:N materials to high nitrogen sites—If soils are high in nitrogen, applying a high C:N material (e.g., wood products, coir mulch, or pine straw) to the surface of the soil, or mixed into the soil, is another strategy to reduce the amount of available nitrogen for annual weed growth (REAP 1991; St John 1999). Over time, the organic material will break down and release nitrogen for uptake by perennials.
- Retain shade—Most weeds require full or partial sunlight to thrive (Penny and Neal 2003). To reduce the vigor of undesirable plants, retain as much shade as possible.
3.11.5 Keeping Weed Sources from Entering the Project
Weed species are often brought onto the site from outside sources such as construction equipment, mowing equipment, cars and trucks, shoes and socks, and in materials used on the project such as gravel and rock. All too often, weeds arrive during the revegetation efforts in contaminated mulches, topsoil, hydroseeding equipment, and uncertified seed sources. Making the effort to prevent the introduction of weed propagules onto the project is always the preferred strategy because it is easier and more economical to prevent the introduction than to control or eliminate weeds once they have become established. There are many possible entry points for weeds:
- Vehicles and equipment—Weeds seeds and plant parts can arrive during construction on vehicles and equipment. Portable wash stations are often set up at staging areas and/or designated site entrance/exit points to thoroughly clean tires, wheel wells, and chassis of vehicles and equipment to reduce the possibility that weed seeds are brought in from other projects or areas or transferred within a project area.
- Erosion control seeding—Rye grasses are non-native and commonly used for temporary or permanent erosion control seeding. Seeds of these species are also found in material used for wattles. These grasses persist in the landscape and are difficult to eradicate once introduced. Unless these species are required by jurisdiction on the project, other species, such as native grasses used in conjunction with sterile hybrid grass seeds would satisfy erosion control needs and offer greater benefits to the environment. Erosion control matting works well to block the establishment of weeds while preventing erosion and facilitating the growth native plants.
- Hydroseeding tanks, range drills, and other seed delivery systems—Seed delivery systems, such as hydroseeding tanks and range drills, brought in from other projects, can contain plant species that are not wanted on the project. A thorough cleaning and inspection of this equipment are essential to eliminate the potential introduction of unwanted seeds. It is recommended that hydroseeding tanks are washed out and range drills air-blown before this equipment is brought on the project site.
- Seed sources—Commercial grass and forb seed sources, whether native or non-native, can contain weed seeds that were harvested along with the native seeds. The quantity of this material is dependent on the weed control practices and seed cleaning technology implemented by the seed producer. A good means of eliminating the possibility of contamination of native seeds with weed seeds, is to ask for seed tests of the seed being considered for purchasing. The purity seed test identifies the contaminants in a seed lot, including weed seeds, other plant seeds, and inert material. It is important to ask for tests that determine the presence of state-listed noxious weeds and other crop species (check state and federal lists to determine if any local weed species should be added to the testing list). Testing is conducted through certified seed labs per the standards of the Association of Official Seed Analysts. The seed should be cleaned and retested or reject the seed lot.
- Mulches—Buy mulch that is free of weed seeds. Hay and straw mulches are of special concern (Figure 3-85). Some states certify hay or straw as "weed-free" which means that the material is free of noxious weeds but not necessarily free of all seeds. Some straw comes from the stubble left after a seed harvest of native and non-native grasses, which often includes unharvested seeds. These seeds are viable and will establish into plants when the straw is applied. A site visit or discussion with the vendor prior to purchase is a good way to assess if there are seeds of unwanted species in the material.
- Compost sources—Compost sources are not always free of weeds, especially if the materials were not composted properly. Compost must reach lethal temperatures and remain there long enough for plant seeds to be rendered nonviable. Fresh, moist compost piles, where temperatures are maintained between 140 to 160 degrees F for at least several days, will kill most pathogens and weed seeds (Epstein 1997; Daugovish and others 2006). When obtaining compost, make sure that the supplier complies with standards that meet the time-temperature requirements to ensure destruction of weed seeds. Visiting the mulch sources or testing mulch for weed seeds may also be appropriate prior to purchase (Section 5.2.5).
- Gravel, sand, and rock sources—Prior to acquiring road building materials such as sand, gravel, and rock, determine if this material comes from a source that is free of undesirable weeds. A plant survey of the source area will identify whether the material is suitable.
- Haul routes and waste areas—Haul routes and waste areas can have weeds that may be transported around the project on vehicles and equipment. It is recommended that these areas be treated prior to construction or avoided during construction to reduce this risk.
- Salvaged and purchased topsoil—During the survey for salvaging topsoil, areas of weed populations are also typically identified (Section 5.2.4). To reduce the possibility that these weed populations are spread when topsoil is salvaged and reapplied, weed populations are avoided during topsoil excavation. Salvaged topsoil is stored in a manner that limits weeds from becoming established. For topsoil that is being considered for purchase, prior inspection of the piles will assure that the soil is free of weeds (Figure 3-86).
- Sand, gravel, rock, or topsoil storage—Inspect areas where topsoil, gravel, rock, sand, or other materials to be used on the construction project will be stockpiled. If there are weeds growing nearby, it is very likely that seeds will end up on these piles, especially if the piles are to be stored for over a year. Remove these populations prior to stockpiling materials and require stockpiles to be maintained free of weeds.
- Hedgerows and windbreaks—Planting windbreaks with non-native species, such as autumn olive, privet, honeysuckle, buckthorn, and multiflora rose, introduces fast-growing, highly competitive species (Harper-Lore and others 2013). Use of native species is preferable.
- Livestock grazing—Cattle can bring weed seeds from offsite where they have been grazing. Until native vegetation has become established, it is important to keep livestock out of the area.
3.11.6 Controlling Unwanted Vegetation
Stratifying the project by weed status
Locating weed populations early in the planning phase will help in developing a successful vegetation management strategy. During the initial vegetation assessment (Section 3.6.1) and topsoil surveys (Section 3.10.1), weed populations are typically identified and mapped to produce a weed status map.
The weed status map is typically composed of these 4 mapping units:
- State-listed noxious weeds—Areas where state-listed weeds are present and regulations require their control.
- Species of concern—Areas where species of concern are present. These are plants that are known to occur within the area, are not regulated for treatment by the State or County, but cause concern due to their population density, life strategies that provide them with a competitive advantage over desired native plants, dense growth in waterways, affinity for habitats specific to those required by desired specialist plant species, toxicity or ability to cause injury to livestock, etc. If the resources are available, the implementation of a new project can often provide a great opportunity to treat these species.
- Weed-free, non-functioning plant communities—These are areas where there is no state listed noxious weeds or species of concern present, but the plant communities are composed of exotic species, monocultures, poor pollinator species, or have depauperate plant communities with a large proportion of bare ground.
- Weed-free, functioning plant communities—Areas where a suite of native plant species is present, soils are intact, and there are minimal noxious weeds or species of concern present. These are optimum habitats for pollinators and larger areas may be wildlife refugia as well (Section 3.11.3).
Understanding the life history
Understanding the life history and ecology of the regulated noxious weeds and species of concern of the project area is important in developing a weed control strategy. There are several resources to learn about specific noxious weeds or invasive species. The USDA PLANTS database describes each weed species and has PDF documents called Plant Guides that detail the life history of the weed and how it can be controlled. Another source of information is the Invasive Species Assessment Protocol (I-Rank) website and covers many of the nonnative plants in the United States. This website describes some of the important characteristics of the weed species and ranks each on its threat to native plant communities and its difficulty to control. Because these websites are frequently updated, it is important to check the most current lists. Local County Weed Boards and county and State road maintenance staff are also good sources of weed information, as they track and treat specific weed species, often within the project area.
The important information to have about each weed species in the project area includes:
- Life form
- How it reproduces (i.e. vegetatively, by seed, both)
- Viability and longevity of propagule
- Reproductive period
- Mechanism and distance of propagule dispersal (i.e., wind, animal/bird/insect, explosive seed capsules)
- Life/growth strategy
- Life cycle
- Limiting factors to establishment
- Importance to pollinators
- Treatment options
Based on the ecological requirements and life history information, weeds can be grouped into two treatment groups: (1) species that are treated prior to, and during, construction (Section 3.11.6, see Weed Control during Pre-Construction and Construction Activities) and (2) species that are treated after construction (Section 3.11.6, see Post-Construction Weed Control and Vegetation Management).
Weed Control during Pre-Construction and Construction Activities
Construction activities often occur during the time that seeds are ripening or beginning to disperse, which increases the potential that unwanted seeds will be scattered throughout the project area. To effectively treat some state or county listed weeds and species of concern, it is often optimal to treat prior to, and during, construction. These species include:
Species with wide seed dispersal—These are species that spread seeds long distances, which can be achieved by a number of methods. Some plants have structures that forcibly eject seeds when disturbed or at maturity employ ballistic dispersal (e.g., brooms [Cytisis spp.), bittercress (Cardamine spp.), jewelweeds (Impatiens spp.]). Ballistic seed dispersal can often provide propagation advantage to shorter plant species.
Additional seed dispersal mechanisms that allow transport are gravity and wind dispersal (Figure 3-87). In general, the taller the plant, the further its seeds are dispersed. Plants that utilize wind dispersal frequently have appendages on their seeds (more often this would be fruits) to aid movement via wind. Yellow salsify (Tragopogon dubius) is an example in the aster family that employs wind dispersal, while the elm (Ulmus pumila) is a well-known weedy tree species in many areas that utilizes gravity and wind dispersal.
Bird and insect assisted dispersal is another strategy that facilitates movement of plant propagules across considerable distances. Seeds or fruits may stick to the feet or body of an animal and be transported as they walk or fly. Seeds can also stick to the feet of humans, including restoration designers and implementers. Animals can consume the seeds of plants, depositing them at new locations once the seeds pass through their digestive systems. Animals and insects often actively move seeds or vegetative material from one location to another to utilize them as an immediate or stored source of food or nesting material. Humans move the seeds of many of these species when walking and driving as well.
Any non-native plant species that disperse their propagules substantial distances from their source are ones for which pre-construction treatment should be considered.
Vine species—Species that form vines [e.g., English ivy (Hedera helix) and kudzu (Pueraria montana var. lobata)] are of concern when they are attached to trees that might be transported from one location within the project to another. If the trees are felled and skidded for removal from the project site, or for use for wildlife habitat, stream restoration, erosion control, or other practices on site, pretreatment of the vines is essential. Unless the vining plants are effectively killed prior to felling, the movement of the tree will spread seeds and vegetative parts of the vine across the construction area. This often necessitates treatment for at least one season prior to construction.
Species that propagate vegetatively—Many species spread vegetatively from portions of roots, stems, or both [e.g., Himalayan blackberry (Rubus armeniacus)]. If there is no control prior to construction, whole plants or fragments can be moved during soil excavation, clearing and grubbing activities, and movement of equipment with the result that the species will establish in disturbed areas. Because of the potential spread and tenacity of some species, the objective is to eliminate the risk of spread by the end of construction. Possible treatments include:
Applying herbicides—All state, local, and labeling laws must be followed when using herbicides. In addition, site specific stakeholders might have policies or regulations for herbicide use and their consultation can be very helpful. Herbicides affect plant species through a number of mechanisms such as disruption of cell division, regulation of growth, stopping photosynthesis, and many others. When possible, attacking the weed species through more than one mechanism will prove most effective. Most species that are considered weeds tend to spread readily and respond more vigorously following ground disturbance than many native species. Due to these factors, systemic herbicides are often desirable. The herbicide(s) used will be determined by regulations, the target treatment species, phenology of target species, tolerance for collateral damage to desired species, season of year, deadline for treatment, and more.
Applying herbicides and mastication—A strategy that has proven effective for monocultures of aggressive weedy species, such as Himalayan blackberry in the northwest, is to treat the plants with a combination spray of herbicide, followed by mastication or mowing, and then retreatment with herbicides. The herbicides used combine mechanisms of action as described above and are allowed to translocate for three to six weeks, depending on conditions and herbicides utilized. Once the herbicides travel through the plants' tissues, the above ground shoots are mowed to approximately two to four inches high. The shoots are then allowed to grow until sufficient leaves are developed so that another treatment of herbicide can be conducted. By doing this the plant has been attacked from a number of pathways: 1) multiple mechanisms of action through the herbicides used; 2) reduction of the photosynthetic material; 3) reduction of the seed source; and 4) eventual starvation of root tissue.
Scraping and removing—In cases when herbicide use is not desired or allowed, or when there is not sufficient time to conduct treatments, removal of the non-native weed species can be employed. The undesired plants can be scraped from the site during the clearing and grubbing process and buried at an agreed upon location or hauled off site to an approved facility. If an on-site spoils or waste area is to be used, it is important to include language in the special contract requirements to address the specific needs for containment. This often includes the prevention of further mixing or movement of soil once the weeds are placed, and installation of a "cap" of clean fill dirt on top of the weedy species.
Hand removal—If weed infestations are relatively isolated, have patchy distribution, or are in small populations they can often be removed by hand.
Post-construction weed control and vegetation management
Most unwanted vegetation can be treated after construction through revegetation contracts, however, long-term vegetation management is typically conducted by road maintenance personnel within a statewide Integrated Vegetation Management (IVM) plan or an Integrated Roadside Vegetation Management (IRVM) plan. Treatments, when they are selected through a decision-making process, include mowing, applying herbicides, mechanical removal, hand-pulling, grazing, fire, and biological control. A discussion of post-construction vegetation management is presented in Chapter 7.
For areas that have been identified as weed-free, functioning plant communities (Section 3.11.6, see Stratifying the Project by Weed Status), it will be important to maintain these areas in a sustainable manner. An approach to maintaining these areas is outlined in Section 7.2. Although hand removal can be labor intensive it is a great opportunity to involve community volunteers once construction activities have been completed.
For weed-free, non-functioning plant communities, it is important to decide whether to enhance the current plant community or accept the existing conditions and conduct no additional work. If enhancement is desired, contract components can be included to augment existing native plants to increase competition or to reset community succession, treat surrounding non-native plants, treat the soil to better support native plants, alter the light regimen to better support natives, and improve water drainage.
3.11.7 Designing for Safety and Utility Protection
Planting treatments along roadsides are limited by National Highway System (NHS) design standards and road development best practices guidelines provided by the American Association of State Highway and Transportation Officials (AASHTO) which are adopted by its member State departments of transportation (DOTs). Designers can find guidance on roadside revegetation treatments in key AASHTO guidelines that include the Roadside Design Guide, A Policy on Geometric Design of Highways and Streets (Green Book), Guidelines for Geometric Design of Very Low-Volume Local Roads, and A Guide to Achieving Flexibility in Highway Design. The design guidelines prescribe minimum highway/rural roadside clear zones and urban streetscape horizontal clearances or operational offset distances recommended for motorist safety. The zone distances from edge of travel lane or edge of roadway pavement are determined based on traffic volume, road speed, vertical alignment, and roadside slope conditions. Clear Zones are defined as unobstructed, traversable roadside area that allows a driver to stop safely, or regain control of a vehicle that has left the roadway (Figure 3-88) (FHWA 2008(B)).
Vegetation Treatment Zones (VTZ) are designations used by many DOTs to describe the vegetation coverage and maintenance requirements at certain distances off of the edge of roadway. VTZs typically correspond to highway clear zone distances. The VTZ dimensions, vegetation, and maintenance treatments vary by State and roadside conditions, but generally, DOTs have adopted a 3-zone treatment approach (Figure 3-89), (FHWA 2008(B). Zone 1 generally extends from the edge of pavement to the drainage ditch along the roadway and is composed of low native grasses that are regularly mowed. Some DOTs mow this area less frequently and some choose to spray it with herbicide to eliminate plant growth entirely. Zone 2 extends across the drainage ditch and a few feet beyond, and is composed of grasses, forbs and low shrubs. Maintenance of zone 2 is focused on maintaining drainage and removal of tree species. Zone 3 extends from Zone 2 out to the edge of right-of-way and may be covered with grasses, forbs, shrubs and trees outside of prescribed safety distances. This area is generally not mowed and may contain large shrub and tree species outside of the clear zone. Maintenance for all zones consists of removal and control of invasive plant species and removal of tree species within safety distances.
Clearance for "Line of Sight" and Safety—Trees and shrubs are often thinned or removed in areas where roadway or roadside line-of-sight is impeded. Good communication with the government agency responsible for maintaining the road during the planning phase will help identify those areas not suitable for shrub and tree species. Native grasses, forbs, and low growing shrubs (3 feet tall or less) can provide durable groundcover for these areas and not affect line-of-sight or motorist safety.
Trees Management Location—Trees that reach 4 inch diameter at breast height (DBH) are typically considered immovable objects that can cause heavy damage, injuries, and loss of life in a vehicle crash. According to a study in 2005, more accidents occur between 0 and 12 feet from the travel lane with significantly less between 12 to 30 feet (Mok, Landphair, Naderi, 2006). Tree masses close to the roadway can provide cover for large mammals and can contribute to WVCs. DOTs typically will not allow tree species in clear zones, unless they are protected with guardrail. Best practices for WVC reduction includes full tree and shrub removals or strategic thinning out tree and shrub masses close to the roadway in order to remove desirable cover for ungulates and to open views for motorists to see potential wildlife hazards.
Protection of Utilities—Planting of large trees under power lines can lead to damage of utility lines during wind and ice events and can make line maintenance access difficult and dangerous. Coordination of the roadside revegetation plan with the utility agencies of jurisdiction can identify utility easements, planting requirements, utility maintenance access needs, and additional utility accommodations.
3.11.8 Designing to Isolate Wildlife from Vehicles
According to a 2008 U.S. Department of Transportation study, "there are an estimated one to two million collisions between cars and large animals every year in the United States"..."commonly or typically...with deer (mule deer and white-tailed deer combined)...near forested cover and drainages." (U.S. DOT, 2008). The collisions often kill the wildlife and can cause high damage expenses, injuries, and even loss of life for the driver. New roadway corridors inevitably intersect wildlife crossings. The analysis of the local wildlife population, their traffic patterns, proximity of their habitat fragments, and any data on area wildlife-vehicle collisions (WVCs) in the pre-budget planning stage can inform the designers on the roadway design features to consider in order to reduce the probability of future WVCs along the corridor. It is important to keep animals out of roadway corridors but to also plan for their safe crossing above or below the roadway (Figure 3-90). Preservation and enhancement of wildlife corridors under or over roadways can be an effective method for reducing WVCs. Natural wildlife corridors often occur along creeks, rivers, and along drainage swales, features that can be preserved through the use of natural bottom culverts or clear span roadway bridge structures. Design of wildlife crossings to accommodate the needs, preferences, and tendencies of the animals that may use the crossings can maximize the likelihood of use and increase safety for all involved.
The design considerations vary with each project location. The FHWA has produced two best practices manuals for designers of roadway corridors and wildlife crossings entitled, Wildlife Vehicle Collision Reduction Study (Huijser, 2008) and the Wildlife Crossing Structure Handbook Design and Evaluation in North America (Clevenger/Huijser, 2011). Each contain recommendations based on specific studies of wildlife interactions with various wildlife crossing structures, treatments, and conditions.
Wildlife crossing design for motorist safety will typically focus on accommodations for the largest most prevalent animals in the area, often deer. Designing for a variety of wildlife considerations may require greater culvert or bridge clearance height and span length at each crossing, small tunnels for critter crossings, high-water crossings inside culverts (Figure 3-91), hundreds to thousands of feet of tall wildlife exclusion fencing or wildlife friendly/game fence that allows one-way animal pass through/pass back from the roadway (Huijser, Kociolek, McGowen, Cramer, and Venner, 2015), specific plant material to attract and guide animals to the crossing, or a combination of these measures. As the crossings directly impact the budget for the roadway project, and impact the safety of the public who will use the roadway, it is recommended that their evaluation and planning begin at the early planning stages of a new roadway alignment or bridge/culvert replacement project and that the revegetation expert be involved throughout the process.
Suitable habitat on both sides of the road is a necessary condition for all wildlife to cross, and areas with the highest quality habitat will often have the highest rates of crossing (Barnum 2003). Distance to cover is another factor that affects wildlife crossing use. Small animals prefer plentiful and consistent cover before and, if possible, through an undercrossing. Deer and elk tend to prefer crossing in open areas away from forest cover, especially during the winter (Clevenger and Waltho 2005; Barnum, Rinehart, and Elbroch 2007). Other animals prefer a more balanced composition of cover and open space while most carnivores prefer a dense forest cover.
Roadside revegetation design provides the opportunity to support safe wildlife crossings by creating a roadside planting and maintenance plan that will reduce animal browsing within the ROW, and provide planted conditions that will attract wildlife to the crossing points. Open views to vegetation beyond the undercrossing will encourage animal movement through the crossing to the other side, and continue on to wildlife corridors or refugia beyond. Ungulates, or hoofed mammals, especially deer, are browsers that prefer fresh new growth. Spring growth and new growth after cutting and mowing maintenance will attract ungulates and encourage them to linger and graze. Increasing mowing maintenance at wildlife undercrossings may increase more regrowth periods that can draw browsers to the crossing locations. Reducing the quantity of roadside mowing events away from undercrossings and strategically timing mowing operations to late fall or very early spring, can reduce the number of mass new growth events that draw ungulates to browse along the ROW.
Study results for vegetation control effect on WVCs indicate that flatter ROW side-slopes and lower vegetation along the roadway can improve driver safety and discourage use by large wildlife. Low plant material is less desirable for use as cover by ungulates, and opens up view-sheds so drivers can potentially see animals in the right-of-way (ROW) and allow time to react and slow down to avoid wildlife-vehicle collisions. Plant material 3 feet tall or lower, not counting seed head stalks that may rise taller late in the year is generally considered low. Elimination of large trees from roadway clear zones can remove desirable wildlife cover from the ROW and immovable tree trunks that can damage vehicles and injure motorists that leave the roadway.
Wildlife over-crossings are still rare in the United States but are preferred by ungulates and other species more than under-crossings. Ungulates and other species will use under-crossings if they appear open and free from predator hiding places. They generally prefer level open space leading up to the crossing, a generous tall and wide bridge span or box culvert, natural low vegetation or soil walking surface, and clear views to open space on the other side.
Easy access to wildlife crossing locations can increase their use. If an existing ROW fence exists beyond the wildlife crossing area, it is recommended that portions of fence be removed if possible. In the case of cattle or other livestock use outside of the ROW, a section of fencing may be replaced with a gate that remains open when the field is not occupied. Additionally, wildlife friendly adjustments may be made to the top and bottom strands of barbed wire, such as adjusting the wire spacing and/or replacing with smooth wire on the top and bottom to protect wildlife that jumps over or crawls under the fence.
High-use crossing locations and those with narrow rights-of-way may need the addition of protective traffic barrier and guardrail and/or lengths of tall animal fencing to help encourage animals back down to the under-crossing. Resources that provide wildlife control methods to consider for specific site conditions include Best Management Practices for Wildlife Corridors (Beier, 2008), Wildlife Vehicle Collision Reduction Study (Huijser, 2008), the Wildlife Crossing Structure Handbook Design and Evaluation in North America (Clevenger/Huijser, 2011), Construction Guidelines for Wildlife Fencing and Associated Escape and Lateral Access Control Measures (Huijser, et al., 2015), and Implementing Measures to Reduce Highway Impacts on Habitat Fragmentation (Louis Berger Group, 2011).
Experts recommend that wildlife crossings remain open and clear overnight, during the construction process if possible. These areas should be completed and planted as early as possible in order to reduce animal stress and to keep them from learning new less desirable places to cross the roadway. Supplemental hay and salt licks may also be used to encourage continued crossing use during construction; supplemental feeding may then taper off over a few months after construction.
Plant selection for wildlife crossing locations can be approached as seeded roadside pollinator habitat development supplemented with low native forbs and grasses that are preferred by wildlife for food and browse.
3.11.9 Designing for Disturbances
Disturbances that affect roadside vegetation often occur after plants have become established. While some of these disturbances are unforeseen, others can be expected. The designer may want to consider what disturbances can be expected and how they may be mitigated within the design of the roadsides and revegetation plan.
Deicing for Winter Safety
Approximately 70 percent of the roads in the US are in snow regions (FHWA 2012) which may require deicing practices to make them safe and passible during winter periods. Most deicing materials contain chloride-based salts which, when applied road surfaces, lowers the freezing point and melts snow and ice. Solid salt (NaCl) is the most common product used, following by calcium chloride (CaCl2), magnesium chloride (MgCl2), potassium acetate (KAc) and calcium magnesium acetate (CMA) flakes for bridges (AASHTO 2013). The suggested rate of NaCl is 100 to 300 pounds per lane-mile (Salt Institute 2008).
Potential Impacts to soils and vegetation—Deicing materials can pose a risk to soil properties and plant growth. Salt concentrations in roadside soils correlate positively with salt application rates (Jones and others 1992) and high levels of sodium disperse soil organic and inorganic particles, reducing soil permeability and increases runoff (AASHTO 2013a). The negative effects on plant growth, however, are often associated with road spray on plant foliage rather than presence of salts in the soil. Roads that are treated with deicing materials does not preclude that plants on adjacent roadsides will be affected. The effects of road salts on soils and plant depend on the:
- Sensitivity of plant species to salt—At very high salt levels in the soil, germination of native plant species can be reduced (Harrington and Meikle 1992, Fulbright 1988) or delayed (Ungar 1992) though there is considerable variability between native species. In general, shrubs and grasses tolerate salt concentrations better than trees (Sucoff 1975, Bryson and Barker 2002). And sensitivity among tree species ranges from sensitive to tolerant. In the Lake Tahoe Basin, for example, two and three needle pines (e.g. Jeffrey, ponderosa, and lodgepole pines) show salt damage more frequently than white and red fir (University of Nevada 2009).
- Amount of salt applied annually—The amount of salt damage is related to the total amount of salt applied during the winter. The higher the quantities of salt applied, the greater the effects to soil and plant quality. The University of Nevada (2009) found that the proportion of trees affected by salt damage coincided with the annual quantity of salts applied to the road surface.
- Distance from the road—The highest amount of salts occurs closest to the road and diminishes moving away from the road surface. Ninety percent of salt deposition from road spray often occurs within 65 feet of the roadside (Blomqqvist and others 1999) and it is in this zone where foliar damage such as needle necrosis, twig dieback and bud kill on trees happens.
- Type of salt—The type of deicer may affect plant species differently. Trahan and Peterson (2008) found that MgCl2 was more damaging when directly applied to tree foliage than NaCl. Calcium magnesium acetate was found to be less toxic on certain grass species than sodium based salts (Robidoux and Delisle 2001) and may even improve soil properties by increasing permeability and providing calcium and magnesium for soil fertility (Fritzsche 1992).
- Precipitation—In areas of high precipitation, salts will become diluted and move through the soil profile, reducing the potential negative effects to seed germination and plant growth. In areas of very low precipitation however, salts and sodium will remain at the soil surface and accumulate.
- Soil type—Soils with low pH values may benefit from some addition of road deicers. By raising soil pH, certain nutrients become more available (Section 3.8.4). Soils already high in sodium could become even more toxic to plant growth with NaCl additions.
Assessing potential impacts of deicing practices —During the planning phase, it may be important to assess the level of impact deicing practices have on native plant establishment and growth after construction. The type and amount of deicing material used on the highway project can be obtained from maintenance and operations records. If the quantities are considered high (Figure 3-47), then it may be beneficial to conduct a field survey of soils and vegetation along a stretch of road in or similar to the project area. The survey area can be stratified into zones parallel to the road alignment because the effects of salts on vegetation composition and health grade from most affected to least affected, moving away from the road. Specific monitoring procedures that may be helpful include Soil Cover (Section 6.3.1), Species Cover (Section 6.3.2), and Species Presence (Section 6.3.3), using a Rectilinear Sampling Area design (Section 6.3.6, see Rectilinear Areas), because the narrow width of each sampling area is not conducive to using transects.
Comparing the differences in each zone may show the effects of deicing practices on vegetation. If there are no differences between each zone, then it can be assumed that deicing practices have no effect. If there are differences, then those differences are taken into consideration when developing seed mixes or soil improvement practices. It may be important to determine if these differences are due to deicing practices or due to other factors, such as mowing, surface contaminants, soils, air pollution, drought, or tree diseases or pests. One method to make this determination is to collect soil samples in each zone and measure soil conductivity using a pH meter (Section 6.3.1). If conductivity readings recorded in the zone next to the road are higher compared to the zone furthest away it would indicate that deicing practices maybe responsible. Refer to Figure 3-47 for interpreting conductivity values that affect plant growth.
Mitigating for deicing practices—If road deicing salts are determined to be detrimental to plant growth, the designer may want to select a species mix that has a higher tolerance for soluble salts. Selecting the tolerant plant species can be determined from information collected during the vegetation assessment (Section 6.3.1). Desirable species growing in the deicing zone are good species to consider in species mixes. The ERA tool may also give some guidance on those species most adapted to high salt environments.
Gravelling for Winter Safety
In areas where gravels are frequently applied to road surfaces during snow or icy conditions, there can be a buildup of gravels on the road shoulders. Vegetation that has been established in these areas is often completely covered with gravel after snow melt. In addition, road maintenance often excavates these gravels to reuse and in the process, removes established plants growing in this zone. The designer will want to design revegetation treatments in these areas according to the expected disturbance. In areas where gravels are not salvaged, the designer may want to select plant species that survive and grow well in this unique growing environment. Some species respond favorably to being covered by gravels. These include species such as manzanita and willows that root from their stems when covered by soil or gravel. Tap-rooted species, such as lupines, can take advantage of such conditions because they can access moisture deep in the gravel deposits (Figure 3-92). A vegetation assessment (Section 6.3.1) of the road shoulders of the project area will identify species that have adapted to these conditions. In areas where gravels are annually removed, the designer will want to identify the width of this disturbance and remove the area from the revegetation plan.
Annual roadside maintenance
Ditches at the base of steep cut slopes are depositional areas for rock and soil that have moved down from the slopes above. This material fills ditches, disrupting the flow of water and creating potential road drainage problems during storms. Blading is the removal of material that has filled in the ditchline and is a normal maintenance procedure for erosive cut slopes. This operation not only removes plants that were established in the ditchline, but also destabilizes the surface slopes immediately above the ditch which can affect the revegetation of the entire slope. Designing cut slopes so that they are stable is one method of reducing the need to blade ditches. This includes reducing slope gradients near the ditches and establishing a good vegetative cover that resists slope movement.
The road corridor is sometimes used for recreational purposes that can disturb established vegetation. This recreation is not usually sanctioned or intended, but it exists in certain areas nonetheless. Recreational disturbances include off-road vehicle travel, mountain bike use, trails to recreational sites, parking, and Christmas tree cutting.
It is important for the design team to identify the public's demand for recreational activities and to determine how these activities might affect short- and long-term vegetation goals. For example, abandoned roads that have been revegetated are often desirable places for off-road vehicles because they are open and flat. Roads bordering recreation destinations, such as favorite fishing spots, may have demands for access trails or scenic views that the public does not want blocked by tall vegetation. Public scoping often identifies these needs. There are several approaches to mitigating the effects of recreational impacts, most of which are forms of awareness, protection, and exclusion. Intruders can be excluded physically with barriers, such as ditches, fences, down trees, and large rocks outside of the clear zone. Before these measures are put in place, communication should be tried first. For example, a sign explaining native revegetation efforts may help make potential users aware that they should take their activities elsewhere. Local residents are often great sources for ideas on how to approach these problems; off-road vehicle clubs are another. Educating the public on the purpose for the revegetation treatments can go a long way toward protection. Short paragraphs in the local newspapers or on the FHWA website for each project may help. Using local contractors to implement the revegetation work and engaging local residents brings ownership to the project. In addition to exclusion measures, designers should consider desire paths to accommodate foot traffic that is bound to occur. Incorporating and planning for such access can reduce the impact to the surrounding established vegetation and keep traffic confined as much as possible.
Damage to revegetation projects can be high in areas that are intensively grazed by cows or sheep. In areas with large livestock populations, planted tree seedlings can be injured by rubbing and trampling. Newly establishing native grass and forb cover can be harmed through grazing and by the high-pressure hoof marks tearing up the new roots and surface soil, leaving the site exposed to non-native annual species.
Restricting the entry of livestock for several years after planting or sowing, or until native grasses and forbs have established, is the best prevention measure. This is typically accomplished by fencing the entire area being revegetated. Fencing is most effective when it is installed prior to establishing native vegetation. For this reason, having the fence installed as part of the road contract will ensure that livestock is controlled prior to revegetation work. Working with the local USDA Forest Service or USDI Bureau of Land Management range conservationist will be necessary to ensure that damage by livestock is kept to a minimum.
3.11.10 Designing for Carbon Sequestration
Carbon sequestration is an important environmental and public health benefit that is a result of revegetating disturbed landscapes with native plants. It is often overlooked as a revegetation objective and as part of a vegetation management strategy. The "carbon sequestration capacity" is a quantifiable volume of carbon that can be estimated for existing roadside revegetation and compared to the proposed revegetation plan. Knowing the values in the planning stage can help the design team as they make decisions about design, implementation, and maintenance throughout the project development. Selection of plant material and the ongoing vegetation maintenance procedures have a dramatic effect on the carbon sequestration capacity of a revegetation project.
Carbon sequestration is a process in which CO2 is transferred from the atmosphere into plants through photosynthesis and stored in long-term carbon pools. These pools consist of above-ground biomass (e.g. live trees, shrubs, grasses, and standing dead trees, branches, litter, and duff) and below-grown biomass (e.g. soil organic matter, roots, organisms). Roadside management practices that maintain or increase these carbon pools may reduce atmospheric concentrations of CO2 and mitigate the effects of climate change (Proudfoot 2015). With the large land base of the US in roadsides, the current and potential capacity to capture CO2 is considerable (Ament 2014). For example, roadsides along US highways and federal lands (10.5 percent of all public roads) currently capture nearly 2 percent of the total US transportation carbon emission (Lavelle 2014). Another way that atmospheric carbon can be reduced is by decreasing or changing roadside maintenance operations that generate greenhouse gas emissions such as mowing and mechanized pesticide applications. In combination, practices that reduce carbon emission and increase carbon pools can reduce atmospheric carbon while reducing maintenance costs (Proudfoot 2015).
Minimizing Soil Disturbance
Soils contain large amounts of carbon fixed in soil organic matter. When soils are disturbed, CO2 is released through the oxidation of organic matter. One of the best strategies for maintaining carbon in soils is to minimize soil disturbances. In planning road construction projects, this is accomplished by minimizing the footprint of the project. Another strategy is to create and maintain a resilient plant community that resists disturbances associated with soil erosion and landslides. When disturbances do occur, immediate action assures the quick recovery of native plants and carbon sequestration processes (Section 3.11.10, see Create a Good Growing Environment).
Revegetating with Trees and Shrubs
Establishing and maintaining trees and shrubs along roadsides can be a cost-effective means of capturing carbon (Brown 2010). Compared to other vegetation, trees sequester larger amounts of carbon for longer periods of time (an average of 120 years, FHWA 2010). Trees also shade ground surfaces, reducing the amount of heat generated from roadsides. Shrubs have less capacity to store carbon than trees but greater capacity than grasslands (Ament 2014).
There is an opportunity to use vegetative plantings to create or replace wind breaks, shelterbelts, and snow fences (see Inset 3-4). Using trees and shrubs for these purposes will not only reduce blowing and drifting snow, but can increase wildlife diversity, pollinator habitat, and capture carbon. Using a thick vegetative barrier composed of shrub species near the roadway may also have the added benefit of slowing out-of-control vehicles from roadsides impacts (Ament 2014).
It is important that trees and shrubs are planted in areas that meet road safety and maintenance objectives. When choosing tree and shrub species to plant, species that live longer and are larger at maturity have a greater capacity to store carbon than shorter lived, smaller plants (Proudfoot 2015). In addition, increasing the complexity of a forested site, by planting a multilayer of trees, shrubs, grasses and forbs, has the potential of increasing carbon sequestration (Ament 2014).
Revegetate with Perennial Grass, Forb, and Wetland Species
Perennial grasses store more carbon in the soil than annual grasses (Cox and others 2006) and can sequester carbon for up to 50 years (FHWA 2010). In addition, perennial grasses have greater ground cover than annual grasses which protect soils from surface erosion, water loss, and nutrient loss (Glover 2005), important for optimizing carbon capture. Wetland species capture more carbon than grasses and forbs because of the higher productivity of wetland sites. For this reason, wetland swales are preferable to dry swales (Bouchard 2013).
Create and Maintain a Good Growing Environment
Revegetation planning that promotes healthy function ing plant communities is good for carbon capture and maintenance (Ament 2014). Restoring soils with organic amendments, tillage, mulch, and nutrients will increase the rooting depth and productivity of roadsides to store more carbon. Other practices that maximize slope stability and minimize surface runoff reduce the potential that soils are disturbed which maintains soil carbon. Where road sections are being abandoned or recontoured, restoring the soils and reestablishing perennial vegetation, such as shrubs and trees, have the potential to capture and store carbon while increasing pollinator habitat.
Utilize Site Resources
Land clearing during road construction often creates woody material that is placed in piles and burned. This material can be processed and placed on constructed roadsides as a soil amendment or mulch. Unprocessed woody material, such as logs, can be used as wildlife structures in areas where they meet maintenance objectives. Depending on site factors, processed and unprocessed materials can last from years to decades, temporarily storing carbon before they decompose. How salvaged topsoil is removed and stored may also make a difference in how much soil carbon is oxidized during road construction. Removing topsoil when it is dry and keeping it dry during storage reduces the potential for oxidation of organic matter.
Changing Mowing and Pesticide Practices
Changes in mowing and pesticide practices can directly reduce carbon emissions and increase carbon storage (Dunn 2013). A review of maintenance practices, when developing a vegetation maintenance strategy (Section 3.11) and an Integrated Vegetation Management plan (Chapter 7), can highlight areas where changes can be made. In addition to decreasing carbon emissions, changes to practices can also lower maintenance costs because of reduction in fuel and wages, and can be beneficial to pollinators (Section 7.3.2). Maintenance practices that can be adapted to reduce emissions and increase carbon capture include:
- Mowing times—Shifting mowing times from active growing periods (when mowing disrupts the flow of carbon to the soil) to times of the year when plants are more dormant (early spring, fall, and winter), will increase carbon capture (Dunn 2013).
- Frequency of application—Cutting back on the frequency of mowing or pesticide applications reduces carbon emissions.
- Height of mowing equipment—Raising mowing equipment several inches higher can save fuel costs and reduce the effects of carbon flow to the soil.
- Treatment widths—Reducing the widths of mowing and pesticide applications reduces travel time, amount of pesticide used, and carbon emissions.
Reducing Road Salts
On road systems where applications of deicing salts are detrimental to roadside vegetation (Section 3.11.9, see Deicing for Winter Safety), minimizing the quantity of salt applied or the frequency of application, can reduce the effects on plant productivity and carbon sequestration (Ament 2014). These changes will also result in less carbon emission.
Highway Carbon Sequestration Estimators
Numerous highway carbon sequestration estimators can help calculate the quantity of carbon being captured on a roadside and estimate the potential volume of carbon offsets. These tools do not necessarily provide estimates required for full project development and should be used only to provide a sense of scale (Proudfoot 2015). One such program is the Highway Carbon Sequestration Estimator. This tool is intended to help DOTs assess the return on investments for carbon sequestration practices based on state-specific considerations (FHWA 2010).