ROADSIDE REVEGETATION

An Integrated Approach to Establishing Native Plants and Pollinator Habitat

1.2 The Ecological Context of Roads

Our road system infrastructure is large, covering over 17 million acres (Ament, Begley, Powel, Stoy 2014), widespread, and affects all but our most protected lands (NRC 2005). The total road corridor (paved road plus roadside or right-of-way) covers over 1 percent of the nation's land surface, an area equal to the size of South Carolina (Forman and Alexander 1998). If unpaved roads are also included, the percentages increase (FHWA 2008). The ecological effects of roads extend into a zone far beyond the edge of the pavement, with effects including habitat fragmentation, wildlife mortality, noise and chemical pollution, disruption of hydrologic cycles and water quality, increased erosion, and the potential creation of transportation corridors for noxious and invasive weeds that can invade adjacent lands. With these considerations, an estimated 15 to 20 percent of the United States is ecologically affected by roads (Forman and Alexander 1998). The enormous challenge of understanding and mitigating the ecological effects of roads deserves attention and dedication on local, regional, and national scales.

1.2.1 Past Oversights

Much of the existing road network in the United States was designed and constructed prior to the 1970s, in an era before ecological health became a widespread concern among American citizens and before ecological science had evolved to address large-scale issues (Forman and others 2003). Safety and efficiency were the primary goals of transportation programs in the past, and the ecological context of roadways were largely overlooked in planning, construction, and maintenance efforts. The effects of roads on natural systems (habitat fragmentation; interruption of natural flows of water; and disturbances to animals, plants and their pollinators, soils, and other resources) were not well understood or considered. Lack of awareness about these factors led to a largely antagonistic perception of the relationships between natural systems and road systems.

For example, without effective revegetation of the road disturbance with desirable plants, undesirable vegetation can encroach on the roadway. Undesirable vegetation can disrupt safety and visibility, leading to expensive and potentially hazardous maintenance measures. Undesirable vegetation can invade and wipe out areas of desirable native plants that pollinators need for food and cover to survive. Conflicts with neighboring land uses could result if corridors for invasive weeds are established or if vegetation control measures are viewed as a health or safety concern by the community. These are all issues that arise when ecology and revegetation science are not considered during road design, construction, or modification. Eventually, poorly integrated or addressed natural processes can threaten the function and structural integrity of the road itself, leading to premature deterioration of the road's infrastructure (Berger 2005).

1.2.2 Present Awareness

For over 20 years, the ecological effects of roads have been increasingly recognized by the Federal Highway Administration and by state and county transportation agencies (NRC 2005). Today, road effects on ecological processes are major concerns among private citizens, land management agencies, and the transportation community. Consequently, an emphasis has been placed on the integration of ecological considerations into all phases of road design and construction processes. For example, legislation in some areas now requires road modification and construction projects to restore aquatic connectivity; fish passages have been built to reconnect natural water flows under roads. Other projects have modified roads that were deemed particularly dangerous to endangered species. These roads are being made more permeable to wildlife, greatly reducing losses by improving habitat connectivity, ensuring better visibility for drivers and animals, and creating safer underpasses or overpasses for wildlife (Forman and others 2003). Given the recent decline of pollinator species, there is now a greater emphasis on supporting pollinators by creating habitat on roadsides. A National Pollinator strategy (FR 2014) has recently been released, tasking federal agencies to do more with their land-holdings, development practices, and maintenance operations to support pollinators in order to reverse pollinator declines (Section 1.5.4). Efforts to limit inappropriate road expansion and to obliterate unnecessary roads remain important. Where modification and increased capacity are needed, ecological health, safety, and efficient transport should not be seen as mutually exclusive goals. Understanding roadside environments, how they interface with adjoining lands, and how to minimize environmental impacts has become a key focus of the Federal Highway Administration. Given political will and proper levels of attention, integration of environmental concerns with transportation can result in significant gains.

1.2.3 Trends in Road Construction

For the purposes of this report, the primary focus is on Federal, State, and County road corridors, that include highways, interchanges, rural routes, farm to market routes, river roads, and most roads through our National, State, and County parks and preserves. Roads are widespread, fairly permanent fixtures on the landscape and in the culture of the United States. Given current trends, road networks are expected to persist and expand over time. Current modifications predominantly involve updating infrastructure to increase capacity and to improve safety, including widening roads, replacing bridges, and reducing or altering curves and grades to make the road safer for motorists (NRC 2005). The opportunity to integrate ecological goals with transportation was largely overlooked when the road networks were originally constructed. However, as the nation's roads are being updated and modified, the opportunity cannot be ignored. While attempts to integrate ecological factors are positive, much of the potential for improved integration is still largely unrealized. This has been due, in part, to a shortage of practical information and the absence of an integrated approach to the challenge. The question is, what can be done to balance societal desires for safe, efficient transport with requirements for a healthy environment? In other words, what can be done to help road systems function better with natural systems?

1.2.4 Challenges and Opportunities

The fact that the nation's road networks are in varying states of updates, repairs, and maintenance presents an opportunity to improve road systems so that they integrate better with natural systems. Planners and designers strive to understand detrimental effects associated with roads and how to mitigate them by minimizing the ecological footprint of roads and maximizing potential ecological benefits. Many groundbreaking resources have emerged to support these efforts. Road Ecology: Science and Solutions (Forman and others 2003) places the challenges into comprehensive frameworks, illuminating goals and principles for an ecological approach to transportation issues. Multiple intervention points are identified to help road systems function better with natural systems, integrating transportation goals for safety and efficiency with approaches to protect water, soil, vegetation, wildlife, and aquatic life. The Federal Highway Administration published a landmark book called Roadside Use of Native Plants (Harper-Lore and Wilson 2000) that brought the issue of native plant communities on roadsides to the forefront in the transportation community, highlighting the importance of native plants and their broad utility. The National Research Council of the National Academies of Science expanded on the frameworks identified in Forman and others (2003) in its publication, Assessing and Managing the Ecological Impacts of Paved Roads (2005). Processes within the Federal Highway Administration, state departments of transportation, and other agencies are being improved for better integration.

Figure 1-2 | Recently planted trees on an obliterated section of highway in Oregon

Most road projects today do not involve building new roads, but rather modifying or obliterating existing roads. This photograph shows an abandoned road where the soils were restored and seedlings planted.

Photo credit: Lynda Moore, USFS

In addition to these advances directly related to roads, many advances in the field of restoration ecology have distilled essential principles applicable to severely degraded sites (e.g., Munshower 1994; SER 2004; Clewell and others 2005; Claassen 2006). In addition, vegetation specialists from a variety of organizations have come to consensus about what truly defines a "native" plant and have developed seed collection, transfer, and propagation guidelines to ensure that locally adapted materials are used for optimum results (Withrow-Robinson and Johnson 2006, Johnson and others 2010, Basey and others 2015). In both the public and private sectors, seed and plant producers and installers have been developing innovative methods to meet unique site conditions.

Recently, the opportunity to support declining pollinator species through habitat creation on roadsides has been addressed with great success (Hopwood and others 2015, Hopwood and others 2016a, and case studies in this report). Designers have created "pollinator-friendly" roadside habitat, and agency maintenance departments have altered procedures to better maintain these habitats with a focus on the needs of pollinators.

While the publications above assess the best available conceptual and theoretical information, each also recognizes extensive needs for further work, particularly in developing practical approaches to integrating ecological needs with transportation goals. Central to ecosystem function is native vegetation (SER 2004). However, much of the pertinent information related to protecting and establishing native plants on roadsides has been scarce, scattered, unexamined, or not translated into practice. This report is intended to bridge some of the informational, technical, and organizational gaps to facilitate successful roadside revegetation with native plants. An integrated approach is offered to support both designers and field-based practitioners in successfully revegetating roadsides and obliterated roads with native plant communities (Figure 1-2).

1.2.5 Why Revegetate Roadsides with Native Plants?

Long-term economic and ecological advantages can be gained by establishing desirable native plant communities on roadsides (Berger 2005). Roadside vegetation can support safety goals by reducing headlight glare, reinforcing the road alignment, serving as crash barriers, protecting view planes and visibility, controlling snow drifts, and reducing wind speeds (Forman and others 2003). Pollinator-friendly plants species, many of which are showy flowering plants, can improve the experience of the road user by creating natural beauty via plant form and color diversity along the roadside, in addition to improving driver performance by reducing monotony and stress. Importantly, creating pollinator habitat along roadsides can directly support imperiled pollinators such as the iconic monarch butterfly (Danaus plexippus), economically important managed species such as the European honey bee (Apis mellifera), as well as a wide variety of native pollinators including wild bees, butterflies, moths, flies, beetles, and wasps. A self-sustaining native plant community on a roadside stabilizes slopes, protecting water and soil quality. In addition, the establishment of healthy native plant communities is often the best long-term defense against invasive and noxious weeds. Maintenance costs for managing problematic vegetation are reduced, as is the pollution and controversy that sometimes results from roadside herbicide use (Berger 2005). Establishing healthy roadside vegetation can also help sequester carbon dioxide, one of the factors responsible for global climate change (Palumbo and others 2004, Ament and others 2013).

Using native vegetation supports every aspect of the goals identified as best management practices by the transportation community for road design. These include goals to:

  • Produce a safe, cost effective, environmentally friendly, and practical road design that is supported by and meets the needs of the users
  • Protect water quality and reduce sediment loading into water bodies
  • Protect sensitive areas and reduce ecosystem impacts
  • Maintain natural channels, natural stream flow, and passage for aquatic organisms
  • Minimize ground and drainage channel disturbance
  • Control surface water runoff and stabilize the roadbed driving surface
  • Control erosion and protect soil
  • Implement slope stabilization measures and reduce mass wasting
  • Stormproof and extend the useful life of the road (Keller and Sherar 2003)
  • Create and maintain pollinator-friendly habitats (Hopwood and others 2015)

Figure 1-3 | Unsuccessful roadside revegetation on steep slopes

Steep slopes are often difficult to revegetate and many past attempts at roadside revegetation did not succeed.

Photo credit: Lynda Moore, USFS

Clearly, the goals of safe and efficient transportation and the goals of establishing and protecting native vegetation overlap; when properly integrated, native vegetation supports road objectives. At the same time, considering vegetation as part of road planning processes aids in minimizing and mitigating the ecological footprint of roads during and after construction. Native plants can provide wildlife habitat and improved connectivity for the length of the road (Forman and others 2003). Understanding vegetation and forage preferences, and careful design that accounts for visibility and safety, can guide animals to safe passageways for travel while minimizing dangerous interactions with vehicles. The presence of birds and small animals can be enhanced when appropriate plant species are established. Processes that work for roadside revegetation are also applicable to the process of obliterating roads where roads are no longer needed.

Despite the potential benefits, many past attempts at roadside revegetation have not succeeded. Although revegetation was considered important, some efforts emphasized seeding of exotic plants; these species were perceived as inexpensive, readily available, and easy to establish on disturbed sites, however, this practice has not been effective or self-sustaining on many projects; either the exotic plants spread to become problematic, or failed to persist because they were not locally appropriate species. Once established, exotics may preclude reintroduction of desirable natives. In other cases, little consideration was given to establishing roadside vegetation during or after construction; if vegetation was considered, it was often as an afterthought. A short-term approach to revegetating roadside disturbances often predominated past efforts, while efforts toward long-term development of native plant communities did not receive adequate consideration. The ineffectiveness of revegetation efforts in the past has resulted in such problems as soil erosion and landslides that affected water quality (Figure 1-3). Visually, unvegetated road disturbances diminish the experience of the road user and economically translate into high costs associated with ongoing maintenance.

Past shortcomings may be attributed to past approaches that were often piecemeal and lacking the cooperation and coordination of disciplines necessary to fully integrate native vegetation into the road planning and construction processes. Revegetation specialists typically worked in isolation from engineers, and sometimes even the biological specialists (soil scientists, botanists, wildlife biologists) failed to coordinate their knowledge and efforts. Success will require both practical and technical information and a systematic, comprehensive approach.