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SUMMARY OF KEY FINDINGS

1. Harm to Wildlife

Roads displace species sensitive to disturbance or dependent on forest interior habitat. For example, species like grizzly bears, wolves, and elk avoid otherwise suitable habitat near roads. They may modify their home range, and they have been shown to select areas with lower road densities than the average on the landscape. As a result, high-quality habitat becomes effectively unavailable to them.

Roads also create barriers to the movement of many species. In particular, small animals such as salamanders, frogs, and mice will rarely cross roads or are killed by vehicles when crossing. As barriers to dispersal, roads isolate populations on one side of the road from those on the other. Biologists fear a likely consequence of this isolation and the resultant loss of gene flow between populations is, over the long-term, increased vulnerability of some species to inbreeding or environmental catastrophes. Larger species such as moose, white-tailed deer, and mule deer also have high rates of mortality due to roadkill. The impact of high roadkill rates on species’ total population number is not thoroughly understood, but roads may act as ecological "sinks" (areas of net loss) for some species, endangering their continuing viability.

Species that live in the forest interior are also adversely affected by logging, which fragments habitat by destroying old-growth and mature forest and by creating poor quality habitat "edges" in otherwise continuous forest. Species that prefer old-growth and closed canopy forest, such as martens, California red-backed voles, northern flying squirrels, and red-backed salmanders, to mention a few, have declined in abundance in logged forests relative to unlogged forests. These species’ decline in turn has impacts on other parts of the ecosystem. For instance, a reduction in salamander populations (which are an integral part of the forest food chain) affects bird and mammal species that rely on them as food, as well as forest floor ecology and nutrient cycles.

Research has also shown that forest interior birds are sensitive to disturbance, whether in the form of roads, logging, or the increased access they provide to nest predators or parasitic cowbirds. Breeding success has declined in many of these affected areas. For example, bald eagle nesting success is lower near clearcuts. In eastern forests, cowbirds have invaded the edges (and in some cases, the interior) of many forest fragments. This species parasitizes the nests of other bird species, destroying their eggs or young and laying its own eggs in the nest instead. Research has shown that this behavior may be partly responsible for the population declines of many neotropical migrant bird species.


Displacement of wildlife

  • Grizzly bear use of suitable habitat in Montana declined as road density and road traffic increased.

  • Grizzly bears used habitat near roads less than expected in the Northern Rockies, resulting in less habitat available in roaded areas.

  • Black bears crossed roads with higher traffic volumes less frequently than roads with lower traffic volumes.

  • Habitat occupied by wolves in Minnesota had a lower road density than unoccupied habitat.

  • Wolves showed a preference for areas with low road density rather than high road density when establishing packs in the northern Great Lakes region.

  • Wolves in Alaska avoided roads that were open to regular public use.

  • Roadless areas are important reservoirs for maintaining wolf populations in adjacent, high-road-density areas.

  • Mountain lions avoided improved dirt roads and hard-surfaced roads and selected home range areas with lower densities of these road types.

  • Female Roosevelt elk reduced their daily movements, core area size, and home range size, and therefore, their energy needs, when disturbance due to vehicular access on roads was limited by gate closures.

  • Mule deer and elk avoided roads and areas within 200 m of roads.

  • Columbian black-tailed deer were displaced from their usual home ranges by increased vehicular traffic during the hunting season.

  • Eastern massasauga rattlesnakes avoided roads in all seasons.

  • Prairie voles and cotton rats tended to move away from a narrow dirt road rather than toward it.


Barriers to dispersal

  • Roads were a barrier to movement by the eastern chipmunk and the white-footed mouse.

  • Highways were a barrier to movement for seven of 10 rodent species studied.

  • A narrow dirt road was a significant barrier to movement by prairie voles and cotton rats.

  • A highway in southern Nevada acted as a barrier to crossing for all eight rodent species studied.

  • Roads impeded movement by amphibians and could result in population isolation. Despite some speculation, road ruts and ditches have not been shown to provide successful amphibian breeding habitat rather than acting as ecological traps. Amphibians play a key role in the forest ecosystem, affecting nutrient cycling and also serving as high quality prey for many species.

  • Roads impeded dispersal of all six amphibian species studied.

  • Road mortality of 7 amphibian species, 10 reptile species, 21 mammal species, and 62 bird species was documented during four years of study, exceeding 32,000 individuals on a 3.6 km stretch of highway.

  • Frog and toad density near paved roads decreased with increasing traffic intensity.

  • Frog and toad mortality on roads increased with increasing traffic intensity.

  • Road mortality of 20 species of snakes was recorded along a 44-km stretch of highway passing through Organ Pipe Cactus National Monument, Arizona.

  • Mortality due to roadkill was documented for northern saw-whet owls and eastern screech-owls over a 10-year period in New Jersey.

  • Road mortality along a highway in Ohio was surveyed for one year and included 11 species of mammals, 12 species of birds, 11 species of amphibians, and at least 249 species of insects.

  • Collision with a vehicle was the highest cause of death for female moose studied in Alaska.

  • Mortality of white-tailed deer due to roadkill was documented for 18 months along an interstate highway.

  • Road mortality rates of white-tailed deer were documented after the construction of an interstate highway through their wintering area.

  • Mortality of mule deer due to roadkill was documented for two years along a highway and two state roads.


Loss of habitat

  • Marten capture rates declined as forest fragmentation increased, and the animals were rarely detected in sites with more than 25% non-forested area in a total 9 km2 area.

  • Mountain lions avoided logging areas and established home ranges in areas with lower road densities than the average in the area.

  • Northern flying squirrels, the primary prey of northern spotted owls, occurred at lower densities in logged, shelterwood stands than in unmanaged, old-growth forest.

  • California red-backed voles were more abundant in old-growth forest and naturally regenerated stands than in young, managed stands. Their higher abundance correlated well with the deeper organic soil layers measured in unmanaged stands.

  • California red-backed voles were adversely affected by habitat fragmentation: they were absent in clearcuts, had low densities at the edges of forest remnants, increased in density toward the forest interior, and had higher abundances in large forest fragments compared to small fragments. Truffles, the primary food source of red-backed voles, were absent in clearcuts and near the edges of forest remnants, but occurred in forest interiors.

  • Red-backed salamanders, sensitive to forest moisture and temperature levels, were more abundant in old-growth forest and 60-year-old second-growth than in clearcuts or selectively logged forest. Salamanders are a critical part of the forest food chain: they are important food sources for birds and mammals, and as predators themselves, they cycle large amounts of energy through the forest ecosystem.

  • The abundance of amphibians was significantly lower in clearcuts, plantations, and forest edges than in mature forest interior sites.

  • Lungless salamanders, such as the red-backed salamander, are particularly vulnerable to population declines due to clearcut logging.

  • Clearcuts had a significantly lower abundance and fewer species of salamanders compared to mature, 50- to 70-year-old forest stands in the southern Appalachians. Plethodon salamanders are unlikely to survive logging because individuals are closely tied to small home ranges and unlikely to relocate to intact forest from logged areas.

  • The relatively abundant land salamander Plethodon jordani, an important part of the food chain, disappeared from forest sites in the southern Blue Ridge Mountains after they were clearcut.

  • In the first two years after clearcutting, salamander numbers, including Plethodon jordani, declined to almost zero on all three forest sites studied.

  • Adult and juvenile wood frog and spotted salamander capture rates declined along a gradient from closed-canopy forest to recently clearcut habitat.

  • Juvenile wood frogs, dispersing from breeding pools at the forest edge, preferred to migrate toward closed-canopy forest habitat and away from open habitat.


Reduced nesting success

  • The density of bald eagle nests in southeast Alaska decreased with proximity to clearcuts.

  • Productivity of nesting bald eagles decreased with proximity to clearcuts.

  • Three of four forest interior bird species declined in abundance after logging, whether clearcutting or lower intensity logging.

  • The brown-headed cowbird, a species that parasitizes other birds’ nests, increased in abundance after logging.

  • As forest fragmentation increased, nests of all nine bird species studied suffered higher rates of parasitism and predation.

  • The reproductive success of ovenbirds, a forest interior species, was significantly lower in forest fragments than in continuous forest, partly due to cowbird parasitism of their nests.

  • The density of breeding male ovenbirds was lower in forest fragments than in continous forest, with birds avoiding habitat within 100 m of the forest edge.

  • All three species of tanagers studied were sensitive to forest fragmentation, with a declining probability of breeding tanagers occurring at a given site as fragmentation increased.

  • Nesting success of forest birds decreased within 50 m of forest edges.

  • In five of six studies, nesting success of forest birds decreased as forest patch size decreased.

  • Nest predation rates in southern Appalachian forest fragments increased as fragment size decreased.

  • Artificial nests had higher rates of predation on the edges of forest fragments than in the interior of fragments.



2. Spread of Tree Diseases and Bark Beetles

Logging and road construction have increased the incidence of damaging or lethal tree diseases, including annosus root disease, Armillaria, laminated root rot, black-stain root disease, and Indian paint fungus. Tree stumps have a high likelihood of infection, become centers of infection in a stand, and facilitate the spread of a disease to adjacent, living trees. For some diseases, such as black-stain root disease, woody debris left after thinning attracts insects that are vectors for infection. For at least one disease, Port-orford-cedar root rot, road construction and logging equipment have been directly linked to the spread of the disease to new stands. Black-stain root disease has also been found to occur at higher rates along roads and skid trails.

Trees stressed by disease are more susceptible to attack by bark beetles. For example, ponderosa pines infected with black-stain root disease have suffered higher bark beetle attack rates than undiseased trees. Attempts to mitigate some of these problems have not been easy or fully successful. Stump treatments, such as borax, are not one hundred percent effective, while other efforts, such as stump removal, damage essential ecosystem functions through soil compaction.


Increased occurrence of tree diseases

  • Multiple studies have shown that annosus root disease, often fatal or damaging for a number of conifer species, has increased in western forests as a result of logging.

  • The incidence of annosus root disease in true fir and ponderosa pine stands increased with the number of logging entries.

  • The proportion of western hemlock trees infected by annosus root disease increased after thinning, due to infection of stumps and logging equipment wounds.

  • The percentage of western hemlock trees infected by annosus root disease greatly increased after thinning, with infected stumps being the primary source of infection.

  • Annosus root disease was found on 89% of true fir stumps in stands that had been logged five to 10 years earlier.

  • Annosus root disease and Armillaria infected freshly cut stumps of young western hemlock and Sitka spruce in southeastern Alaska.

  • Armillaria is a primary, aggressive root pathogen in western interior forests, where it spreads into healthy stands from the stumps and roots of cut trees.

  • Armillaria root disease was present in stumps of old-growth ponderosa pine, logged up to 35 years earlier. The oldest stumps of ponderosa pine had the highest rate of infection by Armillaria.

  • Mortality of saplings was significantly correlated to the number of Douglas-fir stumps infected with Armillaria and laminated root rot.

  • The pathogenic fungus Armillaria had a threefold higher occurrence on disturbed plots compared to pristine plots at high productivity sites in the Northern Rockies.

  • Infection and mortality from the root disease Armillaria ostoyae was several times higher in forest stands with logging disturbance than in undisturbed stands.

  • Thinning and soil disturbance led to an increased risk of infection and mortality by black-stain root disease in Douglas-fir.

  • The majority of black-stain root disease infection centers were close to roads and skid trails.

  • Black-stain root disease occurred at a greater frequency in Douglas-fir trees close to roads than in trees located 25 m or more from roads.

  • Thinned stands attracted a greater number of black-stain root disease insect vectors.

  • Mechanical wounding of grand fir and white fir by logging equipment activated dormant decay fungi, such as the Indian paint fungus.

  • Port-orford-cedar root rot, a fatal fungus, is spread by logging equipment, road maintenance equipment, and construction equipment, which transport its spores to new areas.


Attack by bark beetles

  • Root disease fungi predispose some conifer species to bark beetle attack and/or help maintain endemic populations of bark beetles.

  • More mountain pine beetles and western pine beetles (two species of bark beetle) were captured on ponderosa pine infected with black-stain root disease than on healthy trees.

  • Two species of beetle were more frequently attracted to wounds on trees that were also diseased than to uninfected trees.

  • Loblolly pines colonized by annosus root disease had a greater probability of being infested with southern pine bark beetle. Trees infected by annosus root disease had significantly less radial growth than trees not infected.

  • A significantly higher percentage of plots attacked by the southern pine beetle were infected by blue-stain fungi.


Problems with mitigation

  • Borax-treated plots did not have lower rates of annosus root disease infection compared to untreated plots 20 years after thinning.

  • Mitigation measures for Armillaria root disease were problematic in several regards.

  • Stump removal, a method of Armillaria root disease control, resulted in high levels of soil compaction in ash-cap soils.

  • Restricting thinning to summer months, a recommended practice for mitigating the spread of annosus root disease in southern forests, was not a reliable form of disease control.

  • Annosus root disease may spread via root systems from stumps to neighboring trees even following treatment of stumps with borax.



3. Promotion of Insect Infestations

Forest edges created by logging or road construction sustain higher populations of tree pest species such as the tent caterpillar, jack pine budworm, and gypsy moth. Possible mechanisms for this include increased larval development due to higher light levels and increased mortality of natural pathogens.

Biologists predict that logging and habitat destruction will increase the severity of a variety of insect outbreaks because the loss of habitat diversity and old-growth forest has meant a decrease in the diversity of natural pest predators. Natural predators of the western spruce budworm for instance include ant and bird species whose habitat needs include large standing and downed logs that are reduced or eliminated by logging.


Insect infestations

  • Forest fragmentation due to cleared forest increased the duration of tent caterpillar outbreaks.

  • Forest edges were predicted to be source populations for tent caterpillars.

  • Mortality of tent caterpillars in the forest understory due to a natural virus (NPV) decreased as forest cover decreased and edge habitat increased.

  • Abrupt edges along mature jack pine stands increased the levels of defoliation by jack pine budworm in Michigan.

  • Trees at forest edges created by roads had 2.4 times more gypsy moth egg masses than trees in the forest interior.


Loss of ecological complexity

  • A diversity of predators is important for preventing pest outbreaks.

  • Old-growth and roadless areas, with their greater diversity of composition, structure, and predators, are predicted to be less vulnerable to pest outbreaks than forests simplified through management.

  • Species diversity and functional diversity of arthropods were much higher in old-growth stands than regenerating logged stands.

  • Old-growth forests, which have a greater diversity of insect predators, are predicted to help control pest populations.

  • Ant and bird predation reduced adult western spruce budworm densities by approximately 10- to 15-fold at low budworm densities, and by approximately twofold at high budworm densities.

  • Thatching ants play an important role in suppressing insect defoliator populations.

  • Ants, important predators of the western spruce budworm, require sufficient down wood in a range of sizes and decomposition stages.



4. Invasion by Harmful Non-native Plant and Animal Species

Roads facilitate the spread of invasive non-native (exotic) plants, animals, and insects. For example, vehicles can transport the seeds of exotic plants to new areas. Reduced canopy cover (with correspondingly higher light levels) and increased soil disturbance along roads have favored numerous exotic plant species, including Oriental bittersweet and spotted knapweed, as well as exotic ant species such as the red imported fire ant and the Argentine ant. Over time, some exotic species spread from roadsides into adjacent, undisturbed areas.

Exotic species disrupt natural ecosystem processes and the species that depend on them. For example, exotic plants have been shown to replace native understory vegetation, inhibit seedling regeneration, and change soil nutrient cycling. Some weedy species can cause higher erosion rates or change fire regimes. The abundance and diversity of native ants has decreased in areas invaded by the red imported fire ant or the Argentine ant. This can change the entire food base for other species. The decline in other species, such as the northern bobwhite, has been directly documented in habitat infested with exotic ants.


Invasion by non-native species

  • Non-native plant species occurred on high-use, low-use, and abandoned forest roads, with the greatest frequency on roads with the highest level of disturbance and lowest percentage of canopy cover.

  • Exotic annual plants invaded an ecological reserve in California along a pipeline corridor and were still dominant in the corridor 10 years after the disturbance occurred.

  • Oriental bittersweet, an exotic vine of the eastern United States, responded vigorously to increased light intensity after disturbances such as road construction, logging, or windthrow.

  • Spotted knapweed invaded new areas along roadsides.

  • Spotted knapweed preferred open canopies and disturbed areas.

  • Spotted knapweed and diffuse knapweed, two exotic species, preferred open, disturbed habitat, including roads, over shaded areas.

  • Exotic weeds spread along logging roads in forests at all elevations in western Montana.

  • Exotic weeds invaded clearcuts in mid-elevation forests.

  • In a regional survey, a greater proportion of anthropogenically disturbed plots in the southeastern and northeastern United States contained at least one exotic species compared to undisturbed plots.

  • The red imported fire ant, an exotic pest in the southeastern United States colonized roads, power lines, and forest gaps created by logging.

  • The density of red imported fire ant mounds was correlated with the degree of soil disturbance and direct sunlight exposure.


Spread into undisturbed areas

  • Exotic annual plant species invaded adjacent undisturbed oak woodland, coastal sage, and grassland communities from a pipeline corridor in an ecological reserve in California.

  • Exotic weeds spread outward from roadsides in lowland forest and rangeland in Montana, invading relatively undisturbed areas.

  • Originally confined to roadways and abandoned farmland, cheatgrass now invades shrub, ponderosa pine, and pinyon-juniper ecosystems.

  • The red imported fire ant, an exotic pest in the southeastern United States, is believed to disperse into forest gaps from adjacent roads and power lines.


Damage to ecosystem processes

  • Oriental bittersweet, an exotic vine in the eastern United States, inhibited seedling regeneration and damaged young hardwood stands through stem girdling.

  • Tree seedling density decreased with increasing cover of an exotic honeysuckle, Lonicera tatarica.

  • The diversity and density of herbaceous species declined as honeysuckle cover increased in three of four northeastern forest stands.

  • In shrub-steppe ecosystems, invading weed species, which were usually non-mycorrhizal, disrupted succession by native species, 99% of which were mycorrhizae-dependent.

  • An exotic weed, bull thistle, reduced growth rates of ponderosa pine seedlings by up to 33% in a forest plantation.

  • Forest litter depth and soil organic layers were lower and pH was higher in sites invaded by two exotic plant species (Japanese barberry and a Japanese grass species), when compared to adjacent uninvaded forest sites.

  • Native oaks and shrubs occurred at a lower density in forested sites invaded by Japanese barberry and a Japanese grass species than in uninvaded sites.

  • Surface runoff and soil erosion were greater from spotted knapweed-dominated sites than natural bunchgrass-dominated sites.

  • Fires have become more common and extensive in pinyon-juniper woodlands and sagebrush ecosystems invaded by cheatgrass, an exotic grass.

  • The incidence of fire has increased in ponderosa pine forests and pinyon-juniper woodlands where cheatgrass, an exotic annual, has invaded. This grass has proven difficult to control.

  • Invasion in Texas by the red imported fire ant resulted in a 90% decrease in native ant abundance and a 70% decrease in ant species richness.

  • Native ants had a lower abundance and diversity in areas invaded by the Argentine ant, an exotic ant.

  • The trophic structure of invertebrate communities changed in areas invaded by Argentine ants, with higher numbers of scavengers at the expense of herbivores, predators, and parasites.

  • Northern bobwhite populations in Texas decreased after invasion by the non-native red imported fire ant. Densities of northern bobwhites increased after treatment to reduce infestation by the red imported fire ant.



5. Damage to Soil Resources and Tree Growth

Logging and road construction compact soils, disturb or destroy organic layers, and cause high rates of soil erosion. Soil compaction, which can last for several decades, is typically measured by changes in soil bulk density or porosity. Trees’ access to nutrients and water is reduced because of restricted root growth in compacted soils, reduced water infiltration rates, and decreased oxygen and water available to root systems. Soil compaction also has a detrimental impact on microorganism communities, which play a critical role in nutrient cycling and tree growth. The loss of organic layers also affects mycorrhizal fungi, which are important to many tree species in accessing nutrients. As a result of this damage to soil resources, trees can suffer from moisture stress, reduced growth rates, inability to establish seedlings, and reduced resilience in the long term.

Compacted soils are also more susceptible to surface erosion. The frequency of mass erosion events, such as debris slides, also increases in landscapes that have been roaded or logged, thus increasing total soil loss. In forests where overland (surface) water flow is unlikely, roads have been documented to intercept water at road cuts, converting subsurface flow to surface flow. This greatly increases runoff-related erosion.

This loss of soil due to erosion not only reduces the productivity of the local site by removing top, nutrient-rich layers of soil, but the sediment that is generated often runs into streams, where it has a range of detrimental impacts on aquatic ecosystems.


Damage to soils

  • Logging resulted in soil compaction, displacement of surface mineral soil, loss of organic matter, and loss of nitrogen, an essential nutrient.

  • Logging on volcanic ash soils in the Pacific Northwest caused soil compaction, as measured by increased soil bulk density.

  • Average soil bulk density was 15% greater on skid trails than on undisturbed soils in a ponderosa pine site 23 years after logging, and 28% greater on a lodgepole site 14 years after logging.

  • Compacted volcanic and granitic soils were slow to recover on skid trails in western Idaho, and after 23 years, only the bulk density of the granitic soil’s top few centimeters had returned to undisturbed values.

  • Stump removal, a method of Armillaria root disease control, resulted in high levels of soil compaction in ash-cap soils.

  • Soil bulk density increased, aeration porosity decreased, and water conductivity decreased in the upper layers of soil after logging in the Piedmont region.

  • Logging was documented to cause soil compaction in a variety of soil types in the southern United States.

  • Logging resulted in soil compaction and disturbance of organic matter in three New England forests.

  • Logging activity caused significant soil erosion in the Pacific Northwest.

  • Soil compaction leads to surface erosion.

  • Clearcutting and post-logging slash burning were associated with high rates of ravel (upslope erosion) on various soil types in the Northwest.

  • Roads caused debris slides in areas that would be relatively stable otherwise.

  • The likelihood of surface runoff increased on the compacted soils of skid trails.

  • Subsurface flow converted to surface flow by road cuts could trigger soil erosion and mass movement.

  • Soil erosion rates due to debris slides were many times higher on forests with roads, landings, and logging activity than on undisturbed forests.

  • Roads were responsible for 61% of the soil volume displaced by erosion in northwestern California.

  • Clearcutting increased the frequency of mass soil movements from hillsides.


Impacts on tree growth and health:

  • Soil compaction results in root damage and decreased root growth, which decrease plants’ ability to access nutrients and water.

  • Soil compaction and organic matter disturbance cause a decline in mycorrhizal fungi.

  • Soil compaction results in reduced infiltration rates and increased surface erosion.

  • Soil compaction results in a loss in site productivity as measured by tree growth.

  • Soil compaction restricted root growth and increased moisture stress in southern U.S. forests.

  • Soil compaction after logging resulted in a loss of soil pore space and a 33% reduction in water to plants.

  • Soil compaction by logging reduced the movement of water through the soil (saturated hydraulic conductivity), with increases in runoff predicted.

  • Soil compaction reduced growth of young ponderosa pine.

  • Beneficial soil microorganisms and mycorrhizal fungi occur primarily in soil organic layers. Soil compaction and the disturbance of organic layers of the soil due to logging activities alter soil microbial activity and adversely affect mycorrhizal populations.

  • Ectomycorrhizal abundance and diversity on Douglas-fir seedlings were much lower in soils compacted by stump removal than in undisturbed soils.

  • A 20% increase in soil bulk density due to soil compaction significantly reduced the numbers of root tips on Douglas-fir and western white pine seedlings.

  • Ectomycorrhizal root tip abundance and diversity in Douglas-fir seedlings were decreased by soil compaction and organic layer removal.

  • Soil erosion results in the loss of nutrients and water availability, degraded soil structure, and the loss of important soil organisms including mycorrhizal fungi.

  • Erosion of the topmost soil layers, which are the most important for nutrients, water, and soil biota, is the most damaging to site productivity.

  • Roads in mountainous areas affected site productivity upslope and downslope of the road through changes in the groundwater system and through debris slides.


Role of soil microorganisms and mycorrhizae:

  • Healthy ectomycorrhizal populations are important for forest stability and recovery after a disturbance.

  • Mycorrhizal fungi increase nutrient uptake in plants.

  • A healthy population of soil organisms is critical for nutrient cycling.

  • Mycorrhizae increase the uptake of nutrients and water.



6. Impacts on Aquatic Ecosystems

Roads and logging can significantly degrade stream ecoystems by introducing high volumes of sediment into streams, changing natural streamflow patterns, and altering stream channel morphology. The frequency of landslides in steep terrain is higher in roaded areas and in forests that have been clearcut. Much of the resultant eroded soil ends up in streams. Fine sediment from road surfaces runs into streams during storm events. The interception of subsurface flow by road cuts also increases surface flow and therefore surface erosion.

Streamflow patterns can change in watersheds that are roaded and/or have been logged. Roads, ditches, and new gullies form new, large networks of flow paths across the landscape; this changes the rate at which water reaches streams. As a result, peak discharge volumes in some watersheds are higher and after large storms begin earlier than they would in undisturbed watersheds.

These changes in stream habitat affect the health of aquatic organisms. The survival rates of many salmonid species, for instance, decrease as fine sediment levels increase. Deposition of fine sediment on the stream bed degrades spawning areas, reduces pool refuge habitat, decreases winter refuge areas for juveniles, and impedes feeding visibility. For example, survival rates of coho salmon, chum salmon, and steelhead trout fry decrease as stream sediment levels increase. Sensitive amphibian and invertebrate species are also adversely affected by increased sediment loads, decreasing in abundance and/or diversity. Thus, large-sized aquatic invertebrate species may be replaced by smaller-sized species. Changes to aquatic invertebrate communities not only affect the food supply available to other stream organisms such as fish, but also to non-aquatic forest species. As adults, stream invertebrates emerge from streams and occupy riparian forests, where they are an important source of food for birds, bats, and various other mammals.

Changes in stream channel structure lead to decreases in the habitat available to fish and other organisms. In addition, increases in stream peak discharge volumes and/or sediment loads can affect egg survival rates of salmon adapted to natural stream flows or streambed scour rates.


Increases in sediment and altered streamflows

  • Roads degraded stream habitat for aquatic species, including salmonids, by accelerating erosional processes and modifying natural drainage networks.

  • Logging activities degraded stream habitat by changing the amount, quality, and timing of flowing water, increasing erosion rates, and reducing stream habitat diversity.

  • Soil erosion rates due to debris slides were many times higher on forests with roads, landings, and logging activity than on undisturbed forests.

  • Roads were responsible for 61% of the soil volume displaced by erosion in northwestern California.

  • Clearcutting increased the frequency of mass soil movements from hillsides.

  • During storm events in southwestern Washington, average sediment levels in runoff from forest roads ranged from 500 mg/l to 20,000 mg/l.

  • Roads were direct sources of sediment delivery to streams, with approximately 34% of road drainage points entering stream channels.

  • Very fine sediment washed from a forest road surface directly into a stream during rainfall events.

  • Forest road erosion was a source of fine sediment in stormflow runoff, even after mitigation measures.

  • Gravel forest roads generated up to 440 ton of sediment/km/year from surface erosion.

  • The volume of fine sediment present in streams increased in direct proportion to logging in the watershed and stream crossings by roads.

  • Logging and forest road construction led to an increase in landslides and surface erosion, disrupting the riparian vegetation along first- and second-order tributaries of a river in Oregon.

  • Roads intercepted subsurface flow on mountainous slopes in the Idaho Batholith, converting it to surface flow. Subsurface flow converted to surface flow by intercepting roads would be likely to trigger soil erosion and soil mass movement.

  • The peak rate of subsurface flow increased by an average of 27% after clearcutting, and due to its interception by a road cut and conversion to surface flow, was believed likely to lead to increased levels of erosion from the road and the slopes below the road.

  • Roads and clearcut logging increased peak stream discharges and advanced the timing of peak discharges in multiple paired watershed studies, most likely because of subsurface flow being converted to surface flow at road cuts.

  • Even after many years, roads and clearcut logging, both together and separately, resulted in significant increases in stream peak discharges.

  • Roads formed new surface flow paths to natural channels and incised new gullies, so increasing the routing efficiency of water; thereby probably explaining some higher stream peak flows.

  • Almost 30 years after clearcut logging occurred, average and peak stream flows in the watershed studied were still higher than pre-logging flows.

  • Natural streamflow rates during periods of high flow were significantly altered in two watersheds after logging road construction.

  • Subsurface flow intercepted by logging roads was converted to surface flow and was the most likely cause for increases in streamflows during snowmelt runoff and heavy summer storms.

  • Stream peak flows increased significantly in a watershed with 12% of its area in roads, before any logging occurred.

  • Stream peak flows increased as the percentage of watershed area clearcut increased.

  • Forest roads extended the natural channel network, initiated new channels, and increased the susceptibility of steep slopes to landsliding.

  • Road cuts intercepted subsurface flow and diverted it to roadside ditches.


Adverse impacts on aquatic species

  • Salmonid survival rates decreased after logging and road construction as fine sediment levels in streams increased and as important habitat characteristics, including the number of pools and winter cover, decreased.

  • Coho and chum salmon fry survival declined after logging and associated increases in fine sediment deposited in spawning areas.

  • Survival rates of coho salmon and steelhead trout fry decreased as the proportion of fine sediment in spawning gravel increased.

  • Brook trout populations declined significantly after stream sedimentation levels increased.

  • Populations of stream benthic invertebrates (the major food source of brook trout) declined significantly after stream sediment levels increased.

  • Higher fine sediment levels in a stream resulted in a loss of pool habitat, fish cover, changes in stream velocity, and higher summer water temperatures.

  • Salmonids avoided water with suspended sediment in Alaskan streams and lakes.

  • Delivery of fine sediments to streams and deposition on spawning and rearing substrate decreased after a moratorium on logging, but increased again after logging resumed.

  • Fine sediment deposition on cobble substrates decreased the availability of interstitial spaces (used as winter refuges), and winter densities of juvenile chinook salmon decreased correspondingly.

  • Chum salmon eggs were susceptible to mortality from increased streambed scour associated with logging and/or roads.

  • Salmonid embryo survival rates decreased as the proportion of fine particles in stream spawning substrate increased and dissolved oxygen levels decreased.

  • Adult and juvenile salmonids exposed to suspended fine sediment in streams had an increasingly negative response as concentrations and duration of exposure increased.

  • Juvenile coho salmon avoided water with high turbidity levels.

  • Basins with more than 25% of their area logged had lower stream habitat diversity, as measured by the number of pools and pieces of wood, than basins with less than 25% of their area logged.

  • The diversity of juvenile anadromous salmonid populations was lower in basins with more than 25% of the area logged than in basins with less than 25% logged.

  • The density of all three stream amphibian species studied was lower in streams affected by sediment due to road construction than in control streams. Two of three species had significantly lower numbers in all five stream microhabitats.

  • A higher proportion of fine sediment occurred in streams flowing through forest stands with logging than streams flowing through unlogged forest stands.

  • Abundance, density, and biomass of all aquatic amphibian species studied were lower in streams flowing through logged forest than unlogged forest streams.

  • Stream insects preferred fully exposed cobble on the streambed to cobble partly or fully embedded in fine sediment.

  • Adult aquatic insects were an important part of the insect community in forests adjacent to streams, and were believed to be an important part of the food web for forest animals such as birds and bats.

  • The remaining intact watersheds in southeast Alaska are key to maintaining sustainable salmon stocks.

  • Trout standing stocks decreased as the density of road culverts (a measure of the extent to which roads crossed watercourses) increased.

  • During the summer, adult chinook salmon preferred pool habitat and cool stream reaches over other kinds of stream habitat.



7. General review papers on the ecological effects of roads

A few papers have reviewed and summarized research from both within and outside North America on the effects of roads on aquatic and terrestrial ecosystems. These papers cover not only the six issues highlighted in this bibliography, but also additional concerns such as higher poaching pressure along roads, stream pollution from runoff of heavy metals and deicing salts, impaired plant photosynthesis due to roadside dust, and impacts on wetlands over the long-term.

Some researchers have also attempted to quantify the total area of landscape affected either directly or indirectly by roads. They estimate that overall, up to 15-20% of the United States’ land area is affected by roads.


Ecological effects of roads

  • Roads were associated with a diversity of negative effects on the biotic integrity of both terrestrial and aquatic ecosystems.

  • Based on numerous studies on the ecological impact of roads, 15-20% of the United States land area was estimated to be affected by roads.

  • The zone of ecological effects surrounding roads averaged more than 600 m wide, and for some factors extended more than 1 km from the road surface.

  • Roads are a major cause of forest fragmentation because they divide large landscape patches into smaller patches and convert forest interior habitat into edge habitat.

  • Clearcuts and roads affected 2.5 to 3.5 times more of the landscape than the surface area occupied by the actual clearcuts and roads themselves.

  • Road networks affected stream systems, increasing the frequency and/or magnitude of peak flows, debris flows, and landslides.

  • Richness of plant, bird, amphibian, and reptile communities in wetlands decreased as road density within the adjacent 2 km increased, with the full impact on biodiversity not evident for several decades.

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