Forest
Roads: A Synthesis of Scientific Information
General Technical Report
PNW-GTR-509
Forest Service Pacific Northwest Research Station
May
2001
US
Department of Agriculture
Forest
Service
Pacific
Northwest Research Station
Portland,
Oregon
Editors
Hermann Gucinski, Michael J. Furniss, Robert R. Ziemer, and Martha H Brookes
(Abstracts from Document)
Roads are a vital component of civilization. They provide access for people to study, enjoy, and commune with forested wildlands and to extract an array of resources from natural and modified ecosystems. Roads have well-documented, short- and long-term effects on the environment that have become highly controversial, because of the value society now places on unroaded wildlands and because of wilderness conflicts with resource extraction.
The approach taken in this report is to identify known and hypothesized road-related issues and to summarize the scientific information available about them. The report identifies links among processes and effects that suggest both potential compatible uses and potential problems and risks. Generalizations are made where appropriate, but roads issues and road science usually cannot be effectively separated from the specific ecologic, economic, social, and public lands management contexts in which roads exist or are proposed.
General Consideration of Road
Networks
Across a forest or river basin,
the access needs, economic dependencies, landscape sensitivities, downstream
beneficial uses of water, and so on can be reasonably well defined, but these
relations tend to differ greatly from place to place. An effective synthesis of road issues draws local experts
together to thoroughly evaluate road and access benefits, problems and risks,
and to inform managers about what roads may be needed, for how long, for what
purposes, and at what benefits and costs to the agency and society.
Road effects and uses may be
somewhat arbitrarily divided into beneficial and detrimental. The largest
group of beneficial variables relates to access. We identified access-related
benefits as harvest of timber and special forest products, grazing, mining,
recreation, fire control, land management, research and monitoring, access to
private inholdings, restoration, local community critical needs, subsistence,
and the cultural value of the roads themselves. Nonaccess-related benefits
include edge habitat, fire breaks, absence of economic alternatives for land
management, and jobs associated with building and maintaining the roads.
Undesirable
consequences include adverse effects on hydrology and geomorphic features
(such as debris slides and sedimentation), habitat fragmentation, predation,
road kill, invasion by exotic species, dispersal of pathogens, degraded water
quality and chemical contamination, degraded aquatic habitat, use conflicts,
destructive human actions (for example, trash dumping, illegal hunting,
fires), lost solitude, depressed local economies, loss of soil productivity,
and decline in biodiversity.
Issue-Biodiversity is, in simplest terms, the
variety of life and its processes (Keystone Center 1991). Recent syntheses
(Heywood and Watson 1995) emphasize the reciprocal relation between
biodiversity-conceived as genetic and species diversity-and ecosystem
function. The many species comprising the biodiversity of an area play roles
essential to ecosystem function and are the source of variation that enables
an ecosystem to adapt to change. The healthy, functioning ecosystem, in turn,
supports the many species living within it. Appreciating this reciprocity
means that biodiversity can be taken as a natural measure of the ecosystem as
a whole and thus can integrate the many concerns listed.
Some species may play more important roles than others in
the normal functioning of an ecosystem. For example, keystone species may
define the major structural elements of an ecosystem, as Douglas fir does
for forests in the Pacific Northwest, or they may-by virtue of their position
in a complex trophic structure-act to maintain the diversity as keystone
predators do for herbivores. On the other hand, the many species that do not
appear to serve an important role in an ecosystem constitute a reservoir of
potential adaptation to change. Because an ecosystem cannot predict change,
the diversity of species acts as a hedge against it.
Biodiversity is vital to long-term ecosystem function,
and human activities that decrease biodiversity can impair it. Our working
hypothesis, then, is that measures of biodiversity provide the best
integrative assessment of the effects of roads on ecosystems.
Findings-Roads can have major adverse effects on
biodiversity, many of which are described (Forman and Collinge 1996). A recent
review by Forman and Hersperger (1996) usefully distinguishes these aspects of
the road-biodiversity interaction:
· Road density: As road density increases, thresholds may be passed that cause some species to go locally extinct. The probability of extinction depends, in part, on body size, with larger animals requiring larger residual populations to prevent their extinction.
·
Road-effect zone: The effects of roads can extend over some distance
from their centers, such that their "effective widths" can be many
times their actual widths.
Issue-Natural
populations of animal species are reduced by habitat loss caused by road
building and by the animals' avoidance of areas near roads. Populations can be
fragmented into smaller subpopulations, thereby causing increased demographic
fluctuation, inbreeding, loss of genetic variability, and local population
extinctions.
Findings--Habitat
loss has broader effects than just the conversion of a small area of land to
road surface. Roads fragment by changing landscape structure and by directly
and indirectly affecting species. Habitat effects of roads on the landscape
include dissecting vegetation patches, increasing the edge affected area and
decreasing interior area, and increasing the uniformity of patch
characteristics, such as shape and size (Reed and others 1996).
Whenever forest
roads are built, changes in habitat and modified animal behavior will lead to
changes in wildlife populations. Road-avoidance behavior is characteristic of
large mammals such as elk, bighorn sheep, grizzly, caribou,
and wolf. Avoidance distances of 300 to 600 feet are common
for these species. Road usage by people and their vehicles has a significant
role in determining road avoidance by animals. In a telemetry study of
movement by black bear, bears almost never crossed interstate highways, and
they crossed roads with little traffic more frequently than those with high
traffic volumes. Bobcats crossed paved roads in Wisconsin forests less than
expected, possibly to minimize interactions with vehicles and people (Lovallo
and Anderson 1996).
A few studies
have related genetic changes in populations simply to the presence of roads
(Forman and others 1997),
but the distribution of roads in the environment also must be considered. Road
density is a useful index of the effect of roads on wildlife populations.
Wolves in
Wisconsin are limited to places with pack-area mean road densities of 0.7
mile/square mile or less (Mladenoff and others 1995).
Some studies have shown that a few large areas of low road density,
even in a landscape of high average road density, may be the best indicator
of suitable habitat for large vertebrates (Rudis 1995).
Issue-Road building introduces new edge habitat in
the forest. The continuity of the road system also creates a corridor by which
edge-dwelling species of birds and animals can penetrate the previously closed
environment of continuous forest cover. Species diversity can increase, and
increased habitat for edge-dwelling species can be created.
Findings-Roads
and their adjacent environment qualify as a distinct habitat and have various
species, population, and landscape-scale effects (Baker and Knight 2000,
Dawson 1991, van der Zande and others 1980).
Some research has attempted to describe habitat modifications caused
specifically by roads, but most of this work is species and site specific.
Surveys of songbirds in two national forests of northern Minnesota
found 24 species of birds more abundant along roads than away from them (Hanowski
and Niemi 1995). Close to half
these species were associated with edges, including birds like crows and blue
jays that use roads as corridors to find food. Turkey hens in North
Carolina nested near closed and gated logging roads and used them extensively
in all stages of brood development (Davis 1992). One study showed that habitat
in the roadside right-of-way supports a greater diversity of small mammals
than do adjacent habitats, but this finding may not apply to forest roads
with only narrow cuts and fills on either side.
The similarity
between forest roads and transmission-line right-of-ways may be important in
assessing the contribution of roads to habitat. Studies have shown that wide
transmission-line corridors support grassland bird communities of species
not found in the forest, and narrow corridors produce the least change from
forest bird communities (Anderson and others 1977). The same study notes that
increasing edge diversity of birds, for instance, may negatively affect
abundance of interior species.
In general, road building fragments habitat and creates
habitat edge, thereby modifying the habitat in favor of species that use
edges. Edge-dwelling species generally are not threatened, however, because
the human-dominated environment has provided ample habitat for them. Any
habitat modifications attributed to the road may be insignificant compared to
the effects of the activity, such as timber harvest, for which the road was
built.
Issues-A
widely cited generalization about biological invasion is that it is promoted
by disturbance. Building roads and subsequently maintaining them (including
ditch clearing, road grading, and vegetation clearing) in the interior of a
forest represents disturbances that create and maintain new edge habitat.
These roadside habitats can be invaded by an array of exotic (non-native)
plant species, which may be dispersed by "natural" agents such as
wind and water as well as by vehicles and other agents related to human
activity. Roads may be the first point of entry for exotic species into a new
landscape, and the road can serve as a corridor along which plants move
farther into the landscape (Greenberg and others 1997, Lonsdale and Lane
1994). Some exotic plants may then be able to move away from the
roadside into adjacent patches of suitable habitat. Invasion by exotic plants
may have significant biological and ecological effects if the species are able
to disrupt the structure or function of an ecosystem. Invasion also may be
of concern to land managers, if the exotic species disrupt management goals
and present costly eradication problems.
Findings-Although
few habitats are immune to at least some invasion by exotic plants, predicting
which species will become pests usually is difficult. Assessing the scale of a
biological invasion problem is complicated by the lag between when an exotic
is introduced and when it begins to expand its distribution and population
size in a new area. Cowbirds, for example, can be introduced into forested
environments by roads and subsequently affect populations of neotropical
migratory birds through nest parasitism. The spread of pathogens where roads
act as vectors is described in "Forest Diseases," below. Few
environmentally benign approaches to exotic plant control or eradication have
been tested.
Issues-The
effects of roads on aquatic habitat are believed to be widespread and
profound, and evidence is documented through empirical associations and direct
mechanistic effects, although the mechanistic effects become fuzzy when
direct, quantitative, cause-effect links are sought. Several studies correlate
road density or indices of roads to fish density or measures of fish
diversity. Mechanisms include effects of fine sediment, changes in
streamflow, changes in water temperature caused by loss of shade cover or
conversion of groundwater to surface water, migration barriers, vectors of
disease, exotic fishes, changes in channel configuration from encroachment,
and increased fishing pressure. A growing body of work indicates that the
complexity of habitat and the predictability of disturbance influences species
diversity. At the landscape scale, correlative evidence suggests that roads
are likely to influence the frequency, timing, and magnitude of disturbance,
which are likely to influence community structure.
Findings-Roads
contribute more sediment to streams than does any other land management
activity (Gibbons and Salo 1973, Meehan 1991), but most land management
activities, such as mining, timber harvest, grazing, recreation, and water
diversions, depend on roads. Most of the sediment from timber harvest
activities is related to roads and road building (Chamberlain and others 1991,
Dunne and Leopold 1978, Furniss and others 1991, MacDonald and Ritland 1989,
Megahan and others 1978) and the associated increases in erosion rates (Beschta
1978, Gardner 1979, Meehan 1991, Rhodes and others 1994, Reid 1993, Reid and
Dunne 1984, Swanson and Dyrness 1975, Swanston and Swanson 1976). Serious
degradation of fish habitat can result from poorly planned, designed, located,
built, or maintained roads (Furniss and others 1991, MacDonald and others
1991, Rhodes and others 1994). Roads also can affect water quality through
applied road chemicals and toxic spills (Furniss and others 1991, Rhodes and
others 1994), and the likelihood of toxic spills reaching streams has
increased with the many roads paralleling them.
Increased fine sediment composition in stream gravel has
been linked to decreased fry emergence, decreased juvenile densities, loss of
winter carrying capacity, and increased predation of fishes. Increased fine
sediment can reduce benthic organism populations and algal production.
Survival of incubating salmonids from embryos to emergent fry has been
negatively related to the proportion of fine sediment in spawning gravels
(Chapman 1988, Everest and others 1987, Scrivener and Brownlee 1989, Weaver
and Fraley 1993, Young and others 1991). Increased fine sediment in stream
gravel can reduce intragravel water exchange, thereby reducing oxygen
concentrations, increasing metabolic waste concentrations, and restricting
movements of alevins. Survival of embryos relates positively to dissolved
oxygen and apparent velocity of intragravel water, and positively to gravel
permeability and gravel size (Chapman 1988, Everest and others 1987).
Consequently, juvenile salmonid densities decline as fine sediment
concentrations increase in rearing areas (Alexander and Hansen 1986, Bjornn
and others 1977, Chapman and McLeod 1987, Everest and others 1987, Shepard and
others 1984). Increases in fine sediment also can reduce winter carrying
capacity of streams by loss of concealment cover and by increasing the
likelihood of predation. Pools function as resting habitats for migrating
adults, rearing habitats for juveniles, and refugia from natural disturbances.
Pools that lose volume from sediment support fewer fish, and fish that reside
in them may suffer higher mortality (Alexander and Hansen 1988). Similarly,
populations of tailed frogs can be severely reduced or eliminated by increased
sedimentation, presumably because of their dependence on unembedded
interstitial areas in the stream substrate where they hide and overwinter
(Brown 1990, Daugherty and Sheldon 1982). Increased sediment reduces
populations of benthic organisms by reducing interstitial spaces and flow used
by many species and by reducing algal production, the primary food source of
many invertebrates (Chutter 1969, Hynes 1970).
The effects of
roads are not limited to those associated with increases in fine-sediment
delivery to streams; they can include barriers to migration, water temperature
changes, and alterations to stream flow regimes. Improper culvert placement at
road-stream crossings can reduce or eliminate fish passage (Belford and Gould
1989), and road crossings are a common migration barrier to fish (Clancy and
Reichmuth 1990, Evans and Johnston 1980, Furniss and others 1991). In a large
river basin in Washington,
13 percent of the historical coho habitat was lost as a result of improper
culvert barriers (Beechie and others 1994). Roads built adjacent to stream
channels pose additional effects. Changes in temperature and light regime from
removing the riparian canopy can have both positive and negative effects on
fish populations. Sometimes increased food availability can mitigate negative
effects of increased summer water temperatures (Bisson and others 1988).
Beschta and others (1987) and Hicks and others (1991) document negative
effects, including elevation of stream temperatures beyond the range of
preferred rearing, inhibition of upstream migrations, increased disease
susceptibility, reduced metabolic efficiency, and shifts in species
assemblages.
Issue-Effects of roads on
vertebrate populations act along three lines: direct effects, such as habitat
loss and fragmentation; road use effects, such as traffic causing vertebrate
avoidance or road kill; and additional facilitation effects, such as
overhunting or overtrapping, which can increase with road access.
Findings-In
recent research in the interior Columbia River basin, Wisdom and others (2000)
identify more than 65 species of terrestrial vertebrates negatively affected
by many factors associated with roads. Specific factors include habitat loss
and fragmentation, negative edge effects, reduced densities of snags and
logs, overhunting, over-trapping, poaching, collection, disturbance,
collisions, movement barriers, displacement or avoidance, and chronic,
negative interactions with people. These factors and their effects on
vertebrates in relation to roads are summarized from Wisdom and others (2000)
as follows:
Road
construction converts large areas of habitat to nonhabitat (Forman 2000, Hann
and others 1997, Reed and others 1996); the resulting motorized traffic
facilitates the spread of exotic plants and animals, further reducing quality
of habitat for native flora and fauna (Bennett 1991, Hann and others 1997).
Roads also create habitat edge (Mader 1984, Reed and others 1996); increased
edge changes habitat in favor of species that use edges, and to the detriment
of species that avoid edges or experience increased mortality near or along
edges (Marcot and others 1994).
Species
dependent on large trees, snags, or logs, particularly cavity-using birds and
mammals, are vulnerable to increased harvest of these structures along roads (Hann
and others 1997). Motorized access facilitates firewood cutting, as well as
commercial harvest, of these structures.
Several
large mammals are vulnerable to poaching, such as caribou, pronghorn antelope,
mountain goat, bighorn sheep, wolf; and grizzly bear (Autenrieth 1978, Bruns
1977, Chadwick 1973, Dood and others 1986, Greer 1985, Gullison and Hardner
1993, Horejsi 1989, Knight and others 1988, Lloyd and Fleck 1977, Luce and
Cundy 1994, Mattson 1990, McLellan 1990, McLellan and Shackleton 1988, Mech
1970, Scott and Servheen 1985, Singer 1978, Thiel 1993, Van
Ballenberghe and others 1975, Yoakum 1978). Roads facilitate this poaching
(Cole and others 1997).
Gray
wolf and grizzly hear experience chronic, negative interactions with humans,
and roads are a key facilitator of such interactions (Mace and others 1996,
Mattson and others 1992, Thiel 1985). Repeated,
negative interactions of these two species with humans increase mortality of
both species and often causes high-quality habitats near roads to function as
population sinks (Mattson and others 1996; Mech 1973).
Carnivorous
mammals such as marten, fisher, lynx, and wolverine are
vulnerable to overtrapping (Bailey
and others 1986, Banci 1994, Coulter 1966, Fortin and Cantin 1994, Hodgman and
others 1994, Hornocker and Hash 1981, Jones 1991, Parker and others 1983,
Thompson 1994, Witmer and others 1998), and overtrapping can be facilitated by
road access. Movement and
dispersal of some of these species is believed to be inhibited by high rates
of traffic on highways (Ruediger 1996). Carnivorous mammals such as lynx also
are vulnerable to increased mortality from highway encounters with motorized
vehicles (as summarized by Terra-Berns and others 1997).
Reptiles
seek roads for thermal cooling and heating, and in doing so, these species
experience significant, chronic mortality from motorized vehicles (Vestjens
1973). Highways and other roads with moderate to high rates of motorized
traffic may function as population sinks for many species of reptiles,
resulting in reduced population size and increased isolation of populations
(Bennett 1991). Roads also
facilitate human access into habitats for collecting and killing reptiles.
Many
species are sensitive to harassment or human presence, which often are
facilitated by road access; potential reductions in productivity, increases in
energy expenditures, or displacements in population distribution or habitat
use can occur (Bennett 1991, Mader 1984). Examples of such road-associated
effects are human disturbance of leks of sage grouse and sharp-tailed grouse,
nests of hawks, and dens of kit fox. Another example is elk avoidance
of large areas near roads open to traffic (Lyon 1983, Rowland and others
2000), with elk avoidance increasing with increasing rate of traffic (Wisdom
and others 2000, Johnson and others 2000).
Bats
are vulnerable to disturbance and displacement caused by human activities in
caves, mines, and on rock faces (Hill and Smith 1984, Nagorsen and Brigham
1993). Cave or mine exploration and rock climbing are examples of recreation
that could reduce population fitness of bats that roost in these sites (Nagorsen
and Brigham 1993, Tuttle 1988). Such activities may be facilitated by human
developments and mad access (Hill and Smith 1984).
Ground
squirrels often are targets for recreational shooting. This is facilitated by
human developments and road access (Ingles 1965). Many species of ground
squirrels are local endemics; these small, isolated populations may be
especially vulnerable to recreational shooting and potentially severe
reductions or local extirpations of populations.
Roads
often restrict the movements of small mammals (Mader 1984, Merriam and others
1988, Swihart and Slade 1984), and consequently can function as barriers to
population dispersal and movement by some species (Oxley and Fenton 1974).
Many granivorous birds are attracted to grains and seeds along roadsides and as a result have high mortality from collisions with vehicles (Vestjens 1973). And pine siskens (Carduelis pinus) and white-winged crossbills (Loxia leucoptera), for example, are attracted to road salt, which can result in mortality from vehicle collisions (Ehrlich and others 1988).
Terrestrial
vertebrates inhabiting areas near roads accumulate lead and other toxins that
originate from motorized vehicles, with potentially lethal but largely
undocumented effects (Bennett 1991).
In summary, no terrestrial vertebrate taxa seem immune to the myriad of road-associated factors that can degrade habitat or increase mortality. These multifaceted effects have strong management implications for landscapes characterized by moderate to high densities of roads. In such landscapes, habitats are likely underused by many species that are negatively affected by road-associated factors. Moderate or high densities of roads sometimes create areas that function as population sinks that otherwise would function as source environments were road density low or zero.
Issues-Large numbers of
animals are killed annually on roads. In selected situations, as with some
amphibians with highly restricted home ranges, populations of rare animals may
be reduced to dangerous sizes by road kills.
Findings-An estimated 1 million vertebrates a day are killed on roads in the United States (Lalo 1987). Studies show that the number of collisions between animals and vehicles is directly related to the position of the nearest resting and feeding sites (Carbaugh and others 1975). Because most forest roads are not designed for high speed travel, and the speed of the traffic is directly related to the rate of mortality, direct mortality on forest roads is not usually an important consideration for large mammals (Lyon 1985).
An exception is
forest carnivores, which are especially vulnerable to road mortality because
they have large home ranges that often include road crossings (Baker and
Knight 2000). Forest roads pose a greater hazard to small, slowly moving,
migratory animals, such as amphibians, making them highly vulnerable as they
cross even narrow forest roads (Langton 1989).
Nearly all species
of reptiles use roads for cooling and heating, so many of them are killed by
vehicles. Highways and other roads with moderate to high-speed traffic
function as population sinks for many species of reptiles, resulting in
reduced and increasingly isolated populations (Wisdom and others 2000).
Predators and scavengers are killed while they feed on road-killed wildlife, as are other species attracted to roads because of salts or vegetation, or because roads facilitate winter travel (Baker and Knight 2000). Although countless animals are killed on roads every year, documented road-kill rates are significant in reducing populations of only a few rare species in North America, and these kills generally are on high-speed highways (Forman and others 1997).
Issue-In general, the existence of roads seems
to have little effect on forest tree diseases, but there are some examples
where building or using roads caused significant local effects. The negative
effects can be ameliorated through simple modifications in how roads are built
and used.
One negative effect includes the movement of people
on the roads, which allows the pests to be introduced. Road building also may
set the stage for an insect attack that further stresses the trees and then a
disease outbreak that kills them (Boyce 1961).
The one benefit of roads, as it pertains to tree
diseases, is to provide access for silvicultural activities that protect
resources, such as the ability to inoculate decay fungi into trees to create
wildlife habitat (Bull and others 1997).
Findings-A significant
forest disease problem associated with roads is Port-Orford cedar root
disease. This disease (Chamaecyparis Iawsoniana (A. Murr.) Pari.) is a
root disease caused by the fungus Phytophthora lateralis. Spores of the
fungus are carried in water or contaminated soil to uninfected areas. Roads of
any sort in the very limited geographic range of the primary host provide a
way to move soil-along with the fungus-from infected to uninfected areas.
Spread of the fungus can be
checked by careful planning to reduce entry to uninfected areas, road
closures, partial road closures during wet weather, attention to road surfaces
and drainage of possibly contaminated water to streams, wash stations to
remove soil from vehicles before entry to uninfected areas, and sanitation
strips to remove host plants from near roadsides (Kliejunas 1994, Roth and
others 1987, Zobel and others 1985).
Building and maintaining roads may
exacerbate root diseases. Wounded trees and conifer stumps created and not
removed during road building provide infection courts for annosus root
disease; the disease may then spread through root contacts to kill a patch of
trees (Otrosina and Scharpf 1989).
Trees damaged or stressed by road
building-through direct wounding of stems and roots, covering of roots with
side castings, or compacting of soil over roots-become susceptible to various
tree diseases. Armillaria root disease is benign in deciduous stands where
only injured trees are attacked, but more serious in conifer stands where
pockets of disease are initiated (Shaw and Kile 1991).
Oak decline is associated with
poor sites, older stands, and road building or other disturbance (Wargo and
others 1983). Black stain root disease (Leptographium wagneri) attacks
stressed conifers associated with disturbance, especially compaction caused by
road building; in pinyon pine (Pinus monophylia), it is associated with
roads and campsites (Hansen 1978, Hansen and others 1988, Hessburg and others
1995).
Droopy aspen disease is associated
with road building and compaction, but the pathogen identity is unknown (Jacobi
and others 1990, Livingston and others 1979). Sap streak disease in sugar
maple is associated with compaction from roads and from direct injury to trees
(Houston 1993).
Road building can be planned to
help reduce the spread of some forest tree diseases: mistletoe is spread by
the forcible ejection of the mistletoe seeds. In young plantations or
pole-sized stands, roads can subdivide an area to prevent mistletoe seeds from
reaching a healthy stand (Hawksworth and Wiens 1996). In Texas, roads could be
planned to separate a portion of a stand with oak wilt from healthy trees. The
act of building the road (if extensive enough) severs root connections and
prevents tree-to-tree movement of the pathogen (Appel and others 1995,
Rexrode and Brown 1983). In other areas, new or established roads may have the
unintended effect of breaking the continuity of host roots and thus halting
the spread of laminated root rot (Phellinus weirii) and other root
diseases (Hadfield 1986, Thies and Sturrock 1995).
Roads indirectly contribute to
disease spread by giving people access to remote forests and ways to transport
material long distances. New pockets of both oak wilt and beech bark disease
(Houston and O'Brien 1983) may have resulted from moving firewood from the
forest to a homesite (Appel and others 1995, Rexrode and Brown 1983).
Pitch canker (Fusarium
subglutinans) was recently reported on Monterey pine (Pinus radiata) in
California; previously, it had been found on little-leaf and slash pines in
the South. A single introduction is thought to be responsible; 117 vegetative
compatibility groups are found in Florida but only 5 in California, and 70
percent of the isolations in California are from a single group, likely
carried on a tree transported as an ornamental (Correll and others 1992,
Storer and others 1995).
Campers who use roads to get to
remote sites in Colorado and other states have caused significant mortality by
carving on aspen and birch, which provides pathways for various fungi that
cause cankers and quickly kill the trees. Many trees are unintentionally
damaged, for example when campers hang a gas lantern on a branch too close to
the trunk of a tree, thereby causing heat damage.
One abiotic disease has caused
significant damage. In the Lake Tahoe basin in California, trees were killed
by salt put on the roads to reduce ice. This problem also has appeared in some
areas of the Midwest and east coast (Kliejunas and others 1989, Scharpf 1993,
Scharpf and Srago 1974). Needle and rust diseases spread long distances by
spores and do not appear to be influenced by roads or road building.
Issue-The introduction of roads into the closed
forest environment creates corridors by which predators can enter and affect
native populations.
Findings-Forest roads create corridors by which
predators, especially people, can enter the forest environment and affect
wildlife populations. Nest depredation of songbirds may increase by predators
attracted to edges. Evidence for edge effects, however, is highly variable (Paton
1994). Although evidence has been found for local edge effects in cowbird
parasitism and nest depredation, their effects on bird populations is not
documented. Geographic location and large-scale patterns in the amount of
forest and non-forest habitats may be more important in determining the
reproductive success of forest songbirds (Donovan and others 1997, Robinson
and others 1995). Forest carnivores apparently travel on roads in winter when
snow is deep, and thus the road system alters and enhances their ability to
move (Paquet and Callaghan 1996). Wolves and grizzly bears are two key species
that have chronic, negative interactions with people, and roads are a key
facilitator. Repeated, negative interactions of these two species with people
increase mortality of both species and often cause high-quality habitats near
roads to be population sinks (Wisdom and others 2000). High road densities
are associated with a variety of negative human effects on several wildlife
species (Brocke and others 1988). People directly affect snakes by collecting,
harassing, and killing them (Wisdom and others 2000). Increases in illegal
hunting pressure, facilitated by roads, also negatively affect populations.
Moose, wolves, caribou, pronghorn antelope, mountain goat, and bighorn sheep
are particularly vulnerable to this kind of predation (Lyon 1985, Wisdom and
others 2000).
Issue-Roads provide access
to and increase the opportunity for applying a variety of chemicals in
national forests. Some applications target the roads, such as with road
surface treatment; other chemicals are intended for adjacent ecosystems to
control pests and fertilize vegetation. Materials also are added to roads by
traffic, such as asbestos from brake linings, oil leakage, and accidental
spills. Some portion of applied and spilled chemicals eventually reaches
streams by drift, runoff, leaching, or adsorption on soil particles. Roads
also increase the nutrient delivery to streams by removing vegetation,
rerouting water flow paths, and increasing sediment delivery. And roads
increase the likelihood of toxic spills associated with accidents along
streamside corridors.
Findings-Chemicals applied on and adjacent to roads can enter
streams by various pathways. The likelihood of water quality deterioration
from ground applications is a function of how much chemical is applied, the
proximity of the road to a stream, and the rainfall, snowmelt, and wind events
that drive chemical and sediment movement. The risk is a function of the
likelihood of water-quality deterioration and exposure of organisms, including
people, and how susceptible the organisms are to the pollutants. (A large
proportion of Forest Service roads are low standard and few if any chemicals
are applied, so the risk of chemical contamination for most Forest Service
roads is relatively low.)
Chemicals are applied directly to roads and adjacent rights-of-way for
various purposes, including dust abatement, stabilizing the road surface,
deicing, fertilizing to stimulate plant growth on road cuts and fills, and
controlling weeds and the invasion of non-weedy plants onto the roadway (Furniss
and others 1991, Norris and others 1991, Rhodes and others 1994). Applied
chemicals can enter streams directly when they are applied, but little is
known about the effects of these chemicals on stream biota (Furniss and others
1991). Norris and others (1991) provide a comprehensive review of the types
and amounts of fertilizers, pesticides, and fire retardants applied to forests
in the United States, although little information is given to distinguish
road-related from aerial applications. They report that most herbicides are
applied by ground-based equipment, presumably using roads for access; that
ground-based applications in or near aquatic zones can result in chemicals
entering streams by drift or direct application; and that these problems are
more serious when the chemicals are applied from the air. Movement of sediment containing adsorbed chemicals is possible, and
the risk increases with increasing persistence (Norris and others 1991). The
amount of input by this pathway is thought to be small, however; it is a more
likely pathway for entry of salts applied for deicing and of fertilizers
applied to road fills.
Increased nutrient supply to
streams from roads is proportional to the area disturbed and maintained free
of vegetation and the amount of sediment delivered. Increased nutrients rarely
have detrimental effects on stream water quality, but they may modify the
composition of aquatic biota (Hawkins and others, in press). Few studies
examining watershed responses to logging separate the effect of road building
from those of the broader disturbance associated with removing timber. In one
such study, Swank (1988) monitored stream chemical composition during the
pretreatment, road building, logging, and post-treatment phases in a
cable-logged watershed in the southern Appalachian Mountains. No stream
chemical response was found to result from the road-building phase of the
watershed treatment.
Nutrient movement to streams often
increases significantly after timber harvest operations (Frederiksen and
others 1973, Hombeck and others 1973, Likens and others 1970, Pierce and
others 1972, Swank and Waide 1988). The primary intent of these studies was to
assess onsite nutrient losses, with changes in water quality a secondary
concern. All cited studies report increases in nitrogen and phosphorus
concentrations in streams after treatment. In general, nutrient loss to
streams is roughly proportional to how much vegetation was removed. For
example, three studies at Hubberd Brook in New Hampshire compared three
treatments: clearcutting with a herbicide treatment to suppress vegetation
regrowth (Likens and others 1970), clearcutting without suppressing regrowth
(Pierce and others 1972), and strip cutting of one-third of the forest (Hornbeck
and others 1973). The three studies found nitrogen concentrations in streams
reduced-most by the first treatment, less by the second, and least by the
third. These findings suggest that residual or reestablished vegetation
immobilizes released nutrients, thus diminishing the disturbance effect.
Although roads might not respond in the same way because of drainage
rerouting, we expect that nutrient mobility is proportional to the area
maintained in a disturbed, revegetated state.
Hazardous chemical spills from
vehicle accidents can pose a direct, acute threat of contamination to streams.
The risk of hazardous chemical spills resulting from vehicle accidents
adjacent to waterways is recognized and documented by the National Forest
System and by state transportation departments (IDT 1996). Risk-analysis
models of accident-related chemical spills are available, but they are
designed for paved roads in non-mountainous terrain. Models take into account
risk to human health, traffic frequency, vehicle type, and proximity to
water. Possible contaminants include any substance being transported, such as
fuel, pesticides, chemicals used in mining, fertilizers, and fire retardants.