Contribution of Roads to Forest Fragmentation

Adapted from: Reed, R.A., Johnson-Barnard, J., and Baker, W.A. 1996. "Contribution of Roads to Forest Fragmentation in the Rocky Mountains." Conservation Biology 10: 1098-1106.

Logging truck and new logging road in Roundy Timber Sale area, west slope Boulder Mountain, Utah. Photo © 1999 by Ray Wheeler.

Increasingly, previously extensive, continuous tracts of forest are being reduced to widely dispersed patches of remnant forest vegetation by logging and road-building, but few measures of the effects of roads on forest fragmentation are available. Fragmentation affects animal populations in a variety of ways, including decreased species diversity and lower densities of some animal species in the resulting smaller patches. This study seeks to quantify the effects of roads and logging activities on forest habitat.

Roads precipitate fragmentation by dissecting previously large patches into smaller ones, and in so doing they create edge habitat in patches along both sides of the road, potentially at the expense of interior habitat. As the density of roads in landscapes increases, these effects increase as well. McGurk and Fong (1995) considered the additive effects of clearcuts and roads, but did not measure the amount of associated edge habitat. Thus a more direct measurement of the impacts of roads on landscapes is needed.

The objectives of this study were to determine the importance of roads in delineating and quantifying landscape structure, including the proportion of the landscape occupied by edge habitat, and compare the effects of roads and clearcut logging on forest fragmentation. Though this particular study was conducted in subalpine conifer forest habitat in southern Wyoming, the effects of roads and logging in similar montane and subalpine forestlands at higher elevations on the Colorado Plateau may be similar.

Interpretation of Edge-Effects

Using USFS Resource Information System maps and a geographical information system (GIS), a comparison was made between the 1950 landscape and road network and that of 1993. A significant increase in edge habitat, due to clearcutting and road construction, was seen throughout the study area.

Figure 1. Number of forest patches.

The landscape measurement of total perimeter increased considerably (approximately 20%). The number of patches (areas of intact forest) in the study area increased 179 and 158% respectively (see Figure 1), indicating extensive fragmentation of previously contiguous areas. The mean patch area decreased 65 and 62%, indicating  much smaller patches of undisturbed forest. Measurements comparing roads to clearcut logging indicate that the construction of roads causes even greater fragmentation than clearcutting alone (see Figure 1). An implication of these results is a significant reduction in the amount of interior forest habitat available to requisite interior species, such as pine martens, brown creepers, spotted owls, or gray wolves.

The apprehension concerning the introduction of edges onto landscapes results from potentially detrimental microclimatic and biological changes, relative to intact forest, which occur along edges (Noss and Cooperrider 1994).  Edge habitats along roads and clearcuts experience microclimatic changes, including increased evaporation, increased temperature, increased incident solar radiation, and decreased available soil moisture. Road edges and clearcuts may not have equal magnitude and impact on edge environments however. For instance, clearcuts have greater depth of influence in windy conditions.

Figure 2. Road densities in the southeastern Jemez Mountains, New Mexico in 1935 (left) and in 1981 (right). Images courtesty of Allen, Betancourt, and Swetnam. 1997. Southwestern U.S. LUHNA pilot project. USGS Biological Resources Division.

Road edge habitat is unique in many respects from natural edges or edges produced by clearcuts. Whereas natural and clearcut edges will become progressively less defined as the forest regenerates to a patch, road edges tend to exist long-term and be disturbed more frequently. Road edges increase air pollution, soil erosion, noise, disturbance by human activity, and exotic species introductions, and may induce populations changes in the vegetation and animal communities included in the areas of edge influence. These factors combine to create particularly deleterious habitat situations, and endanger the existence and perpetuation of all native species on the landscape.

The simple appearance of the landscape, the actual area in clearcuts and roads, and the density of roads are incomplete indicators of the impacts of logging.  Even though roads occupy a smaller fraction of the landscape in terms of land area, their influence extends far beyond their immediate boundaries. The amount of land area that is edge habitat created by clearcuts and roads is 1.5-2.5 times the land area actually occupied by the clearcuts and roads, so the cumulative area affected is approximately 2.5-3.5 times the actual area of clearcuts and roads (see Figure 2).

Applications to Forest Management

This study provided strong evidence that roads associated with logging activities can have, and often do have, potentially detrimental impacts on animal and plant communities. Considering the findings in this study, the investigators recommend forest-wide systematic obliteration and revegetation of a significant number of forest roads.  Not only should existing, little-used roads be revegetated, but road obliteration and revegetation should become part of the overall harvesting process on National Forest land.  Timber harvests should be planned to minimize impact on the landscape and exacerbation of the current landscape fragmentation problems on many forestlands.

Literature Cited:

McGurk, B.J., and D.R. Fong, 1995. Equivalent roaded area as a measure of cumulative effect of logging. Environmental Management 19: 609-621.

Noss, R.F., and A. Cooperrider. 1994. Saving nature's legacy: protecting and restoring biodiversity. Island Press, Washington D.C.