Why access routes matter so much
To harvest, thin or manage a forest, people and machines have to get in and wood has to get out. That requires an access network, and the network often has a larger effect on soil and streams than the cutting of trees. Bare roadbeds, compacted ground and channels that concentrate water are among the most common sources of sediment in forested watersheds, according to the general pattern reported by forest managers and researchers in many regions.
A forest floor is normally covered by leaves, needles and roots that soak up rain and hold soil in place. A road replaces that surface with hard, packed ground that sheds water quickly. Understanding this simple change explains most of the problems and most of the fixes.
The parts of an access network
- Main haul roads carry trucks and are built to last. They usually have a gravel or compacted surface, ditches and culverts.
- Secondary or spur roads branch off for specific harvests and may be closed afterward.
- Skid trails are lightly built or unbuilt routes along which machines drag or carry logs from the stump to a landing. They are used more or less temporarily.
- Landings are cleared areas where logs are gathered, sorted and loaded. They tend to be heavily used and compacted.
- Stream crossings are the points where any of the above meets water and where care matters most.
What goes wrong
Erosion. Water running down a bare road or trail picks up soil, and the longer the stretch of uninterrupted slope, the more energy it has. The result can be gullies, washouts and muddy runoff. The general processes are covered in the discussion of soil erosion in forests.
Compaction. Heavy machines squeeze soil particles together, reducing the air spaces that roots, water and soil organisms need. Compacted soil takes in less rain, so more runs off the surface, and tree roots may grow poorly. Soils that are wet and fine-textured are generally the most easily compacted, and recovery can take many years.
Sediment in streams. Fine soil carried into streams can cloud the water and settle on gravel beds used by fish and insects. This affects forest streams and, downstream, water quality for people. Poorly built crossings are a frequent culprit.
Altered water flow. Roads and ditches can intercept water moving through the soil and send it rapidly into channels, changing how a catchment responds to storms.
Slope failure. On steep ground, cutting into a hillside or piling loose fill can contribute to landslides, especially during heavy rain. How roots help hold slopes is explained in the article on slope stability.
Planning comes first
Most of the benefit comes from decisions made before any machine moves.
- Use the least amount of road needed. More roads mean more disturbance, so planners try to reach the whole area with a modest, well-placed network.
- Choose good locations. Stable ridges and gentle, well-drained slopes are generally better than steep, wet or unstable ground. Roads that run along contours tend to be easier to drain than those that run straight up a slope.
- Keep away from water. Roads and landings are usually kept well back from streams, with protective strips left in place. These strips work in a similar way to riparian buffers.
- Minimize crossings, and make the necessary ones at right angles to the stream with proper structures.
- Reuse existing roads where they are in good condition, rather than starting new ones.
- Mark skid trails ahead of time, so operators follow a set pattern instead of making their own routes. Using the same trails repeatedly confines compaction to a small part of the stand.
A written management plan often includes a map of planned roads, trails and landings.
Building and maintaining roads
Good practices commonly include:
- Surfacing with gravel or other material in places where traffic is heavy or the soil is soft.
- Shaping the road so water drains off, using a slight crown or slope to the side.
- Cross-drains, broad-based dips and water bars, which break up the flow of water and turn it off the road before it gains force. Spacing normally gets closer as slopes steepen.
- Culverts sized and placed to carry runoff without washing out, with outlets protected by rock where needed.
- Bridges or other crossings that do not block fish passage or constrict the stream.
- Seeding or mulching cut banks and bare soil to help vegetation return.
- Regular inspection, especially after heavy rain, because small drainage problems can grow quickly.
Working with skid trails and landings
- Avoid wet periods. Harvesting when the soil is dry, frozen or covered by snow reduces rutting and compaction. Many operators halt work in wet spells.
- Use equipment suited to the ground, such as low-ground-pressure machines, or lay slash (branches and tops) on trails to spread the load.
- Keep trails off steep slopes or run them at a gentle angle across the slope, never straight down.
- Limit landing size and place landings on level, well-drained ground away from water.
- Fell trees toward trails so logs can be moved with less driving across the stand.
Closing out after the harvest
When work is finished, closing the job properly is as important as the work itself. Skid trails and landings are usually smoothed and drained, with water bars put in and bare soil covered with slash, mulch or seed. Temporary roads may be blocked or closed, and permanent ones are left drained and stable. Leaving a road without drainage is a common source of problems years later.
Related methods
Planning of this kind is a core part of reduced impact logging, which treats the layout of roads and trails as a basic step. Many regions publish best management practices for forestry, and forest certification standards generally require attention to road construction and soil protection. For small landowners, planning trails and access in a woodland and working with a professional forester are useful ways to get the layout right.