Preventing Soil Compaction
The health of forest soils is vital to plants. Soils provide habitat for symbiotic fungi, nitrogen-fixing bacteria, decomposers, and many other animals. Yet the health of soils is fragile. They can be damaged by high-intensity fires. Restoration treatments can prevent such fires, yet treatments themselves may damage soils. This is a guide to minimizing soil impacts during restoration treatments.
Forest fires can cause severe damage to soils. Depending on the severity of the fire and distribution of fuel, damage can include the loss of organic material near the soil surface, the formation of a water-repellent layer, and the destruction of soil structure (Selmants et al. 2003). As a result, erosion and long-term degradation of forest ecosystems can occur.
Many of fire’s negative effects on soils can be minimized with restoration treatments aimed at preventing catastrophic fires. However, mechanical thinning projects can introduce their own damage to forest soils, especially soil compaction. Compaction can reduce soil porosity and decrease the rate of water infiltration (Selmants et al. 2003). These effects can lead to increased runoff and erosion, similar to that seen following fires.
In addition, prescribed fires after thinning can cause extensive soil heating and, when burning in thick litter, can damage roots and cause tree mortality. Slash pile fires can cause intense soil heating, destroying the soil seed bank and mycorrhizal fungi (Korb and Springer 2003). For these reasons, soil effects should be considered when planning post-thinning burns.
Soil impacts should be considered during all stages of a restoration project: in the planning process, during implementation, and after thinning is completed. These steps can aid in minimizing impacts:
- Make prescriptions reasonable for easier cutting. Leaving a few small trees where a dense doghair thicket stands makes it difficult for the logger to cut the rest down, resulting in more time needed and greater impacts to soil. If a thicket is retained for wildlife habitat, it may be better to cut trees from its edge while retaining the thicket’s integrity.
- Designate skid trails ahead of time. Use a pattern, such as a zigzagging line, that occupies as little surface area as possible (Garland 1997). Take advantage of areas least susceptible to compaction, such as areas with some surface rocks.
- Consider manual treatments such as hand felling, especially in ecologically sensitive areas.
- Consider a mosaic of treatments that leaves some areas undisturbed.
- Work when soils are least sensitive. Avoid wet periods. One good option is to work when soil is frozen or dry. A layer of snow can provide an additional buffer against compaction.
- Maintain soil organic matter and keep surface soil, litter, and slash in place while harvesting. This can increase resistance to compaction and protect soils from erosion. The tradeoff is that this can also increase the severity of burns, controlled or otherwise, that follow mechanical treatments – so removing slash before burning may be a good idea.
- Use a cut-to-length (CTL) harvesting system that processes trees in the woods. The resulting slash can be disposed of in front of the machine and driven on to buffer soils from compaction. The use of a CTL system also reduces the need to drive to each individual tree by harvesting those within a set radius of the machine, as opposed to harvesting requirements for a drive-to-tree feller buncher.
- Use a forwarding system rather than a log skidding system in conjunction with a CTL harvester to collect logs piled in the woods by driving on the same slash mats. Later, the forwarder may be used to collect and remove the slash itself.
- Fell trees to skid trails in order to minimize skidding distances, soil disturbance, and damage to vegetation (illustration; Garland 1997).
- Consider hot loading, or loading logs directly onto trucks rather than decking them first.
- Consider ripping skid trails to rehabilitate them after use.
- Burn slash where soil is already disturbed, such as on loading areas or roads.
- Lop and scatter slash before burning in order to prevent intense soil impacts under slash piles. The tradeoff is that excessive broadcast slash can greatly increase the intensity of prescribed fires and make them difficult to control. In particular, hot spots may occur under larger branches.
- Consider alternatives to burning slash. It can be removed for chipping, or transported to a biomass or pellet plant. If they are to be removed, slash piles should be placed along roads or skid trails for easy loading.
- Severely disturbed areas – such as former roads and sites of slash pile fires – can be remediated with native fungus. Spreading chipped wood and bark on such areas after inoculating it with fungi speeds the ecological recovery process and may help prevent the spread of noxious plants.
- Construct fire control lines with an eye toward minimizing soil compaction and erosion problems. Locate control lines away from erosive soils. Minimize the time control lines are in place, and rehabilitate them quickly after burning. Install water bars on steep slopes. Strike a careful balance between the straight control lines preferred by fire managers and zigzagging lines that may better prevent soil compaction and erosion.
- Seeding with native herbaceous plants can help stabilize soils after fire. Applying seed into the ash bed immediately after burning helps hold the seed in place, reducing the need to drill or otherwise disturb the surface.
- Carefully consider the frequency of future prescribed fires. They are crucial in maintaining forest structure but, if repeated as often as every two years, may cause declines in soil nitrogen available to plants (Wright and Hart 1997). Somewhat longer intervals may be preferable.
- DeBano, L. F., D. G. Neary, and P. F. Ffolliott. 1998. Fire’s effects on ecosystems. New York: Wiley.
- Garland, J. J. 1997. Designated skid trails minimize soil compaction. Oregon State University Extension Service publication EC1110.
- Korb, J. E., and J. D. Springer. 2003. Understory vegetation. Pp. 233–250 in Ecological restoration of southwestern ponderosa pine forests, ed. P. Friederici. Washington, D.C.: Island Press.
- Selmants, P. C., A. Elseroad, and S. C. Hart. 2003. Soils and nutrients. Pp. 144–160 in Ecological restoration of southwestern ponderosa pine forests, ed. P. Friederici. Washington, D.C.: Island Press.
- Wright, R. J., and S. C. Hart. 1997. Nitrogen and phosphorus status in a ponderosa pine forest after 20 years of interval burning. Ecoscience 4:526–533.
This is a modified version of ERI's publication Working Paper 05: Limiting Damage to Forest Soils During Restoration.