Restoration thinning of ponderosa pine forests often results in large quantities of slash that can be challenging to treat. As is true of most aspects of forest restoration, there is no one-size-fits-all approach for dealing with slash. In fact, there are several options commonly used in southwestern forests, each with its own advantages and disadvantages. It is important for land managers to understand the long-term implications of slash removal methods on ecosystem processes. This paper presents standard methods for disposing of slash, as well as the ecological and logistical tradeoffs associated with each method.

Ecosystem Considerations

Soil

Healthy soil is the building block for all other ecosystem processes. The use of mechanized equipment for thinning a restoration site and piling slash can cause soil compaction and degrade the overall quality of the soil (Elseroad et al. 2003). High-intensity fires, whether broadcast burns or in slash piles, can also severely damage soils, leaving them sterile and unproductive.

Fungi

Mycorrhizal fungi are part of the natural biota in southwestern forest ecosystems and often serve as a good indicator of ecosystem health. Found in the soil, these fungi promote the growth and diversity of native plants, bind soil particles, and inhibit the growth of invasive plant species (Marler et al. 1999). Approximately eighty percent of vascular plants form a mutualistic relationship with arbuscular mycorrhizae (Korb and Springer 2003). Research suggests that mycorrhizal fungi can be disrupted when slash is burned, even at temperatures as low as 80°C (Selmants et al. 2003).

Seed bank

Existing seeds in the soil are important for the recolonization of a restoration site. If the seed bank has adequate numbers of viable native seeds, a plant community may be able to reestablish itself without manual seeding (Korb and Springer 2003). Seed banks vary according to location, site history, and the degree of disturbance. Low-intensity short-duration burns generally do not severely impact the seed bank; in fact, such fires appear to stimulate the germination and growth of many native herbaceous plants (unpublished data, Scott Abella, Ecological Restoration Institute). However, invasive plants are likely to colonize both areas disturbed by mechanized equipment and areas where high-intensity fires have burned, resulting in reduced regeneration of native plants.

Plants

The method by which slash is treated site directly influences understory plants. Low-intensity broadcast burns of slash release nitrogen and can result in rapid plant growth. High-intensity burns and heavy use of mechanized equipment can disturb soils and result in widespread colonization by invasive plant species (Korb et al. 2004; Haskins and Gehring 2004).

Wildlife

Southwestern ponderosa pine forests are home to many birds, mammals, reptiles, and amphibians (Chambers and Germaine 2003). If left at a restoration site, larger coarse woody debris can serve as habitat for a number of species, especially small mammals (Randall-Parker and Miller 2002).

Slash Treatment Methods

Leaving some slash on a restoration site can result in ecological benefits. Course woody debris can protect forest soils and new seedlings, provide habitat for wildlife, and release valuable nutrients such as nitrogen or phosphorus into the ecosystem over time (Graham et al. 1994). Some research suggests that leaving 8 to 13 tons per acre of course woody debris is appropriate in southwestern ponderosa pine forest ecosystems; however, this value is only a guideline and will not be suitable for every restoration site (Graham et al. 1994).

Unfortunately, leaving too much slash can increase fire risk and lock up nitrogen. In dealing with slash managers must balance fire risk with the ecological benefits of leaving some slash in place. They also must assess what is logistically and financially feasible. Each of the following slash treatment methods has advantages and disadvantages.

Pile and Burn

Unmarketable wood and debris can be mechanically or manually gathered and piled in small stacks throughout the restoration site. The piles are then burned when conditions are acceptable – usually when overall fire risk is low.

• Advantages. Piles can be burned in a controlled manner. This method is relatively inexpensive.
• Disadvantages. Piled slash can provide habitat for beetles that attack pine trees (Parker 1991). Slash pile fires can burn as hot as 700°C at the soil surface and at 250°C four inches under the surface (DeBano et al. 1998). Heated soils experience changes to pH, total nitrogen, and organic carbon. The physiochemical modifications become permanent when soil temperatures reach 180°C (Giovanni et al. 1988). Burning slash piles with high-intensity fire significantly reduces densities of arbuscular mycorrhizal propagules (Korb et al. 2004); in fact, these fungi are disrupted at temperatures as low as 80°C (Selmants et al. 2003). Slash pile scars often remain unvegetated for a long time unless seeds are manually distributed. In some cases, invasive species take hold at slash pile sites and spread from them (Korb et al. 2004).
• Recommendations. Build small slash piles on existing roads or disturbed areas to minimize damage to undisturbed soils. If it is not possible to build piles on an existing road, revegetate pile sites with native seeds and inoculate them with arbuscular mycorrhizal propagules. This can be done by scooping some nearby soil onto them; avoid soil from areas infested with invasive plants. Research suggests that these linked efforts greatly minimize slash pile scars.

Broadcast Burn

If funding is limited or a minimal amount of slash has been produced, scattering and broadcast burning the slash can be a viable option.

• Advantages. Low-intensity fires release nitrogen into the ecosystem and can stimulate understory plant growth. Prescribed burns reduce the risk of catastrophic wildfire by decreasing the overall forest fuel load.
• Disadvantages. Broadcast burns feeding on heavy loads of slash can be destructive to soils, fungi, the seed bank, and plants, and can kill trees remaining after thinning. Smoke from large broadcast fires may be burdensome to the public.
• Recommendations. Scatter slash away from leave trees in order to protect them during prescribed fire. If large amounts of slash remain, broadcast burn during cooler, wetter periods when the slash is damp. Ensure that slash quantities are small enough to retain some open burn windows.

Lop and Scatter

• Advantages. Compressed slash breaks down faster than undisturbed course woody debris. Mechanical compression can reduce the intensity of prescribed burns (Jerman et al. 2004). Compressed slash holds less oxygen and is not in a vertical arrangement, resulting in lower tree mortality when broadcast burned.
• Disadvantages. If slash is mechanically compressed, increased use of machinery and operator time results in higher expense. Soil compaction and disturbance can result. Tightly packed slash can cause high soil temperatures during fire.
• Recommendation. Rake slash away from old-growth trees to further ensure that they are not damaged by broadcast burning (see Protecting Old Growth for more information).

Haul It Away

This method is generally used only if the restoration project is small and minimal slash has been produced. The slash is loaded on trucks and taken to another location, where it is usually burned.

• Advantages. Since the slash is not piled or burned on the restoration site, damage to the soil, fungi, or seed bank will be minimal.
• Disadvantages. This can be a very expensive method for removing slash.
• Recommendation. Try to limit soil impacts at the restoration site by driving and parking trucks only in designated areas.

General Recommendations

• To minimize buildup of pine engraver beetle (Ips pini) populations, try to avoid creating slash from January until the summer monsoon season begins. If slash is created during the late winter through early summer, treating it within 6 weeks will reduce the risk of beetle-caused pine mortality.
• Remove as much slash from a restoration site as the budget allows, while still taking into account the ecological value of retaining some on site. Removing most or all material over 4 inches in diameter can help mitigate beetle outbreaks (Parker 1991).
• Minimize soil damage by understanding what soil type is present and adjusting the season of treatment and use of mechanized equipment accordingly see Preventing Soil Compaction).
• Use native plant seeds to reseed slash pile scars and, if necessary, areas where slash has been broadcast burned.
• In areas of severe soil disturbance – such as slash pile sites – inoculate the soil with arbuscular mycorrhizal propagules or undisturbed soils (see Restoring Forest Roads).

This is a modified version of ERI's publication Working Paper 13: Treating Slash After Restoration Thinning.