Forest soils play a key role in tree growth. They provide for storage and transport of nutrients and water, air space for tree roots to breathe, and the foundation for holding a tree in place. Protecting soils during forest management activities is crucial to a soil’s ability to provide these necessary components of tree growth.
Let’s take a quick look at a few soil characteristics and how they affect forest productivity. Soil texture refers to the relative amounts of sand, silt, and clay particles in a soil. For example, a soil high in silt and clay particles is referred to as a silty clay; a soil with roughly equal components of silt, sand, and clay is referred to as a loam. Texture affects a soil’s water-holding capacity, drainage, and root growth. A soil’s texture is not really impacted by forestry practices.
Soil structure is the arrangement of soil particles into clumps or aggregates, ranging from single grains (for instance, a very sandy soil) to masses of stuck-together particles (such as a heavy clay soil). Unlike soil texture, a soil’s structure can be easily altered by forest management practices.
Organic matter is derived from dead plant material that accumulates on the forest floor, and is gradually mixed into the soil over time. Organic matter, or humus, provides a source of nitrogen and other soil nutrients. The protection of topsoil – the upper soil layer rich in organic matter – is critical for retaining nutrients important for tree growth. A thick humus layer also holds moisture in the soil, providing much-needed moisture during Oregon’s typical summertime droughts. Humus also inhibits the sprouting of weed seeds.
The slope, or topography, of a forest soil is a factor in the erosion potential of a soil. Steeper topography is more prone to the loss of soil through erosion than gentle topography.
Poorly-planned forestry activities can damage forest soils in several ways: compaction, increased susceptibility to erosion, and loss of topsoil and organic matter. These impacts do not always occur, nor are the impacts equal when they do occur.
Damage to forest soils can be minimized or prevented altogether through the use of Best Management Practices, or BMPs, while planning and implementing forestry operations. BMPs are practical ways of going about your business in the forest, and are a good investment in the future health of your forest. These next few paragraphs address possible negative impacts to your soil, and a few BMPs that will help minimize those impacts.
Soil Compaction an result from the use of heavy machinery, such as crawler-tractors and skidders, on forest sites. Compaction causes a change in soil structure, such as the shift from a granular to a platy, or compact structure. Operating heavy equipment on wet soils can make compaction even worse
Research has shown that compacted soils are less favorable for good plant growth because of high soil density, reduced pore space, and poor aeration and drainage. The high density of compacted soils inhibits root penetration and growth, and can reduce supplies of air, water, and nutrients needed for optimum root growth.
The poor drainage of compacted soils can lead to soil erosion, too. Besides causing the loss of valuable soil nutrients, erosion can deliver sediment to streams, thereby reducing water quality and spoiling salmon spawning habitat.
Best Management Practices
Here’s where the BMPs come in. We can limit the amount of compaction in our forest soils by using low-ground-pressure equipment and designated skid trails during harvest operations. Wider tracks on crawler-tractors, for example, spread the weight of the equipment out over a larger area, thus lowering ground pressure and lessening the potential for compaction. The use of designated skid trails focuses compaction on the trails and minimizes impacts elsewhere. Since most compaction occurs within the first few equipment passes over the same piece of ground, the use of designated skid trails is an especially effective practice for maintaining forest productivity.
Other possible best management practices include skidding on frozen ground or ground covered with several inches of snow. Another technique is “shovel logging,” where excavators designed specifically for logging move logs from the stump to the landing. Shovel loggers travel on limbs and other forest debris, thereby minimizing the amount of compacting pressure on the forest floor.
Susceptibility to erosion is based on a soil’s texture, amount of organic matter, and slope. Soil properties conducive to erosion include a heavy clay content, little organic matter, and steep slopes.
Best management practices to minimize erosion include:
- Minimize ground-based logging on steep slopes.
- Avoid skidding on soils that are unstable or wet.
- Utilize water bars on sloping skid trails.
- Keep road culverts and ditches well maintained.
- Minimize disturbance to the humus layer.
- Leave slash on the forest floor (when consistent with fire management and reforestation objectives).
- Rip or till skid trails and landings to improve water infiltration, especially on steeper slopes.
- Seed noninvasive grasses on disturbed soils, especially those on steeper slopes.
Consider potential impacts to organic matter supplies when planning your forestry operations, too. For instance, while “whole-tree yarding” (where limbs are yarded to the landing along with the merchantable stems) may facilitate follow-up reforestation, long-term organic matter input to the forest floor may suffer. Also, hot slash burns can cause damaging loss of organic matter, thus leaving the soil nutrient-deficient. Such a condition can also leave the soil vulnerable to erosion, especially on steep slopes.
The Oregon Forest Practices Act is designed to protect soils and other forest resources important to Oregonians. Be sure to check out their website for more information.