Mechanical Site Preparation

Mechanical site preparation is designed to clear unwanted vegetation, move logging slash, or cultivate upper soil layers. Techniques include (Nyland 1996):

Bedding: plowing to form cultivated beds with a slightly elevated center.
Blading: using bulldozer-mounted blades to uproot trees and shrubs.
Chaining: dragging heavy chains using bulldozers to remove vegetation and scarify soil surface.
Chopping: using heavy rolling choppers to crush or break apart debris.
Countouring, terracing, leveling: moving soil with bulldozers to change medium-scale topography and drainage.
Disking: plowing with rollers to incorporate organic material into the mineral soil.
Ditching: constructing ditches to improve soil drainage within the rooting zone.
Piling: using bulldozer-mounted blades to push debris into piles or rows (windrows).
Plowing: dragging a plow by bulldozer to scarify surface soil and mix surface litter.
Subsoiling: plowing the subsoil to break or puncture an impervious soil layer and improve drainage.
Shearing: cutting residual trees using a swept-back blade horizontally mounted on a crawler tractor.

Mechanical site preparation techniques are often used in combination. For example, pine plantations in the Southeast are often prepared by blading, piling, and bedding. In some cases, mechanical treatments are used with prescribed burning or herbiciding. For example, logging slash often is pushed into rows to facilitate subsequent burning (Nyland 1996).

The large machinery needed for most mechanical site preparation treatments works best in treating large areas; it therefore is most often used with even-age regeneration systems. Even scarification has proven impractical with partial cuttings. Most often, foresters use mechanical site preparation with artificial regeneration methods to reduce slash and debris and facilitate planting operations. For some species, such as yellow birch in the Lake States or conifer species in the West, mechanical site preparation is used to enhance natural regeneration by improving seedbed characteristics and reducing competition (Nyland 1996).

Advantages and Disadvantages of Mechanical Site Preparation

Mechanical site preparation can provide several benefits (Nyland 1996):

• Facilitating movement through a stand reduces costs of tree planting or other operations.
• Flattening slash alters fuel characteristics and reduces fire hazard.
• Flattening and breaking up slash also promotes decomposition of woody debris and destroys the habitat for many pests.
• Loosening the organic material and upper mineral soil reduces surface compaction, facilitates infiltration, and may enhance seedling root penetration.

Mechanical site preparation methods also have some undesirable side effects (Nyland 1996):

• Removing or disrupting the protective organic covering, loosening upper soil layers, or compacting soil can increase erosion potential on slopes.
• Mechanical methods require large investments in machinery and equipment, and take considerable time to apply across large areas.
• Obstacles like rocks, stumps, and features of the topography hinder movement of the equipment, and some may reduce the effectiveness of a treatment.
• Heavy machines compact surface layers in some soils and during some seasons, thereby reducing infiltration, percolation, and soil aeration.
• The hazard of operating machines on slopes exceeding about 30 percent makes many mechanical methods impractical.
• Surface disturbance creates a good seedbed for many herb and shrub species, and these may interfere with the new trees.
• Mechanical methods will not prevent sprouting of broadleaved shrubs and tree species, and may only temporarily reduce competition.
• Removing organic layers may reduce nutrients in the soil, and may necessitate replacement by fertilizer application.
• Removing and burning the slash and other organic materials prior to bedding mobilizes some nutrients, which then may leach from a site.
• Reducing logging slash removes its shade, increasing the maximum and minimum surface soil temperatures, and possibly reducing seedling survival.
• Reducing coarse woody debris and herb cover can impair habitat of some fauna and flora.