Site Preparation

Site preparation is one part of a silvicultural system that may be carried out prior to or after a reproduction method. Site preparation includes any treatment that modifies existing vegetation or physical site conditions to improve germination, survival, and subsequent growth of desired seedlings. The four main types of site preparation are:

• Prescribed burning
• Mechanical
• Chemical (herbicides)
• Biological methods

(Table: Uses of Mechanical, Chemical, Fire and Biological Methods )

Purpose of Site Preparation

Scarification, which modifies soil surface layers, is a common form of site preparation. Scarification techniques may be used to loosen upper soil, to break up the organic layer, to expose mineral soil by removing undecomposed litter and humus; or to mix surface organic materials with mineral layers. Scarification improves seedbed conditions, and facilitates root penetration and infiltration. Mixing organic materials with mineral soil increases decomposition, nutrient release, and moisture-holding capacity. Site preparation may also be used to (Nyland 1996):

• Add new kinds of vegetation to control erosion
• Alter the habitat for potentially damaging agents, including harmful insects, diseases, and animals
• Enhance conditions for wildlife, and improve forage and browse for domesticated animals,
• Reduce fuels that potentially increase the risks of damage from future wildfire
• Remove, reduce, or rearrange impediments to human activities, and improve access for subsequent operations

Active and Passive Site Preparation

In some cases, logging of forest stands creates conditions beneficial for seedling establishment by disturbing soil surfaces. This effect is considered a type of passive site preparation. Past research has shown that both traditional ground skidding and high-lead yarding will often disturb less than 20 percent of the stand area with partial cuts, and not more than 30 percent with complete stand removal (Marquis and Bjorkbom 1960, Dryness 1965, Miller and others 1974, Nyland and others 1977, Balmer and Little 1978). As a result, foresters may select a skidding or yarding method that causes a greater extent or intensity of ground disturbance (Nyland 1996).

Active site preparation may be needed to kill unwanted vegetation, alter soil properties, or reduce logging debris. Active site preparation can be accomplished by prescribed burning, or by mechanical or chemical methods (herbicides). Foresters may apply these treatments across an entire stand (broadcast treatment), or just to selected parts of it (spots, rows, or bands). With natural regeneration or direct-seeding, site preparation must normally cover most of the surface area in a stand. Also, where interfering plants dominate a site, a fairly complete treatment may prove essential. Whether a method lends itself well to broadcast or spot treatments largely depends upon (Nyland 1996):

• Equipment used
• Proportion and amount of area treated
• Topography and surface conditions
• Conditions of soil drainage
• How much a landowner will invest in site preparation

In some cases, foresters may elect not to use site preparation treatments. For example, many hardwood species in the southern Appalachians, particularly oaks, must have advance regeneration in understories before a regeneration harvest. In these cases, managers should protect this advance regeneration and avoid site preparation. Site preparation also should be avoided where managers want to safeguard ground conditions to inhibit some unwanted species, or to prevent surface erosion. Some species, such as sugar maple and black cherry (Godman and others 1990, Marquis 1990), do not need a scarified seedbed (Nyland 1996).

Selecting a Site Preparation Method

Several factors should be considered when deciding on a site preparation method, including:

• Type of plant community present and desired
• Type, abundance, and distribution of unwanted vegetation and debris
• Terrain, surface relief, and soil physical features
• Subsequent management activities planned
• Costs relative to any expected benefits

In many cases, mechanical, chemical, or prescribed burning methods have similar effects (see table). Mechanical methods requiring large machinery often prove impractical on slopes exceeding 25 to 35 percent, and may not produce the desired effects on rocky surfaces or frozen and snow-covered soils. Chemical treatments may prove ecologically unacceptable in sensitive areas or adjacent to ponds and streams. Prescribed burning must be avoided where there is high risk of escape, and where smoke may create health, safety, and visibility problems. The heat may also kill residual trees a landowner wants to preserve, and a burn may only temporarily affect the targeted vegetation (Nyland 1996).