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Chemical site preparation, which involves the application of herbicides, controls competing vegetation before planting or natural regeneration and during the early stages of seedling establishment. In general, herbicides are effective in suppressing most undesirable vegetation and are well suited for use on many sites. Herbicides, when properly applied, also have little site impact. Proper application implies application according to label instructions. With their minimal site impact, herbicides can offer certain environmental advantages over other site preparation treatments. However, in some areas, application of herbicides are of particular concern to other resource users and members of the public.
Chapter 13 of Regenerating British Columbia's Forests, by J. Otchere-Boateng and L. Herring  suggests that for effective site preparation by chemical methods alone, the site must have the following characteristics:
If these criteria are met, then chemical site preparation may be an appropriate option for achieving regeneration objectives. Examples of situations favoring chemical site preparation include:
Several drawbacks to chemical site preparation must also be considered. Not all undesirable vegetation in forest communities can be controlled with the available registered herbicides. Herbicide operations can cause shifts in competing vegetation creating situations where minor herbicide-resistant species can develop into a major competition problem after a treatment. In most cases, herbicide application must wait until shrubs and hardwoods have sprouted to ensure maximum uptake and translocation. This can delay reforestation efforts. In some areas, early seral vegetation provides a nurse crop that protects seedlings from frost. A herbicide application that removes this vegetation may increase the incidence of frost damage on newly planted seedlings.
Herbicides are regulated and not allowed to be applied on all sites. For example, sites close to sensitive areas, such as potable water sources and fish-bearing streams. Finally, there is growing public concern over the use of chemicals in the environment. This continues to impact the use of herbicides for site preparation.
Where the objective of site preparation is not vegetation suppression alone, but includes, for instance, removal of flammable logging residues or mineral seedbed preparation, herbicides may be used in combination with mechanical and burning methods (i.e., `brown and burn'). For example, on a harvested area invaded by herbaceous and brush species, the green (live) vegetation does not burn readily, and tends to shade logging residues. Shaded residue dries more slowly and may remain too wet to burn. A pre-burn herbicide application to desiccate, defoliate, or kill vegetation can allow the slash to warm and dry, thus allowing a good burn. However, the logistics of combining an effective herbicide treatment with a timely burn may be difficult.
Where brush species on the site are susceptible to herbicide treatments, but removal of the standing dead vegetation and/or logging debris is also required (e.g., for planter access), chemical treatment may best be combined with burning or with mechanical site preparation.
Note that combining herbicides with other site preparation treatments may result in a different and occasionally less diverse vegetation complex than would have been created had either of the treatments been used in isolation.
Herbicide applicators attached to mechanical site preparation implements (e.g., Bräcke patch scarifier or powered disc trencher) are available but are not commonly used in British Columbia.
Otchere-Boateng and Herring [in Lavender et al., 1990] review the possible combinations of treatment that include herbicides for site preparation.
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