Date of this Version
Oregon State University, Agricultural Experiment Station, Special Report 1095, September 2009
A century of fire exclusion in dry forests across the United States has resulted in high fuel loads and increasing dominance by fire-intolerant vegetation. Federal, state, and private agencies have adopted a goal of managing forests to reduce the risk of high-severity wildfire. Forest managers use a variety of tools to create desired conditions within forests; the most common are prescribed fire and mechanical thinning. These two treatments may be used separately or in combination, depending on restoration goals for the forest stand. Before these treatments can be applied, managers must justify their choice by documenting the effects of the treatment on other ecosystem components, such as understory vegetation. Understory vegetation in fire-dominated landscapes often has adapted to regrowing in frequent, low-severity fire regimes. Because fire releases nutrients and, by opening the canopy, allows light to the forest floor, the understory response is positive (e.g., increased growth or reproduction). Scientific reviews of the literature document the effect of fire (prescribed and wildfire) on both native and exotic understory vegetation. However, no synthesis is available on the effects of thinning treatments on understory vegetation. One goal of this document is to synthesize the literature on the effects of mechanical thinning on understory plant species. A second goal is to document the effect of prescribed burning on rare, threatened, or endangered species. We review current literature on studies that address effects of prescribed fire and thinning treatments on understory vegetation. The studies’ outcomes are presented in two sets of tables: (1) functional group results, and (2) species-specific results. Managers often are interested in and need to report the effects of treatments on species; functional group responses can provide a clue to how a species might respond if no other information is available. In general, fire and thinning treatments increased response of understory species. More intense treatments, such as combined thin+burn treatments and greater thinning intensity, had the highest increases in cover and production. Thin-only and burn-only treatments had more moderate increases. In addition, most studies found exotic plants’ response increased as disturbance intensity increased; however, most studies report very low invasive presence even after the treatments. If one of the goals of the forest management plan is to increase presence or cover of understory species in general, then prescribed fire and thinning treatments may be a viable option to restore forest understory. Rare, threatened, and endangered species in dry forest environments often respond favorably to prescribed fires. Many of the species reported in this document increased in abundance or reproduction or were unaffected by fire, indicating that prescribed fire is compatible with (or beneficial for) restoration of these species. The results of this synthesis illustrate several important lessons. First, current forest structure is the result of decades of fire-suppression activities, and so restoration will require multiple treatments to bring forests to within the range of historic variation. Second, while the treatments discussed in this document generally increased native plant responses, the same treatments also increased exotic plant response. Therefore, to avoid spread of exotic plant species, it is important to consider the context of the treatment area, (e.g., nearby roads, wildland urban interface, previous exotic plant invasions) before applying the treatments. Third, applying thinning and prescribed burning treatments in a mosaic pattern of treatment time and type across the landscape will help to maintain a diversity of vegetation (e.g., early-, mid-, and late-successional species across the landscape).