Comparing the effectiveness of thinning and prescribed fire for modifying structure in dry coniferous forests

Forest thinning and prescribed fires are the main practices used by managers to address concerns over ecosystem degradation and severe wildland fire potential in dry forests of the Western United States. There is some debate, however, about treatment effectiveness in meeting management objectives as well as their ecological consequences. This study assesses the effectiveness of thinning and prescribed fire treatments, alone and combined, for modifying forest structure and potential fire behavior in the Eastern Cascade Mountains of Washington State. Treatments were applied to 12 management units (~10 ha each), with each treatment combination replicated three times (including untreated controls). Thinning modified forest structure by reducing overall tree stocking and canopy fuels to =50 percent of pretreatment values. Furthermore, thinning greatly reduced the modeled probability of severe wildfire and reduced stand densities to below critical levels for insect outbreaks. The prescribed fire treatment, conversely, did not appreciably reduce stocking levels or canopy fuel loadings, but was effective for raising canopy height and increasing the density of standing dead trees. Prescribed fire effects were more pronounced when used in combination with thinning. While thinning was a more reliable method for altering stand structure, the spring burns conducted in the experiment were cooler and spottier than were desired and may have led to results that downplay the efficacy of fire to meet forest restoration goals.