Resilience of Forest Soils to Underburning and Thinning.
Darlene Zabowski, Jeff Hatten, George Scherer, and Amanda Ogden. Univ of Washington, College of Forest Resources, Box 352100, Seattle, WA 98195
The fire suppression policy of the last century has resulted in high fuel loads that need to be reduced to lower wildfire hazards. Two methods, thinning and underburning (or both) have been used to reduce forest fuels. Questions remain as to which method has the least impact to soils. This study examined soil properties pre- and post-thinning, underburning, and combined thinning and underburning at the Mission Creek Fire and Fire Surrogate study site in eastern Washington and at a JFSP site near Burns, in eastern Oregon. Both sites are predominantly Ponderosa Pine forests with fertile soils. At the Mission Creek site, a single thinning, burning and thinning and burning treatment were examined along with controls; at the Burns site, a single thinning was done to all plots and some plots were burned twice with a five-year interval. Soils were sampled from the O and A horizons to a 15cm depth and analyzed for pH, C, N, C/N ratio, CEC, and %BS. Soil pH was elevated, N concentration lower, and there was a reduction in O horizon thickness with all burn treatments, but there was little effect on C concentration, CEC or %BS. In general, the soils seemed resilient to all treatments, possibly due to the high soil fertility at both sites. Overall, methods to reduce forest fuel loads may have little impact on soil properties at high fertility sites.