Mark Kimsey

Long-Term Site Preparation Effects on Volcanic Ash Forest Soils and Douglas-Fir

Abstract Long-term site preparation effects on soil characteristics and Douglas-fir (Pseudotsuga menziesii var. glauca) growth and foliar nutrition were measured over a 24-year period following a ground-based harvest in Northern Idaho, USA. Harvest unit soils were classified as Andisols overlaying metasedimentary parent material within a udic-frigid moisture and temperature regime. Douglas-fir site index at base age 50 was 29 m. Site preparation treatments included undisturbed control, broadcast burn, pile and burn, and mechanical scarification. Periodic soil-site measurements were collected on each treatment at regeneration stand ages 6, 14, and 24 years. Six- and 14-year soil bulk density on scarified treatments were significantly higher at 0-15 and 15-30 cm than all other treatments. At 24 years, scarified soil bulk density at 0-15 and 15-30 cm showed recovery to bulk densities observed in non-scarified soils. Scarified soil organic matter (SOM) and N were significantly reduced by 32% and 42% over control levels 6 years post-harvest. After 24 years, scarified SOM and N were significantly lower than that found in broadcast burn (44% and 54%) and pile and burn (33% and 49%). Douglas-fir needle mass and foliar N and P content on scarified soils were significantly lower than on broadcast burn or pile and burn treatments after 24 years (p<0.1). After 24 years, soil and foliar N content was significantly higher on microsites that received a burn treatment (p<0.1). Tree growth on either burn treatment showed significantly greater diameter (35%), height (14%), and volume (92%) when compared to trees growing on scarified soils after 24 years (p<0.1). These results indicate that tree growth on frigid, ash-mantled forest soils of Northern Idaho, USA, can be significantly reduced following soil compaction and displacement of organic matter and nutrient-rich topsoil. Where soil disturbance was minimized and organic matter retention was coupled with a burn treatment, soil and tree productivity was maintained or enhanced.