Monitoring Soil Strength Conditions Resulting from Mechanical Harvesting in Volcanic Ash Soils of Central Oregon

Abstract

Abstract The volcanic ash-derived soils in central Oregon have been shown to compact readily and remain compacted for long periods of time, leading to significant reductions in ponderosa pine (Pinus ponderosa, Laws) tree growth. Concerns over the potential for long-term reduction in forest site productivity on private forestlands in the region led to the development of a soil monitoring process for precisely quantifying changes to the soil environment caused by mechanical harvesting in second-growth ponderosa pine stands. For this study, soil strength was used as an indicator for soil compaction. A recording penetrometer was used to measure soil strength to a depth of 2 ft at intervals of 5 ft along transects established across harvest units. At each sampling point, the number of machine passes was estimated as no disturbance, 1–2 machine passes, 3–5 machine passes, or main skid trails. The penetrometer data were then summarized for each of the four categories. The results show a rapid rise in soil strength with increasing machine activity and 40–70% of the harvest units compacted in a single harvest cycle. The process proved to be an unbiased and practical system for quantifying the extent and intensity of soil strength conditions resulting from mechanical harvesting.