Soil compaction study of 20 timber-harvest units on the Ouachita National Forest
A soil compaction study was performed on 20 timber harvest units on both rocky (15-35% by volume gravel) and non-rocky (<15% by volume gravel) surface soils of the Ouachita National Forest in Arkansas, to determine if these areas met the USDA Forest Service Southern Region (R8) soil quality standards for compaction and affected area extent. The compaction standard states bulk density cannot increase more than 15% from its natural (undisturbed) level and not more than 15% of an activity area can be adversely affected. Eight of the study units exceeded this standard. These eight units generally contained less than 15% rock fragments in the top 8 inches (20 cm) of soil, and seven of the eight had been harvested during the moist season (December-June) using rubber tire skidders. The non-rocky soil units, when harvested during the dry season (July-November), resulted in about 20-50% less compaction than when harvested during the moist season. Non-rocky soils with a sandy loam surface tended to compact less during dry season but more during moist season equipment operation than the non-rocky loam or silt loam soils. Compaction also averaged about 30-50% less on the rocky soils than on non-rocky soils. On the rocky soils, logging equipment operation during either the dry or moist season did not show a difference, and only native surface roads and log decks tended to have a greater than 15% bulk density increase. Compaction due to timber harvest activities that had occurred at least 15-20 years earlier averaged about 9% bulk density increase for the nonrocky soils and 7% for the rocky soils, and indicated that partial recovery had occurred. An analysis of surface infiltration rates found that a 15% density change resulted in more than 60% reduction in infiltration. This study also found that a 15% density change can be visually determined by change in soil structure.