Soil Wetness and Traffic Level Effects on Bulk Density and Air-Filled Porosity of Compacted Boreal Forest Soils

Soil compaction is a common consequence of forest harvesting that has the potential to affect several soil processes and forest productivity. Our objective was to quantify the relationships between soil trafficking, soil wetness, and soil air-filled porosity, and the compacted bulk density and air-filled porosity of 14 boreal forest soils in West-Central Alberta, Canada. Bulk density and air-filled porosity were measured on nontrafficked soil and adjacent areas immediately after the site was subjected to 3, 7, and 12 cycles of skidding with mostly wide-tired skidders. Significant increases in bulk density (P < 0.05) occurred after three cycles at each site when the soil water potential was higher than -15 kPa; the significant increase occurred to a depth of at least 22 cm. The increase in bulk density became asymptotic between 7 and 12 cycles, but the increases were not significantly different from the bulk density at three cycles. The relationship between air-filled porosity and trafficking was the inverse of the level of bulk density and trafficking. but the increase in bulk density of wet soil was limited by an air-filled porosity of about 0.10 m(3) m(-3). Soil compaction only occurred when the soils were at or wetter than field capacity, which can easily be measured in the field with a handheld tensiometer or alternatively, estimated from a field measure of soil consistence. Managing felling operations to maximize transpiration of trees to reduce soil wetness is an effective tactic to avoid significant soil compaction by these types of equipment in this environment.