Ground-based forest harvesting effects on soil physical properties and Douglas-Fir growth

Soil properties and forest productivity can be affected by heavy equipment used for harvest and site preparation but these impacts vary greatly with site conditions and operational practices. We assessed the effects of ground-based logging on soil physical properties and subsequent Douglas-fir [Pseudotsuga menziesii (Mirb) Franco] growth on a highly productive site receiving vegetation control in coastal Washington. We also tested the effectiveness of tillage in maintaining or enhancing site productivity. On average, about half of the area of ground-based harvested plots was affected by vehicular traffic. Sixty-three percent of the trees were planted on microsites with some degree of soil disturbance. Soil bulk density at the 0- to 30-cm depth increased from 0.63 to 0.82 Mg m(-3) in the most compacted portions of traffic lanes. The area-weighted increase in soil bulk density in the 0- to 30-cm depth was 27%. Soil strength in traffic lanes increased at all depths < 55 cm but never exceeded 1300 kPa. Tillage to the 60-cm depth returned the soil to its initial strength condition. Volumetric soil water content in compacted traffic lanes was greater than that in non-compacted soil. Total soil porosity decreased 10 to 13% with compaction, while available water holding capacity increased. In compacted soil, macropore space was reduced 40 to 52%. The study revealed no detrimental effects on tree height and diameter from soil compaction at age 4. At stand age 3, a tree volume index was actually greater for trees planted on traffic lanes than for those on non-disturbed soil.