Influence of Selection Method on Skidder-Trail Soil Compaction in Longleaf Pine Forest

Silvicultural selection systems present trade-offs between concentrating soil impacts in small areas and dispersing them across a stand. We applied three selection methods in longleaf pine forest on coarse-textured soils: single-tree, group (ca. 0.2 ha opening size), and group with reserved trees. Area disturbed was determined from GPS units on skidders, soil compaction was inferred from bulk density and penetration resistance, and tree growth was measured on longleaf seedlings planted in skidder trails. Skidder trails covered an average 3.5 +/- 1.5% (sd) of stand area. Under single-tree selection, more area was affected by single passes than higher numbers of passes, but under group and group with reserves selection more area was affected by higher numbers of passes. Bulk density and penetration resistance went up rapidly as number of skidder passes increased, as did seedling growth. Of concern was that penetration resistance at 20-30 cm depth exceeded theoretical thresholds for root growth after 15 passes. Overall, somewhat more compaction occurred under group methods than the single-tree method. Despite absence of predicted compaction impacts on early longleaf pine growth, foresters should stay alert to the possibility of later productivity declines in heavily used skid trails due to subsurface soil compaction and consolidation.