Soil compaction and conifer growth after tractor yarding at three coastal Washington locations

Abstract

We measured soil density and tree growth after wet-season, ground-based yarding on fine-textured soils at three clear-cut sites. Four treatment conditions were sampled on or near four skid trails (replicates): nontilled and tilled primary skid trails, and adjacent slash-treated areas; the fourth treatment was secondary skid trails at two locations and a logged-only control at the third location. The 16 treatment plots were split into 4 subplots, each randomly assigned to a species–stock type and planted with 30 seedlings. Tree data through year 8 after planting were analyzed as a randomized block, split-plot design. Compared with nontrail areas, bulk density in the 0- to 8-cm depth on primary skid trails after logging averaged 41–52% greater. Eight years later, bulk density in the 0- to 30-cm depth of primary skid trails still exceeded that outside trails by about 20%, yet tree survival was similar except for western hemlock (Tsugaheterophylla (Raf.) Sarg.) having poorer survival on nontilled trails at one location. Average tree height and volume of Douglas-fir (Pseudotsugamenziesii (Mirb.) Franco var. menziesii), Sitka spruce (Piceasitchensis (Bong.) Carr.), and hemlock (except height at one location) did not differ among treatments. Observed differences among treatment means were small. Power analyses indicated that 20% or larger differences in tree height or volume were detectable with 30–95% power.