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Tillage of compacted soil in a thinned Douglas-fir stand in western Oregon : soil density and residual tree growth seven years after treatment

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  • A set of 119 study trees was identified in a 65 year-old Douglas-fir stand after a thinning operation conducted seven years ago using pre-planned skid trails and groundbased machinery. Some of the trails were tilled using a winged subsoiler after this operation in order to alleviate the effect of compaction caused by the heavy machinery used during logging. The primary objective of this study was to determine whether tilling compacted forest soils after a harvesting operation serves to alleviate the compaction over time, and if tillage has any discernable positive or negative effect on the growth of the residual trees adjacent to tilled skid trails. To clarify growth relationships, soil bulk density measurements were taken around study trees adjacent to undisturbed areas, tilled trails, and untilled trails. In addition, different tree growth parameters and competition indexes were evaluated to help distinguish thinning and other effects. The results showed that although mean soil bulk densities were generally higher around trees adjacent to untilled skid trails, there was no statistical difference between the densities in untilled and tilled zones (95% confidence level). This may help explain the observation that seven years after the thinning and tillage, there was no apparent benefit or damage to the growth response of trees adjacent to tilled trails. Regression analysis of several important stand and site variables showed that just two were meaningful in predicting growth, diameter at the time of thinning and Competition Stress Index after thinning (95% confidence level). This suggests that limiting the area of compacted or tilled soil by the use of planned skid trails may have effectively restricted the treatment growth response to the influence of basic tree and stand characteristics after thinning.
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