AN ABSTRACT OF THE THESIS OF

Robert T. Parker for the degree of Master of Science in Forest Resources presented on August 24, 2000.  Title:  Soil Compaction in Central Oregon Volcanic Ash Soils and the Subsequent Effects on Residual Ponderosa Pine Growth.

Abstract approved:                                                                                                        

                                                            Steven D. Tesch

Although previous research in central Oregon has shown soil compaction can lead to a decline in site productivity, the subject is not understood well enough to predict the growth changes resulting from a given level of soil compaction.  A study was initiated to relate the basal area, height and volume periodic annual increment (PAI) of residual, 70 to 80 year-old ponderosa pine (Pinus ponderosa Laws.) trees to compacted soil conditions, as measured by soil strength. 

            This study was superimposed on the USDA Forest Service Long Term Site Productivity Project research plots in Central Oregon and thus constrained by its design.  Soil strength and tree growth were measured on six of these plots.  Three plots had been thinned with a mechanical harvester and the stems removed with a grapple skidder from the plots (Complete Removal).  Three other plots were thinned to similar stocking levels with the harvester, but the stems were left in place to minimize disturbance (No Removal). No true control existed for these installations as both the Complete Removal and No Removal plots were compacted by the harvester.  A recording penetrometer was used to determine soil strength along systematically spaced grid points, to a depth of 24 inches.  Each tree within each plot was mapped and measured for total height, diameter at breast height, and radial growth increment at diameter breast high (DBH).

            The soil conditions around each residual tree were evaluated using 15, 30-, and 45-foot radius plots.  The penetrometer readings that fell within each of these plots were averaged to represent the overall soil conditions affecting each tree.  The Complete Removal plots had significantly higher soil strength conditions than the No Removal plots (p<0.05). The percent increases in average soil strength of the Complete Removal plots over the No Removal plots were 39, 42 and 44 percent for the 15-, 30- and 45-foot radius plots, respectively.

Potential associations between basal area, height and cubic volume PAI growth rates and replication, treatment, soil strength and other covariates were explored with general linear models.  Soil strength was not a significant factor for basal area PAI or for volume PAI at the 30- and 45-foot radius.  Total height and cubic volume PAI at the 15-foot radius declined significantly (p<0.05) with increasing soil strength.

The volcanic ash soils did compact as a result of the low level of mechanical thinning activity conducted on the study sites.  Tree growth was statistically associated with increased soil compaction.  Lack of a true control prevents full evaluation of the mechanical harvesting-related compaction; however, skidding resulted in a measurable increase in soil compaction in the Complete Removal plots.  Forest management practices that lead to frequent entries appear likely to compact these volcanic ash soils.  Depending on logging patterns, large areas could be impacted without careful planning.  It appears that compaction effects are long-lasting and cumulative, thus the risk of reducing long-term site productivity is a concern.