Ponderosa Pine Growth Response to Soil Strength in the Volcanic Ash Soils of Central Oregon

Mechanical harvesting and associated logging activities have the capacity to compact soil across large portions of harvest units, but the influences of compaction on long-term site productivity are not well understood. Previous research in central Oregon has shown that volcanic ash soils compact readily under both compression and vibration loads, resulting in long-term alteration in soil density and a decline in tree growth. In this study, soil strength (SS) and tree growth were assessed in areas subject to repeated timber harvesting with the objective of quantifying the relationship between ponderosa pine (Pinus ponderosa Laws.) growth and SS. Two thinning treatments (felled only versus felled and skidded) in 70- to 80-year-old ponderosa pine stands were replicated at three sites in 1991. Subsequent 5-year growth in diameter, height, and volume of residual trees were assessed with respect to SS measured by a recording penetrometer. Felled and skidded plots had 44% higher SS values than felled-only plots (P = 0.05). Although no treatment effect on growth was detected at the plot level, diameter, height, and volume growth of individual trees within plots declined significantly as average SS within a 30-ft zone of influence increased from approximately 800 to 2,500 kPa. Results show the potential use of SS measurements for monitoring impacts of harvesting operations on tree growth.