Soil compaction often limits conifer regeneration on sites degraded by landings and roads, but inadequate understanding of the relationship between compaction and tree growth could lead to inappropriate soil conservation and rehabilitation efforts. We tested liquid and plastic limits, oxidizable organic matter, total carbon, particle size distribution, and iron and aluminum oxides on soil samples collected from five forest experiments in interior British Columbia. These data were used to estimate soil maximum bulk density (MBD) and relative bulk density (RBD); our objective was to relate RBD to tree growth. Height of interior Douglas-fir ( Pseudotsuga menziesii var. glauca (Bessin) Franco) was limited when RBD was >0.72. For lodgepole pine ( Pinus contorta Dougl. ex Loud. var. latifolia Engelm.) and hybrid white spruce ( Picea glauca (Moench) Voss × Picea engelmannii Parry ex Engelm.), RBDs of 0.60–0.68 corresponded to maximum height, whereas RBDs of 0.78–0.87 appeared to limit height growth. The presence of surface organic material mitigated compaction and was often associated with lower RBD. Our results illustrate the usefulness of RBD to assess compaction and suggest that soil rehabilitation should be considered on disturbed sites where soil RBD is >0.80.
The widespread use of heavy equipment during timber harvesting and site preparation can lead to reduced soil productivity and warrants development of new methods to assess compaction. We evaluated the effects of soil particle density, organic matter, particle size distribution, extractable oxides, and plastic and liquid limits on the maximum bulk density (MBD) of forest soils in British Columbia. Soil samples were collected from 33 sites throughout British Columbia, covering the major forest and soil types of the province. The standard Proctor test was used to determine MBD and related parameters, including the gravimetric water content (W MBD) and porosity (f MBD) at which MBD was achieved. The signifi cance levels of single soil properties in predicting MBD were in the order plastic and liquid limits, organic matter, oxalate-extractable oxides, and particle size distribution. For all samples, liquid limit and clay were most closely related to MBD (R 2 = 0.83). Addition of organic matter to the model increased the regression coeffi cients, and oxidizable organic matter caused a greater increase than did total C. Stratifi cation of the sample set into groups based on plasticity led to higher R 2 values in multiple regressions, and different soil properties were important for nonplastic soils than for those with high, moderate, and low plasticity. Prediction with multiple regression explained the most variation in MBD for nonplastic soils, while properties of highly plastic soils explained the least variation in MBD and moderately plastic soils were intermediate. Based on our fi ndings, we propose an approach for using MBD to help better interpret bulk density data in forest soil compaction studies.
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