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Relative bulk density as an index of soil compaction and forest productivity in British Columbia Zhao, Yihai (Simon)

Abstract

Soil compaction often limits conifer regeneration on sites degraded by construction of landings and roads, but inadequate understanding of compaction characteristics has sometimes led to inappropriate rehabilitation efforts. This warrants development of new methods to assess compaction and its relation to tree growth. The objective of this study was to develop a high-level integration indicator that will characterize compaction of forest soils and that could be correlated to tree height growth. Mineral particle density of soils from interior British Columbia (BC) forests varied significantly among the geographic locations. Oxalate-extractable Fe- and Al-oxides and particle size distribution (PSD) were related to soil and mineral particle densities, while soil organic matter (SOM) and Al- and Fe-oxides were important soil properties in relation to soil particle density. The significance of levels of single soil properties in predicting maximum bulk density (MBD) were in the order: plastic and liquid limits, organic matter content, oxalate-extractable oxide, and PSD. Stratification of the sample according to Atterberg limits improved the predictability of MBD, and variation in particle density was included in the prediction by other soil properties used in the models. Height growth of interior Douglas-fir (Pseudotsuga menziesii var. glauca [Bessin] Franco) was restricted when relative bulk density (RBD) was > 0.72. For lodgepole pine (Pinus contorta Dougl. ex Loud. var. latifolia Engelm.) and hybrid white spruce (Picea glauca [Moench] Voss × engelmannii Parry ex Engelm.), an RBD of 0.60 - 0.63 corresponded to maximum height growth, while that of 0.78 - 0.84 appeared to limit height growth. The presence of surface organic material mitigated compaction and was often associated with lower RBD. Interior Douglas-fir and lodgepole pine planted in low elevation sites in north-central BC did not grow well and their height growth was weakly related to RBD. The results suggest that soil rehabilitation should be considered on disturbed sites where soil RBD is > 0.80. Findings in this study have implications in assessing forest soil compaction and its effect on site productivity. The results will help predict soil behaviour and associated tree growth in response to timber harvesting and site rehabilitation.

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