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Critical turning moments and drag equations for British Columbia conifers Byrne, Kenneth Earl


Tree winching experiments were conducted for 4 British Columbia (BC) tree species: western redcedar (Thuja plicata (Donn ex D. Don) Spach), western hemlock (Tsuga heterophylla (Raf.) Sarg.), and lodgepole pine (Pinus contorta Doug, ex Loud.), and hybrid spruce (Picea engelmannii Parry XPicea glauca (Moench) Voss). Strong linear relationships between stem mass and critical turning moments were found. Based on the slope coefficients, lodgepole pine (145.6), hybrid spruce (118.6), western redcedar (94.5), and western hemlock (77.4) have different critical turning moments with respect to mass. Comparison with results for trees in the United Kingdom (UK) tree winching database indicated that there is a difference between the slope coefficients for BC pine and pine grown in the UK on equivalent soils. Wind tunnel experiments with juvenile crowns of the hybrid spruce were compared with results from earlier experiments with redcedar, hemlock and lodgepole pine. Static and dynamic drag coefficients, and mass versus drag relationships were examined. Differences in drag coefficients exist for species with different foliage characteristics, however, differences in the mass/drag relationships were less pronounced. Critical turning moment and drag results were used to adjust selected parameters in the UK Forestry Commission's mechanistic windthrow risk model 'ForestGALES' to estimate critical wind speeds for the four BC species and for lodgepole pine and hemlock grown in the UK. UK grown trees have higher critical wind speeds than trees of the same species grown in BC. More work is required to adjust ForestGALES to reflect the differences in tree form and mechanical attributes for BC species and to validate model predictions.

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