Improvements in anchorage provided by the acclimation of forest trees to wind stress

Summary Trees are known to modify their growth in windy climates, but these responses have not previously been linked to alterations in anchorage strength. The effects of stem mass, soil group, rooting depth and wind exposure (Detailed Aspect Method of Scoring (DAMS)) on anchorage were modelled using data from experiments where Sitka spruce ( Picea sitchensis (Bong.) Carr.) trees had been mechanically overturned. Regressions of critical turning moment against stem mass were performed on combinations of soil group (mineral, gleyed mineral, peaty mineral and deep peat) and rooting depth ( 80 cm), with DAMS as an additional factor. Correlations of DAMS with critical turning moment were positive for all combinations, and DAMS had a positive effect on the slope of all critical turning moment against stem mass regressions. When DAMS was added to the model, it increased the variance accounted for, from 66.5 to 71.2 per cent. When revised anchorage coeffi cients were applied to the ForestGALES wind risk model, critical wind speeds and associated return times for a 55-year-old stand were reduced on a relatively sheltered DAMS 14 site and increased on a wind-exposed DAMS 18 site. The implications of trees developing stronger anchorage in locations with greater wind exposure are discussed in relation to management of wind risk.

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