Does steep terrain influence tree stability? A field investigation

The anchorage of 40-year-old Sitka spruce (Picea sitchensis (Bong.) Carr.) trees grown in a plantation on a steep (ca. 30 degrees) slope was compared with that of trees grown on an adjacent horizontal area. There was similar gleyed mineral soil on the sloping and horizontal areas. Trees were mechanically overturned using a winch, and anchorage was quantified by measuring load, stem angle, and tree dimensions. Trees on the slope were overturned upslope, downslope, or across-slope. Critical turning moments were calculated around the tree base and the actual hinge point. Critical wind speeds required to uproot or snap trees in this stand were modelled to compare the vulnerability of trees to upslope and downslope winds. No overall difference in anchorage was found between trees grown on the horizontal and sloping parts of the site. However, for trees on the slope, those pulled upslope showed significantly more resistance to overturning for a given stem mass than those pulled downslope. Critical turning moments calculated at the hinge point were smaller than those calculated at the stem base, but differences were small and had no effect on the comparison between treatments. Critical wind speeds for uprooting were estimated to be 28 m center dot s(-1) for an upslope wind and 24 m center dot s(-1) for a downslope wind on this site. The implications of these results are discussed in relation to windthrow-risk modelling and forest soil conservation.