Experimental Test of Non-Destructive Methods to Assess the Anchorage of Trees

Abstract

More than 280 trees were uprooted in winching tests monitored by high resolution inclinometers at the base of the trees and a forcemeter mounted in the winching line. The dataset comprises trees growing on different urban and forest sites in Europe and North America and mainly consists of fifteen widespread tree species. For the first time, a large number of trees were measured non-destructively prior to uprooting failure, as commonly practiced by consulting arborists in static load tests. With these tests, the bending moment required to cause 0.25° root plate inclination (rotational stiffness) was determined and used to predict the strength of the root system from equations described in two evaluation methods currently used in arboriculture. The predictions were tested against the measured anchorage strength, i.e., the maximum bending moment that was required to uproot the trees. Both methods delivered good estimates, which indicates that rotational stiffness at 0.25° inclination of the stem base is a suitable proxy for anchorage strength. Both equations can be considered valid for assessing the likelihood of uprooting failure, as they systematically underestimate and rarely overestimate the actual resistance to failure of a tree’s root system. As a trend, the differences between predicted and measured anchorage strength were larger for small trees, for which the resistance of the root system was overcome at larger inclination angles. While the quality of the predictions differed for species, it did not differ between sites. The angles of stem inclination at which the anchorage strength was overcome for all trees in our dataset support models for the mechanics involved in uprooting failure that previously have been described only for conifers with a shallow root system.