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    Climbers' CornerTree Academy

    What Do We Really Know about Tree Slenderness As an Indicator for Risk Assessment?


    In forestry, slenderness has long been used as an indicator for the stability of individual trees and stands. Some years ago, this has been transferred to urban tree risk assessment by Claus Mattheck and his co-workers. Tree care professionals were fast to adopt it as it is very easy to apply. But the validity of this application has soon been questioned, causing a heated debate and much confusion among arborists.
    After a review of the empirical evidence for this indicator, results from large-scale allometric studies on the slenderness of urban tree populations are presented, followed by a discussion of results from destructive pulling tests on large trees varying in slenderness. Finally, the biological assumptions used in support of the hypothesis, coined “biological suicide” in the original paper, are critically reviewed on the basis of current botanical knowledge.
    Most studies cited in support for this indicator refer to dense, young conifer stands, often damaged by wet snow or ice. Of the few studies which systematically investigated the impact of slenderness, most found no evidence supporting the hypothesis that slender trees are more likely to fail in strong winds. During some of the devastating hurricanes of the last decades, slender trees were less likely to fail than sturdy trees.
    Allometric studies on urban tree populations show that there are almost no mature slender (h/d > 50) trees on which to base empirical studies on failure. The seminal study for slenderness as a failure criterion for urban trees compared failed slender forest trees with standing sturdy urban trees.
    Destructive pulling tests on mature trees of varying slenderness show, that slenderness has only a very small and often insignificant influence on tree failure. Finally, recent results of studies of the hydraulic architecture, phloem transport, and biomechanics do not support the assumption of “suicidal” trees. In conclusion, we caution against basing decisions on pruning or felling trees on tree slenderness.

    Conference Proceedings Documents


    Steffen Rust is Professor of Arboriculture at the University of Applied Science and Art in Göttingen, Germany. His main field of research are non-destructive methods for tree risk assessment. Steffen is one of the developers of stress wave tomography. Recent studies concentrate on ground penetrating radar, electrical resistivity tomography for stems and root systems, as well as static pulling tests and analysis of tree vibration.

    Climbers' CornerTree Academy

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