City Trees

Without a significant correlation to wood density, resistance drilling profiles cannot be interpreted cor- rectly in terms of wood condition (Rinn 2012, 2016). Despite the fact that the SIA calculation model is wrong for thin-walled or open-tree stems (Rinn 2017; 2018a), there are some aspects of the SIA concept (Wessolly 2005) to be recog- nized and acknowledged. For instance, break- age safety is not only determined by the relative loss in load-carrying capacity of a cross-section but is always a result of comparing load-carrying capacity with the real load the tree is facing. The load to a common urban tree mainly comes from wind (Rinn 2014a) and is (in first order) pro- portional to tree height to the power of three (~H 3 ). Interestingly, the load-carrying capacity of the stem cross-sections depends on their diameter to the power of three (~D 3 ), thus in a similar way as wind load depends on total tree height. In consequence, the ratio D 3 /H 3 can be used as a rough measure for tree safety: not in absolute numbers (as wrongly practiced by SIA), but by observing relative changes of this ratio over time (Rinn 2015). The fact that trees try to keep H/D constant for decades after the juvenile growth phase (Kahle et al. 2008) can thus be interpreted as their desire to maintain a constant safety factor (~D³/H³) for the duration of the tree's life, which makes sense in many ways. However, even when tree height is no longer increas- ing (typically 60 to 80 years of age for common urban trees), trees still annually put on girth. The continuous addition of new tree rings automati- cally leads to an increasing load-carrying capacity and, at the same time, to a correspondingly higher basic stability. Consequently, the older mature trees are, the higher their basic stability and the more defects they can tolerate without becoming hazardous. Taking into account these size-related (positive) aging effects allows arborists to deter- mine the gain in safety for mature trees and thus determine what extent of "defects" can be tolerated without having an increased probability of failure. The practical implication is that many, if not most, mature urban trees of concern have no need for pruning (for wind-load reduc- tion) or for other expensive mitigation mea- sures, resulting in the following benefits: • Less money needed for prun- ing and cabling mature trees; • Less damage to tree vitality and to the tree's ability to resist fungal decay; and >> Figure 5. Examples from typical mature urban trees to be inspected in terms of safety: the cross-sections are commonly not circular and the defects are mostly located off-center. Such cross-sections provide many different radius values and many different shell-wall thicknesses (t), often ranging from zero (t=0) to fully wooded (t=Radius). Consequently, there is no typical or average t/R that could be quickly determined at the tree. Furthermore, minimum, maximum, or average t/R values do not represent the load- carrying capacity of such cross- sections. In consequence, the One-Third Rule simply does not apply to these kind of trees. www.urban-forestry.com 35

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