City Trees

as well. Height and the approximate age of the tree have to be determined (or at least esti- mated). Doing this in absolute numbers (like SIA) can deliver worthless or even dangerous results, an example of which is shown in Fig. 4. Realizing that the VTA One-Third Rule as described here is not applicable for most mature urban trees and that the mathematical model of the competing SIA concept is obviously wrong raises questions. How did such concepts sneak into international standards? Why do corresponding presentations dominate nearly all conferences and journals for years without data proving the claims regarding the typical mature urban tree to be inspected in terms of safety? Understanding the scientific basics shows we should use "relative and self-referencing approaches," looking for changes in the major allometric factors (such as tree height and diameter at breast height) over time for evaluating tree-safety. As soon as trees no longer grow in height, the wind load no longer increases, yet girth continues to increase with the corresponding gains in breaking safety. Even tiny radial increments lead to a significant annual growth of the load-carrying capacity and thus correspondingly higher basic stability. This explains the fact that mature trees can obviously tolerate significantly more and bigger defects without being more likely to break as compared to young (and even intact) trees. Countless num- bers of mature trees, hollowed out for decades but surviving strong winds, prove this as a fact. Fortunately, the increase of the basic breaking safety (as a function of age and growth rate) as well as the loss in load-carrying capacity can be estimated using smartphone applications at the tree without need of external reference data (as used by SIA) and, in many cases, even without any advanced diagnostic technology. Applying this safety evalu- ation concept to mature trees in the last years led to significantly less pruning and cabling recommen- dation because of breakage safety concerns, which is positive in many ways: it saves money and does not further reduce the tree's capabilities to defend against fungal decay (Boddy & Rayner 1983). The director of the urban tree department of a German town recently wrote shortly before retiring: "After having applied this 'new' concept of tree-safety evaluation for several years, we kept many trees much longer than we would have done before and we spent much less money for tree care (pruning and cabling), while preserving a more natural urban envi- ronment and habitat without having more failures occur. That meant more benefits for less money." Literature Cited and Further Reading Boddy, L., Rayner, A.D.M. (1983): Origins of decay in living deciduous trees: the role of moisture con- tent and a re-appraisal of the expanded concept of tree decay. New Phytol. 94(4): 623-641. Fink, S. (2009): Hazard Tree identification by Visual Tree Assessment (VTA): Scientifically solid and practically approved. Arboricultural Journal 32.3, pp. 139–155. Gao, S., Wang, X., Wiemann, M. C., Brashaw, B. K., Ross, R. J., Wang, L. (2017): A critical analysis of methods for rapid and nondestructive determination of wood density in standing trees. Annals of Forest Science. 74(2): 1-13. Gruber, F. (2007): Die VTA-0,32-Restwandstärkenregel, wissenschaftlich unhaltbar und praktisch unbrauch- bar. Agrar – und Umweltrecht 37/1, 7-12. Gruber, F. (2008): Untenable failure crite- ria for trees: I. The residual wall thickness rule. Arboricultural Journal 31:5-18. Gruber, F. (2008): Reply to the responses of Claus Mattheck and Klaus Bethge to my criti- cisms on untenable VTA-failure criteria – Who is right and who is wrong? Arboricultural Journal 2008/12/01. Taylor & Francis. pp. 277-296. Kahle, H.-P., H. Spiecker, R. Unseld, P.J. Pérez Martínez, J. Prietzel, K.H. Mellert, R. Straussberger, K.E. Rehfuess (2008): Temporal trends and spatial patterns of height growth changes in relation to changes in air tem- perature and precipitation, and in relation to levels of foliar nitrogen and nitrogen deposition. In: Causes and Consequences of Forest Growth Trends in Europe – Results of the Recognition Project, Chapter: 4.2, Publisher: Brill, Editors: Kahle, H.P.; Karjalainen, T.; Schuck, A.; Ågren, G.I.; Kellomäki, S.; Mellert, K.H.; Prietzel, J.; Rehfuess, K.E.; Spiecker, H, pp.127-167. Mattheck, C.; Bethge, K.; Erb, D. (1993): Failure cri- teria for trees. Arboricultural Journal, Taylor & Francis, 17 (1993): pp. 201-09. Mattheck, C., Breloer, H. (1993): Handbuch der Schadenskunde von Bäumen. Der Baumbruch in Mechanik und Rechtsprechung. Rombach. Freiburg/Breisgau 1993. Mattheck, C., Breloer, H. (1994): The body language of trees. A handbook for failure analysis. Research for Amenity trees No. 4, HMSO Publications Centre. London 1994. ISBN 0-11-753067-0. >> www.urban-forestry.com 37

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