Possibilities and limitations
The SIA model demonstrates that often small residual wall thicknesses are sufficient to ensure the fracture safety of a tree. The implication is that a visual inspection or tapping with a sounding mallet to gauge hollowing is sufficient to determine whether the calculated minimum residual wall thickness has been maintained or not (Wessoly & Erb 2014). In this way the use of expensive examination methods can be shown to be unnecessary. SIA can also be used as an evaluation method to assess the results of sonic tomography (Detter et al. 2010).
In contrast to a subjective “gut feeling assessment” or the application of rigid rules, such as the one third residual wall thickness rule (Gruber 2017 ), the use of SIA is an objective approach to evaluating the implications of a tomogram on tree safety, based on engineering principles.
SIA can further be applied to reaching management decisions to allow the retention of trees where it is shown that the stem currently has insufficient load-bearing capacity. The impact of pruning can be calculated to significantly reduce wind loads and restore safety factors, and the minimum degree of pruning can be specified with justification. Depending on the vitality of the tree and the prognosis for the development of decay, it is also possible that future adaptive growth can increase the load-bearing capacity of the stem such that safety factors might be restored over time. The application of SIA is a relatively basic assessment and many relevant factors are not incorporated into the model. The available SIA tools should therefore be used conservatively and should not be relied upon where the results are marginal, without considering the effect of factors that have not been considered (Detter et al. 2010). If a more detailed consideration is required for marginal scenarios, then more elaborate assessment methods can be applied that adopt the principles of SIA:
The program TreeCalc (arbosafe GmbH) allows the consideration of cracks and open cavities, which can significantly reduce the bearing capacity of a tree stem.
Probably the most precise calculation of the section modulus of the damaged trunk plane can be achieved with the help of an area analysis of an available sonic tomogram.
Models are also available for wind load estimation that more accurately capture the crown shape, incorporate other tree and site parameters, as well as dynamic responses, allowing for more accurate load analyses.
In many cases, however, the aforementioned time-saving freely available methods are sufficient to provide a safety-conscious tree assessment based on tree height, trunk diameter, and a few other parameters.
Detter, A., Brudi, E., Bischoff, F. 2010: Messverfahren und Bewertungsmethoden zur Verkehrssicherheit, AFZ-DerWald 8/2010, S. 34-35
Gruber, F. 2007: Die VTA-0,32-Restwandstärkenregel, wissenschaftlich unhaltbar und praktisch unbrauchbar
Wessoly, L. 2014: Sicherheitsfaktoren bei der Eingehenden Baumuntersuchung, Beitrag Osnabrücker Baumpflegetage
Wessoly, L. & Erb, M. 2014: Handbuch der Baumstatik und Baumkontrolle, Patzer Verlag, Berlin – Hannover, 287 S