International online event with record turnout
25 March 2021 – The pandemic forces many of us to explore new horizons. This also applies to the association of tree consultants ‘Sachverständigen-Arbeitsgemeinschaft Baumstatik e.V.’, who organised its meanwhile 19th workshop as an online event for the first time. And the new format enjoyed record participation right from the start: On 18 March 2021, 99 tree experts from 17 countries on four continents met for this full-day digital event to discuss the interaction of trees and the wind. The topics on the agenda included current studies and models for the dynamic reaction of trees, which participants were able to follow with simultaneous interpretation into German and English.
The high-level speakers certainly had their share in attracting so many participants. The first speaker was Damien Sellier, a researcher at the New Zealand Forest Research Institute (Scion). Joining the seminar from the other end of the world, he presented his results on the topic of “tree sway frequency and damping – finite element modelling and age effects”. He emphasized the influence of mass distribution on the sway properties of forest trees and was able to show that crown collisions play an important role in damping wind-excited oscillation. For open-grown trees, this must be achieved by other mechanisms. With increasing age, there is a shift of the position with the greatest likelihood of fracture within the tree.
Next on the agenda was a virtual visit to Singapore, where Daniel Burcham, Deputy Director of the Centre for Urban Greenery and Ecology on the National Parks Board, gave a lecture on changes in the dynamic properties of trees in natural wind after pruning. Based on the trees he monitored, Daniel was able to clearly show that reducing the crown height by a mere 10% can lead to a reduction in wind load by 25 to 35 %. Raising the crown by 10 %, by contrast hardly reduced the wind load at all. This fact had so far only been shown on small trees under artificial conditions, and has now been proven experimentally for the first time with fully grown park trees in natural wind.
Toby Jackson from Cambridge, Great Britain, then presented his synthesis of data from all over the world on the motion of trees in the wind. Based on this meta-study, he was able to demonstrate that deciduous trees are in fact more prone to wind-failure in the summer due to their foliage, and that their natural frequency can best be estimated according to the principle of a classic pendulum model. The dynamic behaviour of forest conifers turned out to be particularly uniform, and in this case, the model of a cantilever beam seemed to be more suitable. For his PhD, he had conducted simulations similar to those of Damien Sellier, but based on realistic tree geometries which had been exactly measured by means of LiDAR scanners.
After the lunch break, Nikolas Angelou, researcher in the Meteorology and Remote Sensing Section in the Department of Wind Energy at the Technical University of Denmark, presented the results obtained by Ebba Dellwik, head researcher of the department, as well as his own research. Their experiment focused on an individual oak tree at the Danish coast, and they were able to exactly describe its interaction with natural wind, using strain gauges and wind LiDAR scanners. Nikolas explained how the wind exerts forces on trees and how the trees themselves have a measurable influence on the structure of the wind. This may be illustrated using the example of the measured drop in wind speed between the upwind and downwind side of the tree, which reached up to 90% in that case. In that case, the wind-tree interaction results in a layer of high turbulence along the crown periphery, as it was observed in the near wake flow behind the tree.
The last presentation was by Dirk Schindler, professor for environmental meteorology at the Freiburg University, who presented entirely new approaches to modelling the response of trees to turbulent wind. For evaluating measurements on standing trees, both Fourier and wavelet transformations of the dynamic interaction were applied. Based on his results, Dirk Schindler was able to show that the resonance, i.e. the amplification of tree sway by a matching sequence of wind gusts, does in fact not play a major role in the wind and tree interaction. This way, he illustrated how urgently previous modelling approaches need to be revised and be given a new direction.
The event was hosted by the Chairman of SAG Baumstatik, Andreas Detter of Brudi & Partner TreeConsult in Gauting near Munich, who was able to pass many questions from the audience to the speakers. He concluded: “We are very happy about the large number of participants in the event, and the abundant positive feedback we received. Even if we miss the personal contact and exchange, it is great to bring together researchers and experts from all over the world, making use of the new technical possibilities which have arisen out of necessity. This would not have been possible in this form if we had had a face-to-face event.” The obligatory group photo had to be dispensed with this time, but the audience from all around the globe was able to gain plenty of informative insights. The simultaneous interpretation of the presentations into German was also a clear benefit compared to the previous 18 workshops of SAG Baumstatik which were usually interpreted consecutively. The association is therefore considering to conduct future seminars as hybrid events, allowing speakers and participants to join a small face-to-face meeting digitally. Another workshop of SAG Baumstatik will probably take place this year. As usual, information on this will be available beforehand on the website www.sag-baumstatik.org.
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