University of Freiburg: Improving methods to protect forests against severe weather

Forest scientists Barry Gardiner and Marc Hanewinkel are developing improved models to help trees, and thus infrastructure, withstand more effectively

A team of researchers led by Prof. Dr. Kana Kamimura from Shinshu University/Japan, with the participation of Dr. Barry Gardiner, a forest scientist from Freiburg, Germany, has for the first time been able to measure and evaluate how trees move in an extreme tropical cyclone of category five and what damage they suffer. The high-resolution data will help to develop recommendations on how forests, but also infrastructure such as railroad lines, can be better protected against future storm damage – also in Europe.

Helping neighboring trees

The scientists were helped by chance. In Japan, they had set up two experimental plots in an area with sickle fir for other experiments and equipped a total of 36 trees with high-tech sensors. One of the two areas had been thinned the year before, removing half of all the trees. Then, on October 1, 2018, the area was hit by the extremely powerful typhoon Trami with winds of over 100 kilometers per hour. Thanks to the sensors, the team was able to record the behavior of the trees in both test plots at ten-minute intervals.

“The main message is that trees are more vulnerable after thinning because their neighbors have been removed,” Gardiner says. The team has just published the results of their elaborate data analysis in the journal Science Advances. “On the other hand, if the trees are more densely spaced, the neighbors provide stability to a tree because their crowns are touching,” Gardiner adds, “because the force of the wind is dissipated and reduced.” The team was able to analyze 29 trees in depth, three of which had fallen as a result of the storm. All three were located in the thinned experimental plot.

Severe storms also in Central Europe

“However, this does not mean that forests should no longer be thinned at all,” says Prof. Dr. Marc Hanewinkel from the Chair of Forest Economics and Forest Planning at the University of Freiburg, adding that additional factors still need to be taken into account. Hanewinkel heads the Freiburg subproject of the ClimXtreme research network, which is funded by the German Federal Ministry of Education and Research. The Freiburg team, which includes Barry Gardiner, is primarily concerned with the effects of winter storms in Central Europe. “We have to expect that storm patterns will also change in our region due to climate change and it’s not for the better,” Hanewinkel says. In his view, it’s the reason it makes sense to adapt forest management to this reality.

“The tree species plays a very large role in this,” the forest scientist explains. For example, it might make sense to convert coniferous forests to deciduous or mixed forests because deciduous trees offer less surface for attack in winter , a time when there’s the greatest risk of storms. On the other hand, thunderstorms are more likely to occur at certain points during the summer time. Hanewinkel’s project also takes into account the respective region and its specific storm risk when creating models, for example for railroad lines. To this end, the Freiburg researchers are cooperating closely with meteorologists at Freie Universität Berlin.

Many factors must be considered

And it is precisely the type of thinning that is important: Densely standing trees can stabilize even during a strong storm, as Gardiner has shown in detail. But if the trees are too dense, they remain slender and grow quickly in height, which in turn makes them susceptible to storm damage. Region, location, tree species and spacing need to be considered together, Hanewinkel says, “It’s a complex issue about optimization.”

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