Extreme weather events, including cyclones and tornadoes, can significantly damage forests, resulting in ecological and financial consequences. Tree falls disrupt ecosystems and raise forest management costs. With climate change contributing to more frequent severe storms, insights into how forests respond to wind stress are becoming essential for resilience planning.
Understanding tree failure mechanisms is essential to formulating protective strategies. While previous research has examined tree reactions to wind, there is limited knowledge on how these responses vary with forest spacing and weather conditions.
Associate Professor Kana Kamimura from Shinshu University's School of Science and Technology led a study to observe how trees sway under different forest densities and wind conditions. The research team, including experts from the Forestry and Forest Products Research Institute in Japan and the University of Freiburg in Germany, published their findings in *Forest Ecology and Management* in November 2024.
"Several techniques have been developed to predict wind damage," noted Prof. Kamimura. "However, these methods often rely on empirical data and overlook the processes behind how wind damage occurs. Our study aims to directly observe how winds affect trees and how they cope to survive."
In November 2017, two plots of Japanese cedar were established in the experimental forests of Kasumigaura City, Japan. The first plot, P-100, represented a dense forest with 3,000 trees per hectare. The second plot, P-50, had half as many trees to simulate thinning, with 1,500 trees per hectare. Over two years, researchers monitored 24 trees in the dense plot and 12 in the thinned plot using trunk-mounted sensors during various wind events, including Typhoon Trami in 2018.
The study found that cedar trees switch between two swaying modes depending on wind speed. In moderate winds, trees swayed at 2 to 2.3 cycles per second, with branches absorbing much of the wind's energy, protecting the trunk and roots. In higher winds, the sway frequency slowed to 0.2 to 0.5 cycles per second, with force distributed more uniformly across the entire tree, heightening the risk of breakage or uprooting.
The transition between these swaying behaviors occurred at different wind speeds in the two plots. In the dense P-100 plot, the transition occurred at wind speeds between 1.79 and 7.44 meters per second. In the thinned P-50 plot, the shift began at slightly lower wind speeds, between 1.57 and 5.63 meters per second.
To assess wind resistance, researchers observed an uprooted tree in P-50 during Typhoon Trami over a 10-minute interval. The resistance was only 48% of the expected value from controlled experiments, indicating that strong winds had weakened the roots even before peak wind speeds occurred.
"The 52% discrepancy between actual and expected resistance suggests that root fatigue can set in as trees experience movement without the support of surrounding trees, allowing more wind to penetrate the thinned area," Prof. Kamimura explained. This was a key reason why the denser P-100 plot remained undamaged.
This study highlights the importance of balancing forest thinning and wind resistance for sustainable forest management. While thinning can enhance tree growth, it may increase vulnerability to storms, particularly shortly after the process. Prof. Kamimura emphasized, "With more frequent storms in a changing climate, forest management practices must evolve to ensure resilience."
Research Report:Energy transfer during tree movement for different wind conditions and forest configurations
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