Facing the Wind: How Trees Behave Across Various Forest Settings and Weather Events

Researchers in Japan and France identify two primary tree movement patterns that help them survive high winds and prevent damage

Destructive winds during storms and cyclones often cause tree failures, especially through uprooting and stem breakage. However, how trees respond to wind under various forest configurations and weather conditions remains unclear. A recent study on Cryptomeria japonica plots shows that trees dissipate wind energy by switching between two swaying behaviors at specific wind speeds, offering insights that may help in improved forest management to minimize damage caused by storms.

Image title: The study plot showing the wind damage caused by Typhoon Trami on October 1, 2018
Image caption: Forest configuration, defined by tree density and spacing, affects trees’ resistance against powerful winds. In this study, the thinned plot experienced tree failures due to Typhoon Trami, while the dense plot showed no damage. This finding highlights the need for foresters to adopt safe thinning practices.
Image credit: Kana Kamimura from Shinshu University
License type: Original content
Usage restrictions: Cannot be used without permission.

Extreme weather events, such as tropical and extratropical cyclones and tornadoes, can cause widespread damage to forests, leading to environmental and financial losses. When trees fall during these storms, ecosystems might be disrupted, increasing forest management costs. As climate change worsens, severe storms are expected to become more frequent, making it crucial to understand how forests respond to wind stress.

Grasping the mechanisms behind tree failure is key to developing strategies for mitigation. While previous studies have explored how trees react to wind, it is unclear whether these responses remain consistent across different forest configurations—characterized by tree spacing and density—and weather conditions.

In this vein, a team of researchers led by Associate Professor Kana Kamimura from the School of Science and Technology at Shinshu University, Japan, investigated tree movements under various forest configurations and weather conditions, including how trees resist winds. The research team included Kazuki Nanko, Asako Matsumoto, and Saneyoshi Ueno from the Forestry and Forest Products Research Institute, Japan, and Barry Gardiner from the University of Freiburg, Germany, and the Institut Européen de la Forêt Cultivée, France. This paper was made available online on August 27, 2024, and was published on November 1, 2024, in Volume 571 of Forest Ecology and Management.

Explaining their motivation behind the study, Prof. Kamimura says, “Several techniques have been developed to predict wind damage. However, they largely depend on empirical data and parameters, and overlook how wind damage occurs. Our research aims to shed light on how winds directly impact trees and how trees reduce the stress from winds to survive.”

To achieve this, researchers set up two experimental plots of Cryptomeria japonica trees, commonly known as the Japanese cedar, in November 2017 in the experimental forests operated by the Forestry and Forest Products Research Institute, Kasumigaura City, Japan. In the first plot, P-100 consisted of 3,000 trees per hectare, creating a dense forest. In the second plot, P-50, half of the trees were removed for this research, leaving 1,500 trees per hectare to mimic thinning practices. Over two years, the team monitored 24 trees in the dense plot and 12 in the thinned plot, using trunk-mounted sensors to track tree sway during various wind conditions. The monitoring period included multiple typhoons, such as Typhoon Trami, in 2018, which caused significant damage to the thinned plot.

The researchers found that cedar trees exhibit two distinct swaying patterns depending on wind speed. In light winds, the trees swayed at around 2 to 2.3 cycles per second, with their branches absorbing much of the wind energy, protecting the trunks and roots from wind stress. However, at higher wind speeds, the trees shifted to a slower swaying pattern of 0.2 to 0.5 cycles per second. In this phase, the whole tree swayed together, transferring force across the trunk and roots, increasing the probability of breakage or uprooting.

Interestingly, the transition between these two swaying modes occurred at different wind speeds, depending on the forest density. In the dense plot, the trees switched patterns at wind speeds between 1.79 and 7.44 meters per second. In contrast, in the thinned plot, the transition occurred at slightly lower wind speeds, ranging from 1.57 to 5.63 meters per second.

Image title: Root-soil plate of Japanese cedar after the tree-pulling experiments
Image caption: Tree anchorage against wind connects to the characteristics of the root-soil plate, which can be observed during the tree-pulling experiments. This study explores the mechanism behind trees responding to varying wind speeds during storms and cyclones. The researchers provide new insights into the resistance of trees to natural calamities that may improve forest management strategies against storms and cyclones.
Image credit: Kana Kamimura from Shinshu University
License type: Original content
Usage restrictions: Cannot be used without permission.

Using an uprooted tree as a reference, researchers assessed the resistance to damage in the thinned P-50 over a 10-minute period during Typhoon Trami. They found that the actual resistance was only 48% of the expected resistance estimated through controlled tree-pulling experiments.

Prof. Kamimura elaborates, “The 52% difference between actual and expected resistance values was likely due to the roots weakening because of strong winds, even before the winds became more severe. This root fatigue occurred because the trees moved more due to less support from nearby trees and more wind penetrating the plot.” This also explains why the trees in the dense P-100 were not damaged during Typhoon Trami.

This study offers valuable insights for balancing thinning with wind resistance in forest management to support sustainable forestry practices, and help forests withstand extreme climate changes. While thinning promotes tree growth, it can also make forests more vulnerable to storms, especially soon after thinning. Prof. Kamimura concludes, "With more frequent storms in a changing climate, forest management practices must adapt to maintain resilience.”

Reference
Title of original paper:		Energy transfer during tree movement for different wind conditions and forest configurations
Journal:		Forest Ecology and Management
DOI:		10.1016/j.foreco.2024.122223

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