A new investigation into forests populated by Cryptomeria japonica reveals that these trees disperse wind energy by alternating between two different swaying motions based on wind speed. This discovery can aid in better forest management practices to reduce storm-related damage.
Strong winds during storms and cyclones frequently result in trees failing, often through uprooting or stem breakage. Despite this, how trees react to wind in different forest setups and weather conditions is not well understood. A recent investigation on Cryptomeria japonica plots has shown that these trees switch between two types of swaying behaviors at certain wind speeds, which could lead to improved strategies in forest management to lessen storm damage.
Severe weather events, including tropical and extratropical cyclones, as well as tornadoes, can wreak havoc on forests, leading to both ecological and monetary losses. Tree falls during such storms can disrupt ecosystems, raising the costs of forest management. With climate change intensifying, the occurrence of serious storms is predicted to rise, highlighting the need to comprehend how forests react to wind stress.
Understanding how trees fail is essential for developing mitigation strategies. While earlier research has investigated how trees respond to wind, it is uncertain whether these reactions vary with different forest configurations—defined by tree density and spacing—and varying weather conditions.
To address this issue, a team led by Associate Professor Kana Kamimura from the School of Science and Technology at Shinshu University, Japan, studied tree movements in diverse forest setups and weather conditions to see how trees withstand winds. The research team also included Kazuki Nanko, Asako Matsumoto, and Saneyoshi Ueno from the Forestry and Forest Products Research Institute in Japan, along with Barry Gardiner from the University of Freiburg in Germany and the Institut Européen de la Forêt Cultivée in France. The findings were made public online on August 27, 2024, and published on November 1, 2024, in Volume 571 of Forest Ecology and Management.
Discussing the reasons behind the study, Prof. Kamimura states, “Various methods have been created to forecast wind damage. However, they primarily rely on empirical data and parameters, often overlooking how wind damage actually occurs. Our research seeks to clarify how winds affect trees directly and how trees mitigate wind stress to survive.”
To conduct the research, the team established two experimental plots of the Japanese cedar, Cryptomeria japonica, in November 2017 at experimental forests run by the Forestry and Forest Products Research Institute in Kasumigaura City, Japan. The initial plot, named P-100, contained 3,000 trees per hectare, forming a dense forest. Conversely, in the second plot, P-50, half of the trees were removed to simulate thinning, leaving 1,500 trees per hectare. Over a span of two years, the team observed 24 trees in the dense plot and 12 in the thinned plot, using sensors on the trunks to monitor tree sway under various wind conditions. This observation period included several typhoons, like Typhoon Trami in 2018, which significantly impacted the thinned plot.
The research revealed that cedar trees display two different swaying behaviors based on wind speed. In mild winds, the trees swayed at a frequency of about 2 to 2.3 cycles per second, allowing their branches to absorb much of the wind energy and protecting their trunks and roots from stress. However, with higher winds, they shifted to a slower swaying pattern of 0.2 to 0.5 cycles per second, causing the entire tree to sway, which can increase the risk of breakage or uprooting.
Interestingly, the transition between these two swaying modes occurred at varying wind speeds based on forest density. In the dense plot, trees shifted swaying modes between wind speeds of 1.79 and 7.44 meters per second. In the thinned plot, this transition happened at slightly lower wind speeds, ranging from 1.57 to 5.63 meters per second.
To evaluate damage resistance, researchers used an uprooted tree as a standard, measuring the damage resistance in the thinned P-50 during a 10-minute interval of Typhoon Trami. They discovered that the true resistance was only 48% of what was expected based on controlled tree-pulling tests.
Prof. Kamimura explains, “The 52% difference between the actual and expected resistance values could be because the roots weakened due to strong winds prior to them becoming more intense. This root fatigue was likely increased by the trees moving more because of less support from neighboring trees and greater wind exposure in the plot.” This helps clarify why the trees in the dense P-100 area remained unharmed during Typhoon Trami.
This research provides essential insights into balancing thinning with wind resistance in forestry management to promote sustainable forestry practices and help forests endure extreme climate changes. Although thinning can encourage tree growth, it might also render forests more susceptible to storms, particularly shortly after thinning takes place. Prof. Kamimura concludes, “With storms becoming more common in a changing climate, forest management strategies must evolve to maintain resilience.”
