Catastrophic winds from tropical cyclones and storms cause forest trees to fail, leading to significant economic and environmental losses. Tree failure, particularly uprooting, occurs when wind energy is transferred from the air to the tree’s roots via the stem.
However, the exact mechanisms behind how trees respond to wind under varying weather conditions and whether these responses differ based on forest structure still need to be fully understood.
Previous studies have examined how trees react to wind, but it remains unclear whether these responses are consistent across different forest configurations, such as variations in tree spacing and density and differing weather conditions.
A research team led by Associate Professor Kana Kamimura from the School of Science and Technology at Shinshu University, Japan, investigated tree movements under different forest configurations and weather conditions. Their study focused on understanding how trees resist wind and how these responses vary with changes in forest structure.
In November 2017, researchers established two experimental plots of *Cryptomeria japonica* (Japanese cedar) in the experimental forests of the Forestry and Forest Products Research Institute in Kasumigaura City, Japan. The first plot, P-100, had 3,000 trees per hectare, creating a dense forest, while the second plot, P-50, had 1,500 trees per hectare, simulating thinning practices.
Over two years, the team monitored 24 trees in dense and 12 in thinned plots, using trunk-mounted sensors to track tree sway under varying wind conditions. The monitoring period included multiple typhoons, including 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 2 to 2.3 cycles per second, with their branches absorbing much of the wind energy, which helped protect the trunks and roots from stress.
At higher wind speeds, the trees transitioned to a slower swaying pattern of 0.2 to 0.5 cycles per second, where the whole tree moved together, transferring force through the trunk and roots, which increased the likelihood of breakage or uprooting.
The transition between these two swaying modes occurred at different wind speeds depending on forest density. In the dense plot, this shift happened at wind speeds between 1.79 and 7.44 meters per second, while in the thinned plot, the transition occurred at slightly lower wind speeds, ranging from 1.57 to 5.63 meters per second.
Using an uprooted tree as a reference, the researchers assessed the damage resistance in the thinned P-50 plot over a 10-minute period during Typhoon Trami. They found that the actual resistance to uprooting was only 48% of the expected resistance, estimated based on 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 provides valuable insights for managing forest thinning in a way that balances tree growth with wind resistance, supporting sustainable forestry practices and helping forests better withstand extreme climate events. While thinning can promote tree growth, the research highlights that it can also increase a forest’s vulnerability to storms, particularly in the period immediately following thinning.
Journal Reference:
- Kana Kamimura, Kazuki Nanko et al. Energy transfer during tree movement for different wind conditions and forest configurations. Forest Ecology and Management. DOI: 10.1016/j.foreco.2024.122223