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Tree dynamic response and survival in a category-5 tropical cyclone: The case of super typhoon Trami

In the future with climate change, we expect more forest and tree damage due to the increasing strength and changing trajectories of tropical cyclones (TCs). However, to date, we have limited information to estimate likely damage levels, and nobody has ever measured exactly how forest trees behave mechanically during a TC. In 2018, a category-5 TC destroyed trees in our ongoing research plots, in which we were measuring tree movement and wind speed in two different tree spacing plots. We found damaged trees in only the wider spaced plot. Here, we present how trees dynamically respond to strong winds during a TC. Sustained strong winds obviously trigger the damage to trees and forests but inter-tree spacing is also a key factor because the level of support from neighboring trees modifies the effective “stiffness” against the wind both at the single tree and whole forest stand level.


Fig. S1. Research compartment including positions of the plots and an anemometer with data availability
Research compartment with unthinned (1.8 m mean between tree spacing; P-100) and thinned treatments in 2017 (3.6 m mean between tree spacing, P-50). Green circles and triangles indicate trees; red filled circles were the damaged trees; light blue filled circles were the instrumented trees (either with strain gauges or IMU) with identification labels (shown to the right side of the symbols). There were no trees outside the compartment (roads and open fields). All trees with the circle symbols were genetically related (full siblings). Wind direction (blue arrow) was observed at a propeller vane anemometer located outside the plots and operated by FFPRI (see Fig. S6).

Fig. S2. Wind momentum sweeps and ejections
Wind momentum sweeps (right-lower part of each plot) and ejections (left-upper part of each plots) from the beginning of the typhoon wind period (20:00, 30 September 2018) to the end of observations (1:50, 1 October 2018) with the ultrasonic anemometer located between the P-50 (thinned) and P-100 (unthinned) plots at 12.5 m height, about 1.3 m below the mean tree height (see Fig. S1).      S7. Wind speed at the ultrasonic and 3-cup anemometers 10 minute mean wind speed and standard deviation from the ultrasonic anemometer (inside the compartment) and from the 3-cup anemometer (outside the compartment). The two datasets were significantly cross-correlated with a zero time-lag (r > 0.95).

Table S1. Information on tree sizes in the compartment and within the plots in 2018
Mean and standard deviation (s.d.) of stem diameter at a breast height of 1.3 m (dbh) and height of the trees in the research compartment and the analyzed trees in the two research plots: P-100 and P-50, along with the undamaged and damaged tree groups * Tree height of 3 visible trees measured using a Vertex (HAGLÖF SWEDEN Co., Långsele, Sweden) from outside the plot.

Movie S1. Video of the research plots recorded after damage from super typhoon Trami
This video was taken by the corresponding author on 24 October 2018, who entered the research plots from the north side of the compartment. First it shows the P-50 plot (damaged) on the left side and then the P-100 plot (undamaged) on the right side.
Movie S2. Animation of IMU derived tree displacements for damaged tree S14 during typhoon Trami Large scale displacements of the tree can be seen, particularly from 0:18 on the 1 October 2018 onwards, when the tree was estimated to be damaged. A progressive lean of the tree towards the north east can be observed to develop as damage is accumulated during of the storm. The black marker and line represent the current time point and progressively fade to light orange and then disappear as time passes, leaving a ghost trail. Playback speed of video is x10 the actual speed during the storm. Left animation gives a 3D view of the whole tree stem with the circle marking the location of the IMU. Right animation provides a bird's eye plan view of the displacement at the center of the tree stem (~6m above ground) where the IMU was located on the tree with the large open circle representing the base of the tree. Bending of the tree during wind loading has not been accounted for in the animations.

Movie S3. Crown movements in the unthinned area of the research compartment
The target area in this vertical looking movie includes the P-100 trees. It was recorded by the corresponding author on 30 May 2019, when the hourly maximum wind speed was approximately 4.95 m/s at the 3-cup anemometer (outside the plot).