DOI: 10.5061/dryad.r4xgxd2dj
title: Tree dynamic response and survival in a category-5 tropical cyclone: The case of super typhoon Trami

description: 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.
description: <b>Study site</b> We established
 two research plots in Chiyoda Experimental Forests of the Forestry and
 Forest Product Research Institute (FFPRI) in Japan (36.184°N, 140.216°E;
 42 m above sea level). In 2005, 2-year old genetically similar
 <i>Cryptomeria japonica </i>trees (full siblings) were planted
 in a compartment at 1.8 m spacing (3,000 trees/ha, 25 m
 <i>×</i> 22 m in size) (Mori <i>et al.</i> 2019).
 As a windbreak, 55 <i>Cryptomeria japonica</i> trees were also
 planted at the compartment edge; they were genetically unrelated to the
 subject trees. In November 2017, we thinned some trees in half of the
 compartment and created two plots with a 15 m <i>× </i>6 m in
 size: P-100 (total 24 trees, unthinned since planting, 1.8 m tree spacing,
 15.7 cm mean diameter at a breast height of 1.3 m (<i>dbh</i>)
 with 2.7 cm standard deviation (<i>s.d.</i>)) and P-50 (total
 12 trees, thinned every other tree, 3.6 m mean tree spacing, 15.3 cm mean
 <i>dbh</i> with 3.3 cm<i> s.d.</i>). The means of
 <i>dbh</i> were not significantly different at 5% significant
 level (<i>p</i> = 0.7360) based on a two-sample
 <i>t</i>-test without assuming the equal variance between the
 plots. We also measured the tree heights of the cut trees in P-50: 13.5 m
 mean tree height with 1.2 m <i>s.d.</i> (<i>n</i>
 = 32). A buffer zone with a similar treatment surrounded each plot. When
 the plots were created, all trees were cut down in the other compartments
 located in a northeast and east direction. Our compartment was surrounded
 by an open field and a 3.5 m wide paved road (on the south to southwest
 side). <b>Data collection </b>
 We attached two aluminum strain-transducers (Moore <i>et
 al.</i> 2005) at 0.25 m height on 36 trees in the north and east
 direction (or the west direction when we could not attach the transducer
 in the east direction due to the stem shape). The transducers had
 one-active and one-dummy strain gauge (KFGS-5-120-C1-23, Kyowa Inc.,
 Tokyo, Japan) to make reliable measurements of the strain values in the
 tree stem without becoming detached from the growing trees. The other half
 of the Wheatstone bridge was completed on a circuit board with balanced
 precision resistors, and the whole bridge was then attached to a
 PhidgetBridge 1046 (Phidgets Inc., Calgary, Canada), which supplied power
 to the strain gauges and converted the strain to voltage. We also attached
 an Inertial Measurement Unit (IMU, PhidgetSpatial 3/3/3 Basic: 1042,
 Phidgets Inc., Calgary, Canada) to 33 tree stems in the north direction at
 6 m height, which was the height from the ground surface to the initially
 estimated crown base. We avoided the three smallest trees due to the risk
 of climbing up to 6 m. The IMU includes three sensors: accelerometer,
 gyroscope, and magnetometer. Because the IMU uses the magnetic field, we
 first obtained the calibration parameters based on the local magnetic
 field (PhidgetSpatial User’s guide,
 https://www.phidgets.com/?&prodid=32). The signals from the strain
 gauges and IMU were recorded at 10 Hz and logged by Raspberry Pi3
 (Raspberry Pi Foundation, Cambridge, UK). We used the Linux “cron” utility
 in the Raspberry Pi3 to control the data-logging Python codes.
 Data-logging started running at 0 minutes and stopped at 59 minutes every
 hour and the gyroscope in the IMU was reset to zero at the start of each
 recording period to compensate for drift. An ultrasonic
 anemometer (CYG-81000, R. M. Young Co., Traverse City, USA) was attached
 to the top of a mast (12.5 m height: about 1.3 m below than the mean tree
 height). The position was 12.6 m from the north edge and 10.8 m from the
 west edge of the compartment. It measured three orthogonal components of
 the wind: <i>u</i>, <i>v</i> and
 <i>w</i>, at 1 Hz frequency, which was logged by a GRAPHTEC
 data logger midi LOGGER GL240 (Graphtec Co., Yokohama, Japan). The
 anemometer fell off the mast at around 2:10, 1 October 2018, and we
 confirmed there was useful data until 1:50. We installed a 3-cup
 anemometer (S-WSB-M003, ONSET Co., Bourne, USA) at 11 m height on an
 electronic pylon tower (50 m north of the sonic anemometer), and recorded
 at 0.1 Hz. <b>Wind from tropical cyclone (typhoon
 Trami)</b> The Japan Meteorological Agency
 reported that a category-5 tropical cyclone, super typhoon Trami, made
 landfall in Japan at approximately 20:00, 30 September 2018, and tracked
 north over the main island and changed to a tropical storm by 12:00, 1
 October 2018 (www.data.jma.go.jp, in Japanese). The 3-cup anemometer
 measured 29.71 ms<sup>-1</sup> between 2:00 to 2:10 (the
 highest 10 minute mean wind speed), and the ultrasonic anemometer recorded
 4.67 ms<sup>-1</sup> between 1:40 and 1:50, 1 October 2018 (10
 minute mean wind speed inside the canopy).
 <b>Field survey </b> After the
 passage of typhoon Trami, we surveyed all trees in the compartment (26
 October to 21 November 2018) and found eight damaged trees including seven
 uprooted trees (leaning on neighboring trees) only in the area that had
 been thinned in the previous year. One tree, shaped like a two-tine fork,
 was broken at the connection between the two separate stems at 4.25 m
 height. We categorized trees as damaged if the minimum ratio of its angle
 to the ground (these calculations are explained in the following section)
 was less than 83% between 6:00 and 7:00, 1 October 2018 (low wind speed
 period). For our on-going research project, we conducted tree-pulling
 experiments on 37 undamaged trees in the unthinned side of the research
 compartment including P-100 in 2019. Using the averaged stem angles
 (recorded at 0.25 and 3 m height) at the maximum <i>TM</i>
 from the 32 uprooted trees, we determined the threshold angle for damage,
 which was the ratio of a mean lean angle of 14.9 degrees (4.8 degrees
 <i>s.d.</i>) to the vertical rest position (assumed as 90
 degrees). Stem angles were based on assuming the tree stems were always
 straight without any curvature due to the wind force.
 We cut and logged all P-50 trees (including the windbreak trees),
 and measured <i>dbh</i>, tree height, crown width and length,
 and stem diameter at 0.25 m height. We did not cut the P-100 trees and
 only measured <i>dbh</i> and stem diameter at 0.25 m height to
 continue measuring tree oscillations in the following year. We could not
 measure the single tree heights in P-100 from inside the plot because of
 the high stem density, so we measured the tree heights of three trees from
 outside the P-100 plot using a Vertex (HAGLÖF SWEDEN Co., Långsele,
 Sweden) in November 2018. We estimated the single tree heights in P-100
 using the mean tree height of the three trees in 2018, single tree heights
 measured in November 2019 and mean tree heights of the genetically related
 trees planted in other compartments (next to our compartment) measured in
 November 2017 (Mori <i>et al.</i> 2019). Assuming more
 sunlight reaching P-100 due to the absence of the thinned area and
 windbreak trees in the growing season in 2019, we estimated 1/3 growth
 rate from 2017 to 2018 and 2/3 from 2018 to 2019. The estimated mean tree
 height in 2018 showed a good agreement with the measured mean tree height
 of the three trees. Despite some technical problems during the TC, in
 total tree movement and turning moment data from 29 trees were
 successfully obtained: 20 trees in P-100 and 9 trees (including 3 damaged
 trees) in P-50; 12 trees had both the strain gauge and IMU data. Some
 hourly data sets were missing due to the power conditions during the TC.
 The mean <i>dbh</i>s of the two plots were not significantly
 different at the 0.05 significance level
 (<i>p</i>=0.8852). <b>Data
 processing</b> The strain gauge data were
 filtered and calibrated. First, we filtered the strain gauge output
 voltages using a high-pass Butterworth filter method with a 10 minute
 cut-off frequency (Duperat <i>et al.</i> 2021) and inserted
 10-minute dummy data in the first and last row of the data to avoid
 over-fitting. The filtered data were then transformed to turning moments
 (Nm) at the stem base using the conversion parameters, which we had
 previously obtained in field experiments. We pulled the trees with a small
 non-damaging amount (March and May 2018) or compressed the attached
 transducers (November 2018) to find the linear relationship between the
 strain and turning moment. We rotated the IMU outputs
 to correct their installed x, y, and z-position, which were confirmed by
 IMU behavior tests in the laboratory (August 2020). The 3-sensor data
 (9-axis) were then fused using the 3-sensor orientation method, which
 calculates quaternions of every data set to reduce errors and noise from
 the local magnetic conditions. The Euler ZYX convention then computed x,
 y, and z coordinates at the given height. In this paper, we chose the
 center of gravity (mass) of the crown (assumed as 6 m + crown length/2) to
 describe the tree oscillation. Since the trees had already begun
 oscillating at the beginning of the study period (20:00 on 30 September
 2018), we used outputs from 1:00-2:00, 28 September 2018, with no measured
 wind outside the plots, as the reference data to calculate the tree
 vertical rest positions. The stem angles to the ground were estimated by
 comparing the reference x, y, and z coordinates and those during the TC.
 The detailed calculation procedure is found in the documentation of the
 MATLAB Sensor Fusion and Tracking Toolbox (MathWorks Inc., Natick, USA;
 https://uk.mathworks.com/products/sensor-fusion-and-tracking.html). The <i>u</i>, <i>v</i>, and <i>w</i> wind components, measured by the ultrasonic anemometer (20:00 on 30 September 2018 to 1:50 on 1 October 2018), were rotated to reduce bias from the changeable wind conditions during a strong wind. We used the double rotation method in the coordinate rotation methods (Golzio <i>et al.</i> 2019) to obtain the best value of the vertical component (<i>w</i>) (Finnigan 2004). The process was based on 10 minute averages because of the lack of stationarity in the wind conditions during the typhoon. We calculated the instantaneous momentum flux (<i>u’w’</i>) using the second rotated streamline vector <i>u<sub>2</sub></i> and vertical vector <i>w<sub>2</sub></i>.
description: This dataset includes four compressed folders: the data of the strain gauges ("TM_Filtered.zip") and Inertial Measurement Unit ("IMU_Original.zip" & "Coordinates.zip") measured between 20:00, 30 September to 7:00 on 1 October 2018 and the data of wind inside and outside the plots ("Wind.zip"). Detailed information can be found in the "README.txt" file in each folder. "Trees_2018.xlsx" provides the tree dimensions in 2018 (after the typhoon), which were used for the analysis.