Wind and Tree Interaction | 101 Studies, Methods, and Modern Technologies for Monitoring and Measuring Biomechanics

Trees play a vital role in human life and provide various ecosystem services in forests and urban areas. These services include the provision of resources (such as oxygen and wood), environmental regulation (reducing carbon dioxide and the urban heat island effect), improving quality of life (aesthetic beauty, stress reduction), and supporting biodiversity.

With climate change and the increasing occurrence of strong winds, the risk of tree failure and uprooting is rising. This phenomenon can have widespread economic, environmental, and social consequences. Therefore, regular monitoring of trees is crucial to prevent damage to buildings, infrastructure, and human lives.

Biomechanical studies in this area have two main approaches:

• Static approach: Examining the effect of constant or quasi-static forces on trees.
• Dynamic approach: Analyzing dynamic forces and the effects of tree motion inertia.

The choice of method depends on the research objective, type of equipment, and the quality of the data that can be collected. In the past three decades, various sensors and modern technologies for recording and analyzing tree movement have been developed, and their accuracy and selection method have a significant impact on the final results.

Technological advancements and increased public awareness have made long-term and real-time monitoring one of the key tools in managing tree health and safety, especially in urban areas.

Methods for Monitoring Wind-Tree Interaction

Dynamic Excitation Approaches in Wind-Tree Monitoring

Based on a review of 101 scientific studies, 70.3% of the research (equivalent to 71 studies) have utilized dynamic excitation approaches. Among them, 62 studies were focused on field monitoring of tree responses to wind, which is considered the most common method for investigating wind-tree interactions.

In dynamic experiments, the movement or bending of trees under wind load in various conditions is recorded and analyzed.

Objectives of Dynamic Studies

The studies that employed dynamic approaches had diverse objectives, including:

• Testing and introducing new technologies, even in extreme weather conditions such as tropical storms

• Using a variety of sensors: from older prism-based systems (1998) to low-cost sensors and advanced, precise sensors

• Vibration analysis for a better understanding of wind-tree dynamics

• Continuous monitoring to compare numerical and analytical models with real-world data

• Investigating the effects of turbulent winds, tree crown interactions, snow load, temperature changes, and forestry operations such as pruning and thinning

• Pull and Release Test

One key method in dynamic approaches is the pull and release test, which has been used in 14 studies. In this method:

• The tree is slowly pulled.

• Then, it is suddenly released to create free oscillation.

• The data are analyzed to determine the tree’s natural frequency and damping coefficient.

Field Measurement Technologies and Tools in Wind-Tree Monitoring

Over the past 30 years, a wide range of technologies and field tools have been used to monitor wind-tree interactions. While many studies have not reported complete technical details (such as the brand or specific specifications of sensors), reviewing the available data can provide valuable guidance for selecting tools.

Common Tools

Between 1994 and 2022, the following tools were most commonly used in field monitoring:

• Strain Gauges

• Inclinometers

• Accelerometers

• Load Cells / Force Gauges

These tools can be used in all types of field experiments (both static and dynamic). For instance, an inclinometer can be installed both at the base of the tree for static tension tests and for dynamic monitoring.

Differences in Experimental Design

• Tension Test (Static or Combined):
  The tree is pulled using a rope at a specified height, with the end of the rope attached to a fixed support. The pulling force is applied by a manual or motorized winch and measured using load cells. Tree movement is recorded using strain gauges, displacement sensors, or inclinometers.

• Dynamic Monitoring:
  In this method, natural wind is used as the loading source, making the experimental design simpler and eliminating the need for force application equipment.

Sampling Rate and Sensor Installation Location

• Sampling Rate: A minimum of 20 Hz is recommended for accurately recording dynamic responses, especially in strong winds (> 11 m/s) or when monitoring branches. A 10 Hz rate is insufficient for capturing vibration details.

• Vertical Installation of Sensors:

  • Installation at higher elevations → records larger displacements (especially in coniferous trees)

  • Installation at a relatively fixed height (e.g., 3/7 of the tree height) → enables comparisons between different trees

  • Installation of multiple sensors at various heights → allows analysis of overall movement patterns

Note:

• Root inclinometers are typically installed at the tree base or on the roots.

• Trunk movement monitoring is usually done at heights less than 2 meters.

• For branches, the sensor is directly installed on the branch.

Technologies and Measurement Tools in Wind-Tree Monitoring

In biomechanical studies, various tools are used to record and analyze tree responses to wind. These tools are selected based on the type of approach (static or dynamic) and the study’s objectives.

Strain Gauges:

• Convert the change in length of wood fibers or roots into an electrical signal.

• Require calibration before use (through tensile tests or laboratory experiments).

• Used in static studies to record fiber deformation during failure.

• Applied in dynamic studies to record oscillation frequency and drag coefficient.

Inclinometers:

• Measure the change in angle of the trunk or roots relative to the initial position.

• In tension tests: Record the deformation of the trunk or root plate.

• In dynamic monitoring: Calculate the trunk displacement in meters from angle data.

Accelerometers and Inertial Measurement Units (IMU):

• Monitor dynamic tree movements caused by wind.

• Capable of recording data in one, two, or three axes.

• IMUs include accelerometers, gyroscopes (angular velocity), and magnetometers (to reduce error).

Load Cells / Force Gauges

• In static tension tests, they are used to record the force applied to the trunk.

• They enable the calculation of the tree’s bending resistance torque.

Cameras, Image Sensors, and Other Technologies

• Filming and analyzing tree movement after the test.

• Modern Tools:

  • Doppler laser

  • Prism-based systems

  • LiDAR

  • High-precision GPS

  • Air pressure sensors

  • Fiber optic technology

Trends in the Last Five Years (2018-2022)

Around 50% of the studies reviewed in this paper have been published in the last five years. During this period, the primary research focus has been on the following areas:

• Testing and validating new technologies

• Estimating the physical and mechanical properties of trees

• Examining the effects of forest or green space management (e.g., pruning, thinning)

• Studying the effect of environmental conditions (e.g., storms, snow load) on tree behavior

Introduced Modern Technologies:

• Low-cost sensor networks including IMUs and strain gauges with wireless data transmission and external battery power.

• High-sensitivity accelerometer networks for modal analysis.

• Multi-beam LiDAR for recording tree dynamic features during storms.

• Single-frequency GNSS receivers for monitoring trunk oscillations.

• Piezoresistive pressure sensors for studying the relationship between wind speed and air pressure beneath the crown.

• Fiber Bragg Grating (FBG) technology for monitoring strain and trunk tilt angles.

Risk Monitoring Tools and Tree Failure Warning Systems

In recent years, with the increasing importance of preventing tree failures, especially in urban areas, several scientific studies have introduced innovative risk monitoring tools. These tools, using smart sensors, communication technologies, and even machine learning, enable the identification of potential hazards and the creation of warning systems.

Wide-scale Tree Monitoring System with Smart Sensors

• Monitoring trunk tilt angles for 8,000 trees in Hong Kong.

• Use of accelerometers with a precision of 0.05 degrees.

• Wireless communication network using LoRaWAN and NB-IoT.

• Data recording: Every 1 hour under normal conditions, and every 5 minutes during alerts (e.g., storms or heavy rain).

• Data displayed on an online GIS platform.

• Warning algorithms based on “increasing trend” and “sudden rise” developed on the basis of the previous system.

• Combined sensor: Accelerometer + Gyroscope with a precision of 0.1 degrees.

• Installed at a height of 50 cm from the ground.

• Identification of failure patterns through statistical analysis and computational fluid dynamics (CFD) simulations.

• Monitoring based on “Jerk” (time derivative of acceleration).

• Accelerometer with a precision of 0.02 degrees.

• Real-time signal processing with Fast Fourier Transform (FFT).

• Data sent to the cloud if hazard thresholds are exceeded.

• Data recording: Every 2 hours under normal conditions, and every 10 minutes during alerts.

Fiber Bragg Grating-based Warning System

• Installation of 8 FBG strain gauges and 2 FBG inclinometers at various heights on the trunk.

• Critical threshold calculation based on wood mechanical properties and a safety factor of 70%.

• Powered by solar energy and real-time data transmission via 4G network.

• Two-level user interface for remote data visualization.

Methods for Studying Wind-Tree Interaction

Conclusion and Future Outlook for Wind-Tree Interaction Studies

This comprehensive review provides a clear picture of the development trends in methods and technologies within the field of wind-tree interaction studies. The examination of articles published between 1994 and 2022 shows a significant increase in research over the past five years, a growth that aligns with the overall rise in global scientific production.

Lack of Standard Protocol in Existing Studies

Analysis of the various methods and approaches reveals that there is no standard protocol or guideline for data collection in the current studies. However, each of the four main methods examined in this review offers specific and valuable information:

Static Approaches:

• Non-destructive tensile testing: The most common method for calibrating sensors.

• Destructive tensile testing: The only way to estimate the ultimate resistance of trees against overturning or trunk failure.

Dynamic Approaches:

• Tensile and release testing: Measures the dynamic properties of trees without requiring real wind, but it is limited to instantaneous condition recording.

• Field measurement of wind response: Provides the ability to study tree responses under real conditions, along with the effects of seasonal and temporal variations.

Future Outlook

The future of wind-tree interaction studies is likely to see advancements in both technology and methodology. The use of smart sensors, artificial intelligence, and real-time monitoring systems will significantly enhance our ability to understand and predict tree behavior under wind stress. Additionally, the development of standardized protocols for data collection will allow for better comparison across studies and more reliable results. As the frequency and intensity of extreme weather events increase, these studies will become even more critical in mitigating the risks of tree failure, especially in urban environments.