⚡ electrical engineering

Evaluating Human Thermal Comfort Impact of Urban Tree Pruning through Solar Radiation Simulation using Quantitative Structure Model

This study proposes an automated workflow integrating Terrestrial LiDAR Scanning, Quantitative Structure Models, and solar radiation simulations to quantify how urban tree pruning alters canopy structure and subsequently impacts human thermal comfort, as measured by Physiological Equivalent Temperature (PET).

Original authors: SZU-YI LEE, CHI-KUEI WANG, CHUNG-CHENG LEE, TZU-PING LIN

Published 2026-10-27

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CC BY 4.0

Original authors: SZU-YI LEE, CHI-KUEI WANG, CHUNG-CHENG LEE, TZU-PING LIN

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

In the sweltering heat of a city summer, the difference between a sweltering sidewalk and a cool, walkable path often comes down to the trees above. Urban forests act as natural air conditioners, not just by releasing moisture into the air, but primarily by casting shadows that block the sun's intense energy. The effectiveness of this cooling depends heavily on the tree's structure: a dense canopy with thick foliage intercepts more sunlight, keeping the ground beneath significantly cooler. However, city trees require maintenance. To keep them safe from falling branches and to maintain clear sightlines for pedestrians, arborists must regularly prune them. This necessary work inevitably removes leaves and branches, thinning the canopy. The critical question for city planners and residents is a delicate balance: how much can a tree be trimmed to ensure safety without stripping away the very shade that makes the street comfortable to walk on?

A team of researchers at National Cheng Kung University in Taiwan set out to answer this question with a level of precision that traditional methods could not provide. Instead of relying on rough estimates or assuming that shade is uniform under a tree, they created a digital twin of a real rain tree to simulate exactly how different pruning styles would alter the sunlight reaching the ground. They began by scanning a mature rain tree located on their campus with a mobile laser scanner. This device, carried around the tree, captured millions of points to create a highly detailed three-dimensional map of every trunk, branch, and leaf. Using this data, the team built a quantitative structure model, a computer representation that treats the tree's wood as a series of connected cylinders, accurately reflecting its true shape and volume.

With this digital tree in hand, the researchers developed an automated system to simulate various pruning scenarios based on real-world safety rules. They programmed the computer to remove branches that grew too steeply or unevenly, which could pose a risk of falling, while ensuring the tree retained enough wood to stay healthy. They tested six different pruning intensities, ranging from a light trim that kept 93% of the tree's wood to a heavier cut that left only 80%. For each scenario, they ran a solar radiation simulation, modeling the sun's path on the hottest day of the year to see exactly how much light would penetrate the canopy and hit the ground below. They then translated these light measurements into a metric for human comfort, calculating how much cooler a person would feel under each version of the tree compared to standing in the open sun.

The results revealed a clear and measurable trade-off between safety pruning and thermal comfort. The original, unpruned tree provided a significant cooling effect, lowering the perceived temperature for anyone standing beneath it by 4.1 degrees Celsius compared to an unshaded area. When the researchers applied a moderate pruning style that maintained the tree's natural conical shape, the cooling benefit dropped slightly to between 3.1 and 3.4 degrees Celsius. This suggests that light, form-preserving pruning is a viable option that maintains most of the tree's comfort benefits. However, as the pruning became more aggressive to balance the branches or remove more wood, the cooling effect diminished noticeably. In the most heavily pruned scenarios, where the tree retained only 80% of its original wood volume, the temperature reduction fell to just 2.3 degrees Celsius. This represents a loss of nearly half the tree's original cooling power.

The study underscores that while pruning is essential for urban safety, the intensity of the cut directly dictates the quality of the shade. The researchers found that even when pruning is done carefully to preserve the tree's health, removing too much canopy can significantly degrade the microclimate for pedestrians. Their work demonstrates that city managers can use these digital tools to predict exactly how much cooling a tree will lose before a single branch is cut. By simulating different outcomes, they can choose pruning strategies that keep trees safe without sacrificing the vital thermal comfort that makes urban streets livable in the heat. The findings suggest that the best approach is to prioritize structural safety first, but then carefully weigh the specific amount of wood to be removed against the resulting loss in cooling, ensuring that the tree remains a functional part of the city's climate control system.

Technical Summary: Evaluating Human Thermal Comfort Impact of Urban Tree Pruning through Solar Radiation Simulation using Quantitative Structure Model

Problem Statement
Urban trees are critical for mitigating thermal stress through shading, evapotranspiration, and airflow regulation. However, the cooling benefits of these trees are heavily dependent on canopy structure, specifically the Leaf Area Index (LAI). While pruning is a standard urban management practice to ensure safety, reduce windbreak risks, and enhance aesthetics, it inevitably alters canopy coverage and reduces shading quality. Existing literature insufficiently addresses the specific impact of pruning on human thermal comfort. Current simulation approaches often rely on simplified models that assume uniform cooling beneath tree canopies, failing to capture the spatial variations in solar radiation that occur after specific pruning interventions. Consequently, there is a lack of systematic methods to evaluate how varying pruning intensities influence outdoor thermal comfort.

Methodology
This study proposes an automated workflow integrating Terrestrial LiDAR Scanning (TLS), Quantitative Structure Models (QSM), and solar radiation simulation to assess the thermal comfort impacts of pruning.

Data Acquisition and Classification: A mature rain tree (Samanea saman) in Tainan, Taiwan, was scanned using an Emesent Hovermap ST-X mobile LiDAR system. The resulting point cloud was classified into "wood" and "leaf" points using a Random Forest (RF) algorithm based on geometric features.
QSM Reconstruction: The wood point cloud was reconstructed into a 3D QSM using the open-source TreeQSM algorithm. The model comprised 2,737 cylinders representing the trunk and branches, with a total reconstructed wood volume of 74.9018 m³.
Automated Pruning Simulation: An automated pruning procedure was developed to operate directly on the QSM, constrained by three principles derived from structural safety literature (Mattheck et al., 2015):
- Conical Crown Angle: Retaining branches within a defined conical shape (tested at half-angles of 45°, 50°, 55°, and 60°).
- Uniform Branch Distribution: Balancing wood volume across four quadrants (NE, NW, SE, SW) to prevent directional imbalance.
- Branch Inclination Angle: Removing branches with an inclination greater than 45° relative to the main trunk to mitigate falling risks.
Six pruning scenarios were generated (Configurations A, B, A-1, A-2, B-1, B-2) with wood volume retention levels ranging from 80% to 93.1%. A conservative pruning limit of 20% (retaining at least 80% of wood volume) was applied to align with tree health guidelines.
Foliage Reconstruction and Radiation Simulation: Leaf points associated with retained cylinders were preserved and converted into 0.1 m cubic volumetric elements to approximate canopy opacity. Solar radiation simulations were conducted using Grasshopper and the Ladybug plugin for Rhinoceros. The simulation utilized Tainan's Typical Meteorological Year (TMYx) data for the summer solstice (June 21, 12:00–13:00), calculating cumulative incident radiation over a 50-meter diameter circular domain centered on the trunk.
Thermal Comfort Quantification: Simulated global radiation (GR) values were converted to Physiological Equivalent Temperature (PET) using a locally calibrated simplified model ( $PET = 0.0126GR + 1.2537T_a - 2.5537V - 1.6374$ ). The change in thermal comfort ( $\Delta PET$ ) was calculated relative to an open-sky baseline.

Key Results
The study quantified the relationship between pruning intensity, retained canopy volume, and thermal comfort reduction:

Baseline Performance: The unpruned tree (100% retention) reduced the domain-averaged global radiation to 528.7 W m⁻², resulting in a $\Delta PET$ of −4.1 °C compared to open sky conditions.
Moderate Pruning: Configurations A (55° cone, 93.1% retention) and B (50° cone, 88.5% retention) showed reduced cooling benefits, with $\Delta PET$ values of −3.4 °C and −3.1 °C, respectively.
Intensive Pruning: Further pruning to balance branch distribution (Configurations A-1, B-1 at 85% retention; A-2, B-2 at 80% retention) resulted in higher radiation levels (approx. 637–671 W m⁻²) and lower cooling effects. The most intensive pruning scenarios (80% retention) yielded a $\Delta PET$ of −2.3 °C.
Thermal Comfort Thresholds: The study notes that a 4 °C reduction in PET corresponds to an improvement of one thermal comfort level. The most intensive pruning scenarios (80% retention) resulted in a cooling benefit nearly 1.8 °C less than the original tree, approaching the threshold where shading quality may be considered insufficient for significant microclimate improvement.

Significance and Contributions
The primary contribution of this work is the development of an automated 3D pruning workflow that integrates TLS and QSM with solar radiation simulation. This approach allows for a detailed, spatially explicit assessment of how specific pruning interventions alter the thermal environment beneath urban tree canopies, moving beyond the uniform estimates common in previous studies.

The study demonstrates that while moderate pruning can be tolerated without critically compromising pedestrian thermal comfort, excessive removal of canopy volume significantly diminishes cooling benefits. The authors conclude that pruning decisions should prioritize structural safety and tree health first, but the workflow provides a quantitative basis for subsequently assessing the trade-offs between these safety needs and the preservation of thermal comfort. By bridging pruning principles with cooling benefits, the proposed method supports more informed and balanced urban tree management decisions.

Limitations
The authors acknowledge several limitations: the study focuses on a single tree species (Samanea saman), requiring species-specific calibration for other forms; the simulation covers only a single critical hour (summer solstice noon) rather than full diurnal or seasonal cycles; the radiation model does not account for longwave radiation, heat storage, or surface reflectivity; and the foliage reconstruction uses simplified cubic units rather than detailed leaf geometry. Despite these constraints, the workflow establishes a viable method for integrating thermal comfort metrics into urban tree pruning protocols.

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