行道樹風險評估與管理：整合天氣與事故數據分析

Abstract

隨著全球都市化程度快速提升與氣候變遷加劇，極端氣候事件的頻率與強度日益增加，都市行道樹的風險管理逐漸成為重要且迫切的研究課題。行道樹不僅美化市容景觀，也具備改善都市微氣候、降低都市熱島效應、提升空氣品質以及提供生態服務等重要功能。然而，在強風、暴雨、颱風等極端天候事件下，行道樹倒伏或斷枝事故頻繁發生，對公共安全及交通造成重大影響，甚至直接危及民眾的生命財產安全。 本研究以臺北市大安區、信義區與文山區三個行政區為研究範圍，透過臺北市1999市民通報系統歷年來的517筆行道樹事故數據進行統計分析與風險辨識。研究結果顯示，榕樹、茄苳、白千層與盾柱木等特定樹種的事故發生頻率與嚴重程度顯著高於其他樹種，因而將其明確定義為高風險樹種。 在氣象因子的深入分析中，本研究發現當最大瞬間風速達到9.5 m/s（相當於蒲福風力第五級）時，即為行道樹風險管理的第一階段預警指標，此時樹木可能已開始出現初步的結構受力或枝條搖動現象，需要提高警覺並提前採取預防性措施。當最大瞬間風速超過12.5 m/s（相當於蒲福風力第六級）或單日累積雨量超過70 mm時，行道樹的倒伏與斷枝風險將顯著升高。此外，三日累積雨量達150 mm以上或4小時內最大降雨量超過30 mm時，同樣會對樹木的根系穩定性產生負面影響，提升事故發生機率。本研究進一步觀察到事故的季節性變化，尤其在夏季7月與8月颱風及豪雨頻繁的月份中，事故數量與損害嚴重程度皆顯著增加，顯示季節性氣象變動對行道樹風險影響的重要性，都市林管理需在特定季節加強風險防範措施。 針對目前臺北市公園處所使用的樹木安全評估表，本研究透過專家訪談、資深巡查人員的實務經驗整理以及參考國際評估標準（如ISA TRAQ及QTRA），發現現行評估系統存在評估項目尚有些許優化空間，如：氣象因子考量不足、樹種特性未充分區分、根盤穩定性評估項目過於粗略等問題。 為改善上述不足，本研究提出了一套經過細緻設計與優化的新版樹木風險評估表。新評估表中特別增設了「高風險樹種提示」欄位與「氣象風險指標」（例如風速與雨量的閾值），以強化預警功能。新增的評估項目還包括根盤土壤流失狀況、枝下高度、樹冠密度與通透性、土壤質地等，並調整樹高與胸徑比的判定標準，同時簡化樹皮缺損程度的評估方法，透過更清晰直觀的評估指標，增強巡查人員實務操作的便利性與準確性。 實務驗證方面，本研究選取了臺北市三個行政區具代表性的40株行道樹，進行新舊評估表的比較分析。結果顯示，新版評估表在樹木風險等級的區分能力上更為精細，尤其在界定臨界值附近的樹木風險狀態時有明顯提升，避免過去因主觀判斷而產生的評估誤差。此外，參與實務驗證的巡查人員一致反饋指出，新版評估表更加符合實務需求，操作更為便捷且指標定義清晰明確，有效提升了評估作業效率與準確性。 綜合而言，本研究所提出的優化版樹木風險評估表，在評估結果上相對能夠更客觀地反映樹木真實風險狀態，為管理單位制定有效的預防措施與應變策略提供具體且實用的參考依據。本研究成果可有效提升都市行道樹的風險管理成效，確保都市公共安全，並在氣候變遷的背景下，提供都市樹木管理更加完善的科學依據與決策支持。

With the rapid increase in global urbanization and intensification of climate change, the frequency and intensity of extreme weather events are rising, making urban street tree risk management an increasingly critical research topic. Street trees not only beautify urban landscapes but also provide important ecological services such as improving urban microclimates, reducing urban heat islands, and enhancing air quality. However, extreme weather conditions such as strong winds, heavy rainfall, and typhoons frequently cause tree failure and branch breakage, significantly impacting public safety and transportation, and directly endangering residents' lives and property.

This study focused on three administrative districts in Taipei City—Da’an, Xinyi, and Wenshan—and conducted statistical analysis and risk identification based on 517 street tree incident records obtained from the Taipei City 1999 citizen hotline system. The results revealed certain tree species, such as Ficus spp., Bischofia javanica, Melaleuca leucadendra, and Polyalthia longifolia, had significantly higher incident frequencies and severity compared to others, clearly identifying them as high-risk species.

Through analysis of meteorological factors, the study identified an initial warning threshold at a maximum instantaneous wind speed of 9.5 m/s (Beaufort scale level 5). At this level, trees may exhibit preliminary signs of structural stress or branch movement, indicating the need for heightened vigilance and preventive measures. The risk of tree failure and severe branch breakage significantly increases when wind speeds exceed 12.5 m/s (Beaufort scale level 6) or daily cumulative rainfall exceeds 70 mm. Cumulative rainfall over three days reaching 150 mm or a maximum rainfall intensity exceeding 30 mm within 4 hours also negatively affects root stability, increasing incident likelihood. Seasonal variations were evident, particularly during July and August, characterized by frequent typhoons and heavy rainfall, resulting in significant increases in incidents' number and severity. These findings underscore the critical importance of seasonal meteorological variations in urban tree risk management, necessitating enhanced risk mitigation measures during specific seasons.

Regarding the current tree safety assessment form used by Taipei City’s Parks Department, this study identified several areas for improvement through expert interviews, compilation of practical experiences from senior inspectors, and references to international standards such as ISA TRAQ and QTRA. These areas included inadequate consideration of meteorological factors, insufficient differentiation of tree species characteristics, and overly simplistic root stability assessment methods.

To address these deficiencies, the study developed an optimized tree risk assessment form. The new form includes specific additions such as a "High-Risk Species Alert" and "Meteorological Risk Indicators" (e.g., wind speed and rainfall thresholds) to enhance early warning capabilities. Additional criteria include soil erosion around roots, branch clearance height, crown density and transparency, and soil texture. The form also adjusts the tree height-to-diameter ratio assessment standard and simplifies the evaluation method for bark damage, providing clearer indicators to improve practical operability and accuracy for inspectors.

For practical validation, this study selected 40 representative street trees from the three administrative districts of Taipei City to compare evaluations made using old and new forms. Results showed that the new assessment form significantly improved differentiation of tree risk levels, especially near critical thresholds, reducing subjective judgment errors. Field inspectors consistently reported the new form better met practical requirements, was easier to operate, and provided clearer, well-defined indicators, significantly enhancing evaluation efficiency and accuracy.

Overall, the optimized tree risk assessment form provides a more objective reflection of actual tree risk conditions, offering practical reference information for authorities to develop effective preventive measures and emergency response strategies. This study effectively enhances urban street tree risk management, ensuring public safety, and providing comprehensive scientific support and decision-making tools in the context of climate change.