臺大校園樹木風險評估技術之探討

Abstract

樹木風險評估(tree risk assessment)為識別、分析和評估樹木風險的系統化過程。藉由評估風險等級的高低，依各風險等級之改善急迫性排定處理順序，能有效分配人力資源，做出相對應的維管頻度，以符合經濟效益。 本研究以國際樹藝協會 (international society of arboriculture, ISA)之風險評估法為基礎，彙整臺灣樹木常見風險來源，建立主要風險來源之評估參考標準 ，以及人流、車流和財損分級 之 參考 依據 ，藉此評估臺灣大學總校區 主要的六種 樹種之風險來源。六種樹種包含白千層(Melaleuca leucadendra)、 楓香 (Liquidambar formosana)、正榕 (Ficus microcarpa)、樟樹(Cinnamomum camphora)、小葉欖仁 (Terminalia mantaly)、臺灣欒樹 (Koelreuteria henryi)。 國際樹藝協會之風險評估理論將評估分為三部分，分別為(1)斷裂可能性(likelihood of failure)、(2)撞擊可能性(likelihood of impact)和(3)後果嚴重性(consequences of failure)。藉由前人文獻分析整理，建立臺灣樹木常見風險來源之斷裂可能性參考標準，分析結果顯示(1)斷裂可能性方面：等勢幹之斷裂可能性可依U型等勢幹、V型等勢幹和含內生夾皮的V型等勢幹區分。腐朽可分為外部腐朽、內部腐朽和整體腐朽，外部腐朽之斷裂可能性以腐朽部位佔樹幹周長比例為依據，區分界線為10%、30%、50%；內部腐朽空洞之斷裂可能性以樹幹留存厚度 (t)和樹幹半徑(r)之比值t/r值 為依據，區分界線為t/r值 0.45、0.3、0.2；整體腐朽空洞之斷裂可能性以腐朽空洞佔橫切面之百分比為依據，區分界線為10%、20%、30%。傾斜之斷裂可能性依傾斜角度為依據，區分界線為15°、30°，植株具樹梢回正情形則降一級判定。植株整體長徑比不佳之斷裂可能性以長徑比≥50為依據；枝條長徑比不佳之斷裂可能性以長徑比≥40為依據。樹冠不平衡之斷裂可能性以長冠幅和短冠幅之平衡程度為依據，區分界線為1:0.5、1:0.1。弱連結枝和枯枝之斷裂可能性歸類為可能(possible)。上述缺陷之最終斷裂可能性等級仍需依實際承重情形判別。(2)撞擊可能性方面：撞擊可能性之判別包含滯留率和缺陷部位斷裂方向綜合評估所得，試驗結果以人流率和單車流率加權1.4倍相加作為行人滯留率區分依據 區分界線為0.1、1.5、3 ped·m-1·min-1。汽車滯留率則以汽車流率為依據，區分界線為8.8、17.5、26.3 unit·lane-1·min-1。缺陷部位斷裂方向或植株斷裂擊中之目標區位於綠化帶者，其撞擊可能性降1-2級作為最終之撞擊可能性等級。(3)後果嚴重性方面：後果嚴重性之財損金額以新臺幣10,000、100,000、 1,000,000元為區分界線。 於臺大校園之植株調查結果顯示：校園內白千層之風險來源以具內生樹皮之V型等勢幹、枯枝和腐朽為主；楓香之風險來源以枯枝為主；正榕之風險來源以等勢幹、傾斜為主；樟樹之風險來源以 V型等勢幹、枯枝、腐朽為主；小葉欖仁之風險來源以活冠比不佳、枯枝、弱連結枝為主；臺灣欒樹之風險來源以枯枝、等勢幹、傾斜、樹冠不對稱為主。 此試驗樹種及其所具之風險值高低僅源於此次調查植株，無法一概而論至臺灣其他區域相同樹種之現況。 風險等級評分以單一缺陷最高風險等級作為植株整體風險值，試驗結果顯示白千層之風險值為 24.1，楓香風險值為 15.0，正榕風險值為 22.1，樟樹風險值為 17.9，小葉欖仁風險值為13.0，臺灣欒樹風險值為 22.1，以白千層風險值具最大值，而小葉欖仁風險值具最小值。小葉欖仁和楓香之風險值較低，僅需適當修剪枯枝和避免錯誤修剪即可減少主要風險來源，其餘四樹種之缺陷如等勢幹、傾斜和腐朽等則為長期累積之結果， 必須 藉由修剪或其他補強措施分散風險，才能消減風險。 未來可於颱風過境後調查植株風害程度，驗證此次調查所得之風險值和實際斷裂情形之相關度，驗證分級依據與評估結果是否和實際斷裂情形相關，期望藉由此研究能提升此風險評估表之實用性和客觀性。

Tree risk assessment is a systematic process for identifying, analyzing, and evaluating the risk of a tree. By evaluating the levels of risk and arranging the processing sequence according to the urgency of improvement of each risk level. Human resources can be effectively allocated, and the corresponding maintenance frequency can be made to meet the economic effect. Based on the risk assessment method of the International Society of Arboriculture (ISA), this research summarizes the common sources of risk of trees in Taiwan, establishes the evaluation reference standard for the main sources of risk, the pedestrian flow, the traffic flow and the property damage classification reference. To evaluate and find out the sources of risk of the six main tree species on the main campus of National Taiwan University. The tree species include Melaleuca leucadendra, Liquidambar formosana, Ficus microcarpa, Cinnamomum camphora, Terminalia mantaly, Koelreuteria henryi. The risk assessment of the International Association of Arboriculture is divided into three parts, namely (1) the likelihood of failure, (2) the likelihood of impact and (3) the consequences of failure. The reference standard for likelihood of failure for common sources of risk of trees in Taiwan was established from literature review. The results from the literature review show that (1) the likelihood of failure: the likelihood of failure of codominant stems can be divided into U-shaped codominant stems, V-shaped codominant stems, and V-shaped codominant stems with included bark. Decay can be divided into external decay, internal decay, and overall decay. The likelihood of failure of external decay is based on the proportion of decayed parts to the circumference of the stem, and the criteria are 10%, 30%, and 50%; The likelihood of failure of the internal decay cavity is based on the ratio t/r of the trunk thickness (t) and the tree radius (r), and the criteria are 0.45, 0.3, and 0.2; The likelihood of failure of the overall decayed cavity is based on the percentage of decayed cavity in the cross-section, and the criteria are 10%, 20%, and 30%. The likelihood of failure of the tilt tree is based on the inclination angle, and the criteria are 15°, 30°, and 45°. Downgraded 1-2 levels when the treetops reorientated. The likelihood of failure of the plant with a poor slenderness ratio is based on slenderness ratio ≥ 50; The likelihood of failure of branches with a poor slenderness ratio is based on slenderness ratio ≥ 40. The likelihood of failure of crown asymmetry is based on the ratio of asymmetry, and the criteria are 1:0.5 and 1:0.1; The likelihood of failure of weak union branches and dead branches is classified as possible; The level of likelihood of failure still needs to be judged according to the actual loading situation. (2) the likelihood of impact: the judgment of the likelihood of impact includes the occupancy rate and the failure direction of the defective part. The results are based on the final pedestrian flowrate, obtained by adding the pedestrian flowrate and the weighted 1.4 times the bicycle flow rate, and the criteria are 0.1, 1.5, 3 ped·m-1·min-1 The criteria of vehicle flowrate are 8.8, 17.5, 26.3 unit·lane-1·min-1. If the failure direction of the defective part or the target area hit by the defective part is located in the green belt, the likelihood of impact is reduced by 1-2 grades. (3) the consequences of failure: the criteria at the amount of property loss for the consequences of failure are NT$10,000, 100,000, and 1,000,000. The results from the actual investigation show: the sources of risk of Melaleuca leucadendra are mainly V-shaped codominant stems with included bark, dead branches and decay; The sources of risk of Liquidambar formosana are mainly dead branches; The sources of risk of Ficus microcarpa are mainly codominant stems and tilt stem; The sources of risk of Cinnamomum camphora are mainly V-shaped codominant stems, dead branches, and decay; The sources of risk of Terminalia mantaly are mainly poor live crown ratio, dead branches, and weak union branches; The sources of risk of Koelreuteria henryi are dead branches, codominant stems, tilt stem, and crown asymmetry. This test tree species and its value of risk are only derived from the surveyed plants, and cannot be generalized to the current status of the same tree species in other regions of Taiwan. The risk level score is based on the highest risk level of a single defect as the overall risk value of the plant. The results show that the risk value of M. leucadendra is 24.1; The risk value of L. formosana is 15.0; The risk value of F. microcarpa is 22.1; The risk value of C. camphora is 17.9; The risk value of T. mantaly is 13.0; the risk value of K. henryi is 22.1. The maximun risk value is for M. leucadendra, and the minimum risk value is for T. mantaly. The risk values of T. mantaly and L. formosana are both low, and only proper pruning of dead branches and avoidance of wrong pruning can reduce the main sources of risk. Risk from codominant stems, tilt stems, decay needs to be spread through pruning or other reinforcement measures to reduce risk. The wind damage to plants can be investigated after the typhoon passes in the future to verify the correlation between the risk value obtained in this survey and the actual failure situation. To verify whether the grading standard and the assessment results are related to the actual failure situation. It is hoped that the practicality and objectivity of this risk assessment form will be improved through this research.