台中市行道樹刺桐環根形成之調查

The Girdling Roots Formation of Erythrina variegate in Taichung city

環根係指樹根圍繞著樹幹呈環狀生長，當環根密貼樹幹將產生纏勒現象，樹幹基部被纏勒處因無法增粗而細縮，樹木可能於細縮處彎折倒塌，對周遭環境之人、車與建物將潛藏危機。刺桐為台灣本土喬木，常做為行道樹栽植於各地，種植於人行道植穴內，其地表根系伸展至植穴周邊硬體時，常被迫彎曲而形成環根。環根有礙樹木正常發育，因而亟待調查並有所後續處理，以助樹木後續生長。採實地調查方法，針對台中市4條栽植刺桐為行道樹之街道，調查環根形成狀況，並探討影響因素。結果發現所有調查株數之64.2%形成環根，且有許多環根已密貼樹幹、甚至徑粗高達 52cm，對刺桐生長造成威脅。且發現胸徑與環根形成比率具顯著之正線性相關，表示隨樹齡遞增、胸徑越大，環根形成比率也隨之提高；另外，植穴面積亦會影響環根形成比率。本調查結果可做為未來減緩刺桐環根危害之參考。 Girdling root refers to the roots encircling the trunk. When girdling root tightly entangles the trunk, the trunk is strangled and unable to grow thicker, which eventually leads to form notching belts on the base trunk. It is likely that the trees may bent and collapse from this and endanger people, vehicles and buildings near the falling trees. Erythrina variegata is an indigenous plant of Taiwan and is often planted alongside the street in Taiwan city. But planting the Erythrina variegata in sidewalk small cavity leads to the problem that the taproot can't stretch normally. Whose roots near the planting hole hardware, often forced to bend and form girdling roots that impede the normal development of trees. Therefore, investigation and some follow-up treatment are urgently required to help the sustainable growth of trees. The current study applies field survey methods and focuses on the Erythrina variegata planted alongside 4 streests in Taichung City to investigate the condition which leads to the formation of girdling root and the relevant factors. The results showed that 64.2% of trees had formed girdling roots and the trunk is tightly surrounded by girdling roots. Some of the girdling roots were even as thick as 52cm. This is a serious threat to the growth of the tree. It is also found that there is a significant and positive linear relationship between the diameter at breast height (DBH) and the ratio of forming a girdling root. This means that the older the age and the greater the DBH, the higher the ratio of forming a girdling root. In addition to DBH, another factor which affects the ratio of forming the girdling root is the area of planting pit. The results of this study can be used as a reference in the future to reduce the girdling root of Erythrina variegata.