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We aimed at clarifying an actual state about an accident that a branch or a trunk of a roadside tree fell and hit a victim directly on the Tokyo prefectural road. We investigated and analyzed the data in connection with a roadside tree in Tokyo Prefecture. As the result, it became clear that the kind of trees with many numbers does not necessarily have many accidents. And it was suggested that there are some types of the accident which is easy to cause according to the kind of trees. Especially about a Zelkova Tree (Zelkova serrata), there were many accidents by branch breakage. And the accidents tended to occur frequently in the same route. It was suggested that there is relationship between that accidents by branch breakage occur and that Zelkova Trees became large through years. On the other hand, about a Japanese Pagoda Tree (Styphonolobium japonicum) etc., there were many accidents by collapsing. And it was suggested that decomposition and a cave have a close relation to this kind of accident.

明治神宮表参道の帯状植枡に植栽されたケヤキ大径木の街路樹167本について, その生長が歩道の与える影響を調査した。そのうち標本木5本について歩道に平行および直行する土壌断面においてトレンチ枠法で根系調査を行い, 根系分布と歩道との関係を追求した。その結果次のことがわかった。(1) 根株から50cmの範囲内にある縁石と歩道は根系生長による影響を受けやすい。(2) 胸高周囲が120cm以上の大径木になると歩道の縁石に乱れが著しく増加した。(3) トレンチ断面の根系分布数は大径木ほど表層に多く, 歩道はその間隙に侵入した根系の肥大生長によって盛り上がる。歩道下の砕石層は厚いほど根系による路面への影響が少ない。