Removal of Invasive Trachycarpus Fortunei at Nishinomiya Shrine, Hyogo Prefecture, Japan cost Effectiveness of Active Vegetation Ma Gement in a Shrine Forest

27 Mar Removal of Invasive Trachycarpus Fortunei at Nishinomiya Shrine, Hyogo Prefecture, Japan cost Effectiveness of Active Vegetation Ma Gement in a Shrine Forest

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Abstract

市街地の孤立林では, 数々の侵入植物の逸出・繁茂が報告されている.兵庫県西宮市の西宮神社においても, 社叢林内におけるシュロ (Trachycarpus fortunei Wendl.) の異常繁茂が確認され, 林冠木の更新を妨げている可能性が示唆された.本研究では, 西宮神社の社叢林においてシュロを全て伐倒除去する作業を行い, 繁茂量の定量化と除去前後での林床の光環境の変化を調査した.また刈り取ったシュロの搬出を造園業者に依頼し, 要した費用と繁茂量の関係を調べた.社叢林内のシュロの本数密度は649本ha-1で, 樹高1.3m以上の樹木個体数の23%を占めた.シュロの胸高断面積は1.7m2ha-1で全体の3.4%に過ぎなかったが, 葉面積指数は繁茂の著しい箇所では2.49-4.60に達していた.シュロの繁茂は道路に面した社叢の北側の林縁および駐車場に面した南西の林縁で著しく, 繁殖個体も林縁に多く見られた.特に繁茂の著しかった箇所では林床の開空度が6-11%と暗くなっていたが, 除去後には開空度が平均2.2%, 最大5.5%増加し林床が明るくなった.今後クズノキをはじめとする林冠木の更新が可能となることが期待される.伐倒除去にかかった総費用は139万円で, そのうち人件費が72.0%と最も高かった.シュロの除去に必要な費用は作業量で変化するため, 繁茂が進行するほど除去に要する費用も増加する.よって費用面からも早めの対策が重要であると言える.これまで社叢林では人の手を入れないことが一般的であったが, 自然林に近いかたちで森林を維持することを目的とする場合, 侵入種を制御する積極的な植生管理が必要であると考えられる.社叢林を含め市街地の孤立林においては, 手付かずに置いておくことが自然林に近い状態という考え方ではなく, 持ち主の管理方針に沿って適切に人手を加えていくことが必要なケースもあると考えられる.

English Abstract

Various invasive species and escaped or mentals have been reported to thrive in urban forest fragments. We found Trachycarpus fortunei Wendl. thriving in the shrine forest at Nishinomiya Shrine in southeastern Hyogo Prefecture. In this paper, we report on the complete removal of this species from the forest, the extent of the invasion, and the effect of the removal on the understory light environment. In addition, we assessed the cost of removal in relation to the amount of T. fortunei removed. T. fortunei density in the forest was 649 individuals per hectare, contributing 23% of the total number of trees taller than 1.3 m. Total basal area of T. fortunei was 1.7m2 ha-1, contributing only 3.4% of the total basal area, whereas in high-concentration areas leaf area index of T. fortunei was as high as 2.49-4.60. T. fortunei was especially concentrated in the northern edge of the forest facing a road and in the southwestern edge facing a parking lot. Reproductive individuals were also concentrated near the forest edge. In high-concentration areas, canopy openness in the understory was as low as 6-11%. After removal of T. fortunei, canopy openness increased by 2.2% on average (maximum 5.5%). The total cost of the removal of T. fortunei was 1, 390, 000 yen of which 72.0% was labor costs indicating that the cost of removal increases with the extent of the invasion. Shrine forests have traditio lly been left unma ged. However, this study showed that, active ma gement is necessary to control invasive species if the objective is to sustain near- tural forest conditions. In fragmented urban forests, lack of ma gement does not necessarily result in tural conditions. As such, human intervention may be needed to realize the ma gement objectives of the land owner.