Effect of Suspending Traffic on a Highway in a Mountainous Region on the Succession of Slope Vegetation

Abstract

Gonbei Highway, a winding road in a mountainous region in central Japan, was damaged by typhoons in the autumn of 2004, and a 9-km stretch of the highway has remained closed (without being restored). We established 11 survey sites to examine slope vegetation along Gonbei Highway in the summer of 2004 (just before the area was damaged), and we conducted surveys again in 2016 (12 years after the damage), including at 5 survey sites along the closed section of the road. At each survey site, a quadrat (2 m × 5 m) was established on the mountain side and the valley side of the road. The deterioration of the road surface was also observed. Here, we examine the effects of suspending traffic on roadside vegetation and road surface deterioration. Our results showed that succession to arboreal vegetation was not observed at the roadside sites along the closed section of the road: vegetation coverage increased significantly along the closed section with increasing coverage of shrubs (mainly bamboo grass). In contrast, tree seedlings increased in the passable sections. Thus, the suspension of traffic and the subsequent long-term absence of roadside vegetation management allowed bamboo grass to increase, and the community of bamboo grass prevented invasion and growth of tree seedlings. Obvious deterioration of the road surface was not observed, excepting sparse cracks and weed invasion. Consequently, the dominance of bamboo grass may be one of the criteria by which to judge the necessity of vegetation management along roads closed to traffic.