Twenty-one years of stand dynamics in a 33-year-old urban forest restoration site at Kobe Municipal Sports Park, Japan

To integrate human-disturbed hillslopes with the regional landscape, natural forest restoration has become an important objective of hillslope re-vegetation in Japan. At Kobe Municipal Sports Park (KMSP), seedlings of native species were planted in 1980 to restore semi-natural secondary forest (satoyama) in an urban setting. Here, we present 21 years of stand dynamics based on vegetation surveys conducted in 1992, 2000, and 2013 in two research plots (control and managed) at KMSP in relation to a reference forest to evaluate management effects and restoration success. Total basal area continued to increase in both the plots, but diameter-growth decreased in the control plot, whereas it continued to increase in the managed plot, which had been thinned by volunteers. In the control plot, which was planted at higher initial density than the managed plot, Quercus phillyraeoides (evergreen, mid-canopy tree) dominated the single-layered canopy and vertical development was delayed. In the managed plot, Quercus serrata (deciduous, canopy tree) dominated the upper canopy layer and evergreen broadleaved trees dominated the mid- to lower-canopy layers, resulting in a vertically well-developed canopy similar to the reference forest. The basal area of Robinia pseudoacacia decreased due to shading by evergreen trees, whereas that of Nerium oleander, an exotic species, had increased in the control plot. Ordination results indicated that vegetation of the control plot was diverging away from the reference forest, whereas thinning had directed the managed plot toward it. Our results confirm that simultaneously planting seedlings of native species does not lead to natural forest stand structure. In the future, adaptive management, such as periodic thinning, removal of shade-tolerant, exotic species and enrichment planting of native species, will be needed to integrate forest restoration sites with the surrounding mid-successional, secondary forest.