Initial effects of thinning and concomitant disturbance on the understory woody community in Japanese cedar plantation

Restoring planted forests to more natural forests has become an important goal of recent forest management strategies. Thinning is an effective method of restoration that improves understory light conditions and encourages the growth and establishment of future canopy tree species. However, ambiguous results have been reported depending on location and circumstances. In this study, we examined the trend in the initial response after thinning across a region to consider understory disturbance accompanied by thinning and overgrowth of shrub species that prevent the growth of tree species. Field surveys were implemented in 49, 48, and 31 Japanese cedar plantations at different times before and after thinning (before thinning and at 2 and 5 years after thinning). Compared to before thinning, understory hardwood abundance was significantly lower at 2 years after thinning, whereas it was higher at 5 years. The increase in the abundance of the understory community was conspicuous for tree species with a rapid regeneration strategy after disturbance. These results suggest that understory disturbance resets the understory community structure by increasing light levels, thereby allowing the establishment of pioneer tree species. However, the abundance of understory hardwoods after thinning was correlated with that before thinning. Thus, we suggest that the possibility of colonization of tree species are largely affected by the abiotic or biotic environmental conditions with no direct relationship to the present thinning treatment (i.e. is the factor that remains essentially intact even after thinning).