Clear-cutting triggers regeneration of abandoned secondary forests but risks alternative successional trajectories with high deer density

Questions Does clear-cutting intended to boost regeneration of abandoned secondary forests lead to an irreversible shift to non-forest vegetation under high deer density? Location Warm-temperate forests in Boso Peninsula, Chiba Prefecture, Japan. Methods We designed a factorial experiment including two levels of forest canopy treatment (clear-cutting or intact canopy) and two levels of herbivory control (presence or absence of deer) using 24 plots of 10 m x 10 m. Vegetation dynamics occurring under each experimental treatment were tracked for 7.5 years. Results Ground vegetation under the closed canopy did not change significantly for 7.5 years both inside and outside the deer fences. However, plots were rapidly covered by early-seral plants till 1.5 years after the cutting event. The quick growth of ground vegetation after clear-cutting was supported by plants that are known to have soil seed banks, as suggested by their ubiquity immediately after clear-cutting. Species richness of ground vegetation was maximized at that time and gradually decreased afterward, suggesting biological filtering. This filtering process worked differently inside and outside the deer fences, resulting in different types of vegetation. The clear-cut-and-fence plots became dominated by tree recruits that will recover secondary forests in the future. Contrastingly, vegetation outside the fences lacked many species (e.g., common tree species) that were found inside the fences and became dominated by shrubs and plants unpalatable to deer. This post-cutting vegetation outside the fences was not similar to that inside the deer fences at any seral stage. This means that the vegetation outside the fences could be tracking an alternative successional trajectory rather than a case of arrested succession. Conclusions Here, abandoned secondary forests still seem to maintain a high regeneration potential that would be triggered by clear-cutting, but reducing herbivory pressure by deer is necessary to avoid the risk of alternative successional trajectories.