Scarification under the canopy of Japanese oak: should pre-harvest treatment coincide with the masting year of acorns?

The regeneration of Japanese oak (Quercus crispula Blume) has turned even more pressing in recent years. The pre-harvest treatment, which accumulates advanced seedlings, is essential for its natural regeneration in stands with a dense dwarf bamboo cover. Such pre-harvest treatment is considered particularly effective when done in conjunction with a mast year of acorns, but it is difficult to adjust the operation to specific timing. If there is no mast year for several years after the pre-harvest treatment, there is concern that the "priority effect" from other plant species may inhibit Japanese oak regeneration. The objective of this study was to determine whether scarification under the canopy of Japanese oaks, treated several years before the masting of acorns, is effective for its regeneration. Seedlings and vegetation were surveyed at sites scarified four years before (PRE) and just before masting (MAST) to compare the effects of treatments, i.e. differences in scarification years on the number and/or size of tall trees and other plant species. PRE could limit the predominance of dwarf bamboos and birches, and sufficient number of seedlings (>10 stems m−2) for future development was observed in the fifth year after masting. Since Japanese oak generally has a masting cycle of less than 5 years at most, we can conclude that the timing of pre-harvest treatment is not constrained by the masting year. Further studies are required to determine the details of priority effects of competitive plants on oak seedlings to establish a natural forest management for Japanese oak.