Effect of gravel conditions on seedling emergence of the endangered monocarpic perennial Aster kantoensis (Compositae) on a floodplain

The short‐lived monocarpic perennial Aster kantoensis Kitamura inhabits gravelly floodplains in the Kanto district of Honshu, Japan. Seedlings of this species are often found in unembedded‐gravel habitats where gravels shade the sand surface, and are rare in embedded‐gravel habitats where the sand surface is exposed to sunlight. To examine whether gravel conditions at the ground surface directly affect the seedling emergence of A. kantoensis , seeds were experimentally sown around unembedded gravels in the unembedded‐gravel habitat and around embedded and artificially unembedded gravels in the embedded‐gravel habitat on a floodplain of the Tama River. The cumulative numbers of seedlings that emerged around the unembedded and artificially unembedded gravels were approximately fourfold and sixfold of that around the embedded gravel. Results of this study indicate that gravel conditions at the ground surface directly affect the seedling emergence of A. kantoensis and that unembedded gravel conditions created by flooding provide microhabitats suitable for seedling emergence.