Aster kantoensis Kitam., an endangered flood plain endemic plant in Japan: Its ability to form persistent soil seed banks Original Research Article

We examined dormancy/germination traits of the seeds of Aster kantoensis, an endangered local endemic on the gravelly flood plains of a few eastern central Japanese rivers, to assess its ability to form persistent soil seed banks in its natural habitats. Immediately after the seed collection from the largest remaining population of the River Tama in early winter, the seeds showed a low level of relative dormancy, which disappeared within a month under the prevailing conditions in the natural habitat. After a few monthsʹ dry chilling storage, the seeds became germinable over a wide temperature range, i.e. 6–36°C. The seeds showed neither requirement for temperature fluctuation nor sensitivity to leaf canopy light. The thermal time required for 50% germination at temperatures below 30°C was less than 70 Kd (Kelvin days calculated with a base temperature of 5°C). Since such requirements for germination of seeds naturally dispersed and overwintered will be satisfied by a spell of spring rain, we cannot expect the formation of persistent soil seed banks. Correspondingly, after spring germination, no or negligible firm intact seeds could be detected in the surface soils sampled. Safe-sites for the recruitment of the species are known to be new areas created by fluvial disturbance, which may occur sporadically in space and time. Without persistent soil seed banks, successful dispersal of the seeds to these safe-sites is of crucial importance for the persistence of the metapopulation composed of ephemeral subpopulations. Recent decrease in the colonization rate of new subpopulations due to diminishing seed sources might accelerate the decline of the metapopulation. To break the extinction vortex in which the species has been involved, we propose human-aided seed dispersal to safe-sites in order to strengthen the conservation of this threatened species.