Factors controlling germination and seedling growth of an endangered Saussurea species (Asteraceae) endemic to serpentine areas in Japan

Saussurea ochiaiana Kadota (Asteraceae) is a perennial herb endemic to serpentine areas in western Japan. To date, only two locations are known habitats for this species of unknown ecological requirements. Therefore, this study aimed to clarify the environmental factors that limit the germination and seedling growth of this species, which is an essential requirement for designing a suitable conservation strategy. Our germination experiments, in which the achenes were subjected to a gradually increasing temperature and a gradually decreasing temperature after short (1 month) or long (4 months) storage, indicated that the achenes required a long cold stratification for germination. In an experiment where plants collected at the study site were potted individually, they did not grow under strongly shaded conditions consisting of only 10% relative photosynthetic photon flux density (RPPFD), but grew well under moderately shaded conditions (50% RPPFD). No significant differences in seedling growth were observed among the three pH conditions tested (pH 4, 6, and 8). Although average seedling shoot weight was significantly larger at low Ca:Mg molar ratios (0.6 and 0.3) than at high substrate Ca:Mg molar ratios (15 and 1.5) in pot culture experiments, the difference between treatments was not very large (<20%). Habitat conditions suitable for the regeneration of this species are decreasing because of climate change and the depopulation of rural communities (reduction in management); therefore, urgent conservation strategies, including ex situ conservation, are needed.