Reasons for the Extremely Small Population of putative hybrid <i>Sonneratia × hainanensis</i> W.C. Ko (Lythraceae)

Sonneratia &times; hainanensis, a species once endemic to Hainan Island in China, is now endangered. China&rsquo;s State Forestry Administration lists this species as a wild plant species with an extremely small population. Field fixed-point investigations, artificial pollination, and laboratory experiments, as well as other methods, were applied to study the reproductive system and seed germination of S. &times; hainanensis to elucidate the reasons for the endangerment of this species. The results are as follows: (1) Outcrossing index, pollen-ovule ratio, and artificial pollination showed S. &times; hainanensis has a mixed mating system and mainly focuses on outcrossing with some self-compatibility. (2) Fruit and seed placement tests showed that the fruit predators on the ground were mainly Fiddler crab and squirrel, with the predation rates being 100%. The artificially spread seeds do not germinate under natural conditions. The mean seed destruction rate and remaining rate of were 82.5% and 17.5%. (3) Seeds need to germinate under ambient light conditions, with an optimal photoperiod of 12 h. Seed germination is extremely sensitive to low temperatures because of optimum temperatures from 30 &deg;C to 40 &deg;C. At an optimal temperature of 35 &deg;C, the seeds germinate under salinities ranging from 0&permil; to 7.5&permil;, with an optimal salinity of 2.5&permil;, which shows the sensitivity of seed germination to salinity, with low salinity promoting germination, whereas high salinity inhibits germination. These findings indicate that the limited regeneration of S. &times; hainanensis is caused by the following: (1) Pollen limitation and inbreeding recession caused by the extremely small population of S. &times; hainanensis. (2) Seeds near parent trees are susceptible not only to high fruit drop rate, but to high predation beneath the parent trees&prime; canopy as well. (3) Seed germination has weak adaptability to light, temperature, and salinity.