Growth and population dynamics during early stages of the mangrove Kandelia candel in Halong Bay, North Viet Nam

Abstract

Quantifying the dynamics of the early stages in the life cycle of mangroves is essential to predict the distribution, species composition and structure of mangrove forests, and their maintenance and recovery from perturbations. The growth and population dynamics of two stands of the mangrove Kandelia candel in Halong Bay (Viet Nam) were examined for 1 year. Growth was highly seasonal, with high growth rates and fast internode formation in the summer, dropping to extremely low growth during January–February, the coldest and driest months in the year. In addition, growth and internode formation rates showed important inter-annual variability during the last decade. The complete reproductive period required 7–8 months. Flower initiation was maximal in June and peak propagule maturity occurred in December–January. Only one mature propagule developed for every 67 and 127 inflorescence buds formed at Site 1 and Site 2, respectively. Kandelia candel propagules begun to sink 10 days after being released, and after 18 days all propagules had negative buoyancy. The propagules developed roots within 19–68 days, depending on whether they were held on the water or sediment, and were capable of long range dispersal, for 15–20% of them dispersed more than 100 m within 1 day. The median age of K. candel plants ranged between 8.7 and 5.6 years, with a density of 1900 and 470 plants ha −1, in Sites 1 and 2. Plant mortality was high, with 64 and 74% of the plants surviving after a year at Sites 1 and 2. Life expectancy (i.e. median age-at-death) of only 2.2 and 2.7 years at Sites 1 and 2, respectively, indicates that mortality of young K. candel plants was specially high. Recruitment occurred in early spring, and did not suffice to balance the mortality within the annual period examined. These results suggest that the K. candel stands in Halong Bay might be maintained by a few years of high recruitment which would compensate for generally high mortality rates.