To evaluate the establishment and early growth of the mangrove species Kandelia candel in the intertidal zone, and to develop a better understanding of biotic and abiotic factors influencing the regeneration of its seedlings, we conducted a field experiment in Zhangjiang Estuary in Fujian. Different positions along the intertidal gradient were selected from 20 m to 120 m horizontal distance down the shore, including eight sampling sites in the mangrove areas. Equal numbers (75) numbers of mature propagules of K. candel were planted in each sampling site. The fates of propagules and growth of seedlings were monitored for 3 years.
The rates of rooting of K. candel propagule varied spatially. The lowest rates occurred in sites with an Avicennia marina-dominated overstory (69.7%). The rates were higher in sampling sites with a K. candel-dominated overstory (90.0%), at the fringe of the mangrove forest (89.3%), and on the bare tidal flat outside the mangrove forest (82.7%). After 1 year, the survival rates of seedlings planted under A. marina forest, K. candel forest, at the fringe of the mangrove forest, and the bare tidal flat were 13.7%, 54.7%, 76.0%, and 34.7%, respectively. Among the surviving K. candel seedlings, those at the fringe of the mangrove forest and on the bare tidal flat had greater height, stem diameter, leaf number, leaf area, and biomass than did those under the A. marina and K. candel forests. In general, our experiment demonstrated that establishment and growth of K. candel seedlings occurred successfully at the fringe of the mangrove forest but did worst under the A. marina forest.
We analyzed the factors which translate K. candel seedlings’ performance into significant differences in terms of establishment and early growth among sites. The performance of K. candel seedlings was not correlated with sediment texture, pH, salinity, organic matter, total N, and total P among the sites. However, interspecies competition, propagule predation by insects and crabs, and the incident light had significant effects on seedling survival and growth.
At the early growth stage, the probability of establishment of K. candel propagules planted in the intertidal sediments was influenced by predators and tidal disturbance. Under the A. marina forest, the compact root system of A. marina prevents K. candel propagules from rooting so that the propagules tend to be carried away by tidal currents. Insects and herbivorous crabs can play a considerable role in the predation of mangrove propagules and possibly are a threat to the regeneration of mangroves. We found that rates of insect and crab predation were higher in the intertidal location under intact canopies than at the fringe of the mangrove forest and on the bare tidal flat.
Long-term survival of seedlings and their development into saplings depend on light availability. Analyses showed that correlation between growth parameters of one year old K. candel seedlings and light intensity was significant. Shade reduced seedling growth in the field. Only those seedlings at the fringe of the mangrove forest and bare tidal flat established successfully and grew to maturity. These sites afford better growth conditions than the surrounding understory and, as importantly, provide a refuge from predation by insects and crabs.
Our results also indicate that the rapid growth of Spartina alterniflora reduces the regeneration of K. candel. As a competitive plant to K. candel in the mid intertidal zone, S. alterniflora may be having a large impact on the mangrove composition of our study forests. It is necessary to search for ways to protect this reserve area of mangrove wetland