Abstract:

Due to its rapid growth,Sonneratia caseolaris, a mangrove species indigenous to Hainan, was introduced to Shenzhen Bay, Guangdong for afforestation purpose during the early 1990s. The seedling emergence, early growth and dispersal pattern of S. caseolaris and their responses to environmental factors have not been well studied in the new habitat. In this study, we evaluated the density, height and coverage of S. caseolaris seedlings underneath the canopies of various mangrove forests (including both natural Kandelia candel and Avicennia marina communities and introduced S. caseolariscommunities) and on the mudflats without canopy, in Futian Mangroves Natural Reserve of Shenzhen Bay from September 2006 to September 2007. Line intercept and square intercept methods were used in the survey. Tidal elevation, light intensity, community types and the distance between the sample squares and the nearest adult S. caseolaris were also recorded. The mean densities of S. caseolaris seedlings under the canopies of both the introduced and natural mangrove forests decreased from September 2006 (24.7 seedlings per m²and 19.7 seedlings per m², respectively) to September 2007 (no seedlings survived). No significant differences were found in the seedling density, height or coverage of S. caseolaris among different mangrove communities. However, the density, height and coverage of S. caseolaris seedlings were significantly higher on the mudflats without canopy than under the mangrove canopies, indicating that higher light intensity in on the mudflats without canopy promoted the dispersal and vertical growth of S. caseolaris seedlings. Although the optimal tidal elevation for S. caseolaris seedlings in Shenzhen Bay was between 1.40 m and 1.60 m, an area that falls within the mid-to-high intertidal zones, seedling density and tidal elevation were weakly correlated. The seedling density under the native mangrove canopies was negatively related to dispersal distance. However seedling density were positively correlated with light intensity(P<0.05), and the correlation coefficients for the introducedS. caseolaris forest increased through time with successive surveys. In contrast, there was no significant correlation founded between seedling density and light intensity under native mangrove canopies. We concluded that distance to mother tree was the most important factor determining S. caseolaris seedling density under native mangrove canopies, whereas the light intensity was the most important environmental factor for controlling seedling dispersal pattern under the canopy of the introduced S. caseolaris forest.

Key words: mangroves, Sonneratia caseolaris, seedling dispersal, light intensity, elevation, dispersal distance