Mangrove colonization: Mangrove progression over the growing Pak Phanang (SE Thailand) mud flat

A combination of remote sensing techniques and in situ measurements along a chronosequence was used to elucidate the rate of progression of the mangrove forest in the Pak Phanang Bay (SE Thailand), a large bay with an extended and rapidly accreting mud flat. The examination of black and white aerial photographs of the forest in 1966, 1974, 1989 and 1995, and satellite images in 1985, 1990 and 1994 revealed that the mangrove forest located in the eastern bank of the bay was progressing over the mud flat. The rate of progression was estimated, from examination of changes in the position of the forest edge with time in the series of images, to average 38.6 m year^-1 over the 28-year interval encompassed by the images. Mangrove progression rates were fastest between 1966 and 1974 and slowest between 1974 and 1985, remaining uniform at about 30 m year^-1 thereafter. The in situ examination of vegetation along transects in the area of fastest mangrove progression showed an average progression rate of 53.12 ± 5.86 m year^-1, quite similar to the estimate (48.4 m year^-1) derived from remote sensing techniques for the area where the transects were surveyed. Avicennia alba was found to dominate the vegetation at the progressing edge of the mangrove, followed by Sonneratia caseolaris, with Rhizophora apiculata being present only occasionally. The fast colonization of A. alba over the mud flat was supported by a large export flux of mangrove propagules from the channels draining the mangrove forest. Which averaged 3715 ± 920 and 1900 ± 808 fruits day^-1 in each of the channels examined. Extrapolation of the long-term mean mangrove progression rate observed along the eastern bank of the Pak Phanang Bay suggested that this mangrove forest will increase by 33 ha year^-1. These results provide evidence that natural mangrove colonization can be a rapid process if sufficient propagules of the pioneer species (A. alba and S. caseolaris) are available, and point, therefore, to alternative management plans to promote and enhance natural colonization processes.