Estuarine, Coastal and Shelf Science (1998) 47, 51–61 Article No. ec980343 Mangrove Colonization: Mangrove Progression Over the Growing Pak Phanang (SE Thailand) Mud Flat N. Panapitukkul a , C. M. Duarte b , U. Thampanya a , P. Kheowvongsri a , N. Srichai a , O. Geertz-Hansen c , J. Terrados b and S. Boromthanarath a a Coastal Resources Institute, Prince of Songkla University, Hat Yai, Songkhla 90112, Thailand b Centro de Estudios Avanzados de Blanes, CSIC, Camı ´ de Santa Ba ´rbara. s/n., 17300 Blanes (Girona), Spain c Freshwater Biological Laboratory, University of Copenhagen, Helsingørsgade 51, DK 3400 Hillerød, Denmark Received 25 September 1997 and accepted in revised form 3 February 1998 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 S onneratia 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 su fficient 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.  1998 Academic Press Keywords: mangroves; colonization; remote sensing; SE Asia; Thailand Introduction South-east Asian mangrove forests are declining at alarming rates, due to the increasing demand for land to be allocated to food, industrial production and urban settlements ( Go ´mez, 1988 ; World Resources Institute, 1988 ; FAO, 1989 ; Arrhenius, 1992 ). In Thailand, for instance, 50% of the mangrove forests have been lost over the past 35 years, largely to create shrimp ponds, which cover about 64% of the lost mangrove areas ( Aksornkoae, 1993 ). Changes in land use in SE Asia result in high soil erosion rates, and have yielded a major increase in transport of eroded sediments to the coastal zone. As a consequence, the sediment delivery to the SE Asian coastal zone has increased by an order of magnitude (on average 14 200 tons sediment km 2 , Milliman & Syvistski, 1992 ), now exceeding those measured elsewhere by 10–1000 times ( Milliman & Meade, 1983 ). Increased sediment delivery to SE Asian coastal areas is generating new substrate for the potential expansion of existing mangrove forests (e.g. Thom, 1984; Aksornkoae, 1993 ). This expansion cannot be adequately forecasted due to a lack of reliable knowl- edge on the rate at which it takes place (Macnae, 1968). This knowledge is critical, because mangrove recovery is now a target in the large-scale afforestation programmes supported by most of the countries. In addition, detailed examination of the dynamics of mangrove progression over extending mud flats will help validate the roles of Avicennia sp. and Sonneratia sp. as pioneer species, as has been traditionally assigned in the literature (e.g. Tomlinson, 1994 ; Lee et al ., 1996 ). The progression of mangrove forests can be addressed at different scales. The long-term rates of progression can only be resolved at the large (several km) scale, for information at smaller scales may 0272–7714/98/010051+11 $30.00/0  1998 Academic Press