No abstract

Aerial photographs and satellite images have been used to determine land cover changes during the period 1953 to 2011 in the Mui Ca Mau, Vietnam, especially in relation to changes in the mangrove area. The mangrove area declined drastically from approximately 71,345 ha in 1953 to 33,083 ha in 1992, then rose to 46,712 ha in 2011. Loss due to herbicide attacks during the Vietnam War, overexploitation, and conversion into agriculture and aquaculture encouraged by land management policies are being partially counteracted by natural regeneration and replanting, especially a gradual increase in plantations as part of integrated mangrove-shrimp farming systems. The nature of the mangrove vegetation has markedly been transformed over this period. The results are valuable for management planning to understand and improve the contribution of mangrove forests to the provision of ecosystem services and resources, local livelihood and global interest.

Propagule dispersal of four mangrove species Rhizophora mucronata, R. apiculata, Ceriops tagal and Avicennia officinalis in the Pambala–Chilaw Lagoon Complex (Sri Lanka) was studied by combining a hydrodynamic model with species-specific knowledge on propagule dispersal behaviour. Propagule transport was simulated using a finite-volume advection-diffusion model to investigate the effect of dispersal vectors (tidal flow, freshwater discharge and wind), trapping agents (retention by vegetation) and seed characteristics (buoyancy) on propagule dispersal patterns. Sensitivity analysis showed that smaller propagules, like the oval-shaped propagules of Avicennia officinalis, dispersed over larger distances and were most sensitive to changing values of retention by mangrove vegetation compared to larger, torpedo-shaped propagules of Rhizophora spp. and C. tagal. Directional propagule dispersal in this semi-enclosed lagoon with a small tidal range was strongly concentrated towards the edges of the lagoon and channels. Short distance dispersal appeared to be the main dispersal strategy for all four studied species, with most of the propagules being retained within the vegetation. Only a small proportion (max. 5%) of propagules left the lagoon through a channel connecting the lagoon with the open sea. Wind significantly influenced dispersal distance and direction once propagules entered the lagoon or adjacent channels. Implications of these findings for mangrove restoration were tested by simulating partial removal in the model of dikes around abandoned shrimp ponds to restore tidal hydrology and facilitate natural recolonisation by mangroves. The specific location of dike removal, (with respect to the vicinity of mangroves and independently suitable hydrodynamic flows), was found to significantly affect the resultant quantities and species of inflowing propagules and hence the potential effectiveness of natural regeneration. These results demonstrate the value of propagule dispersal modelling in guiding hydrological restoration efforts that aim to facilitate natural mangrove regeneration

Propagule dispersal of four mangrove species <i>Rhizophora mucronata, R. apiculata, Ceriops tagal</i> and <i>Avicennia officinalis</i> in the Pambala-Chilaw Lagoon Complex (Sri Lanka) was studied by combining a hydrodynamic model with species-specific knowledge on propagule dispersal behaviour. Propagule transport was simulated using a finite-volume advection-diffusion model to investigate the effect of dispersal vectors (tidal flow, freshwater discharge and wind), trapping agents (retention by vegetation) and seed characteristics (buoyancy) on propagule dispersal patterns. Sensitivity analysis showed that smaller propagules, like the oval-shaped propagules of <i>Avicennia officinalis</i>, dispersed over larger distances and were most sensitive to changing values of retention by mangrove vegetation compared to larger, torpedo-shaped propagules of <i>Rhizophora</i> spp. and <i>C. tagal</i>. Directional propagule dispersal in this semi-enclosed lagoon with a small tidal range was strongly concentrated towards the edges of the lagoon and channels. Short distance dispersal appeared to be the main dispersal strategy for all four studied species, with most of the propagules being retained within the vegetation. Only a small proportion (max. 5%) of propagules left the lagoon through a channel connecting the lagoon with the open sea. Wind significantly influenced dispersal distance and direction once propagules entered the lagoon or adjacent channels. Implications of these findings for mangrove restoration were tested by simulating partial removal in the model of dikes around abandoned shrimp ponds to restore tidal hydrology and facilitate natural recolonisation by mangroves. The specific location of dike removal, (with respect to the vicinity of mangroves and independently suitable hydrodynamic flows), was found to significantly affect the resultant quantities and species of inflowing of propagules and hence the potential effectiveness of natural regeneration. These results demonstrate the value of propagule dispersal modelling in guiding hydrological restoration efforts that aim to facilitate natural mangrove regeneration

No abstract

For plants to reproduce successfully, it is crucial that their seeds be adapted to survive the environmental conditions in which they disperse and establish. The buoyant dispersal units (propagules) of viviparous mangrove species seem perfect adaptations for dispersal and establishment within the mangrove environment. However, much remains unknown about the structural changes mangrove propagules undergo between abscission from the parent tree and establishment. Mature propagules of two viviparous mangrove species (Rhizophora mucronata and Ceriops tagal) were submitted to experimental conditions in order to test: (i) how substrate influences propagule mass and volume during dispersal; (ii) if stranding of propagules on solid soil triggers root development and thus establishment. Our results showed that dehydration stimulates root formation and propagule establishment and that the establishment phase is faster at lower rather than higher salinity. Furthermore, it was found that the larger propagules of R. mucronata were less vulnerable to dehydration than those of C. tagal, that their root growth started later and that, once initiated, their roots grew faster. This indicated that Rhizophora propagules are better suited for long distance dispersal than those of Ceriops and that Rhizophora has an advantage for establishment in the lower part of the intertidal zone, where inundation is more frequent and propagules need to anchor more rapidly. This study therefore points out that two co-living species of the same family have different dispersal and establishment strategies, thereby contributing to the understanding of their local and global distribution and of the species-specific dynamics within mangrove forests.

Abstract:The northernmost and most arid mangrove ecosystem of West Africa is found in Mauritania, in the Parc National du Banc d'Arguin (PNBA). The existing global and regional maps of Mauritania's mangroves have little detail, and available estimates of the mangrove area differ among studies. We assessed the use of automated Remote Sensing classification techniques to calculate the extent and map the distribution of the mangrove patches located at Cap Timiris, PNBA, using QuickBird and GeoEye imagery. It was possible to detect the northernmost contiguous mangrove patches of West Africa with an accuracy of 87%˘2% using the Maximum Likelihood algorithm. The main source of error was the low spectral difference between mangroves and other types of terrestrial vegetation, which resulted in an erroneous classification between these two types of land cover. The most reliable estimate for the mangrove area obtained in this study was 19.48˘5.54 ha in 2011. Moreover, we present a special validation procedure that enables a detailed and reliable validation of the land cover maps.