Mangroves mitigate tsunami damage: A further response

This is a contribution to the discussion on the potential mitigating effect of mangroves to tsunami damage. Kathiresan and Rajendran (2005) were criticised by Kerr et al. (2006). We re-analysed of the original data with an ANOVA-model with covariates. We conclude: (a) the original conclusion of Kathiresan and Rajendran (2005) holds, mortality and property loss were less behind mangroves, and literature suggests that this can be generalised beyond the investigated area; (b) relocation of human settlements 1 km inland is not practical, instead a combination of societal preparedness with early warning and disaster response systems is to be preferred. Furthermore, we deduce that mortality was most strongly, and significantly reduced with increasing elevation above mean sea level, whereas property loss was governed by distance to the shore. This could improve coastal risk assessments. © 2006 Elsevier Ltd. All rights reserved

The effect of increasing sediment accretion on the seedlings of three common Thai mangrove species

Three to four-month-old seedlings of three common Thai mangrove species (Avicennia officinalis L., Rhizophora mucronata Lamk and Sonneratia caseolaris (L.) Engler) were experimentally buried using six sediment accretion levels (0, 4, 8, 16, 24 and 32 cm) in a randomized block design. Avicennia was five-fold more sensitive to burial than Sonneratia and the seedlings of the latter species exhibited the lowest mortality as well as the highest growth rate. The numbers of surviving seedlings of these two species were highly affected by burial (P < 0.001) and their survival decreased with increasing sediment accretions. The seedlings receiving 32 cm of sediment had the highest mortality (100% in Avicennia, 70% in Rhizophora and 40% in Sonneratia). Survival of Rhizophora, however, was not significantly different amongst treatments (P = 0.23). Natural mortality in the control seedlings was substantial in Avicennia and Rhizophora (10 and 40%, respectively). Burial had significant effects on seedling height in Avicennia and Sonneratia only (P < 0.05). The relative growth rate in terms of height was lowest in the 32 cm treatment in both species: 0.30 ± 0.19 and 1.20 ± 0.11 mm c

Coastal erosion and mangrove progradation of Southern Thailand

Approximately 60% of the southern Thai coastline used to be occupied by mangroves according to the first mangrove forest assessment in 1961. During the past three decades, these mangrove areas have been reduced to about 50% with less than 10% left on the east coast. Coastal erosion and accretion occur irregularly along the coast but an intensification of erosion has been noticed during the past decade. This study assessed the relationship between mangrove presence and changes in coastal area. Mangrove colonization rates were assessed using in situ transects and remote sensing time series. Both methods led to comparable estimates ranging between 5 and 40 m

Relationship between sediment conditions and mangrove Rhizophora apiculata seedling growth and nutrient status

7 pages, 4 figures.The growth rate and nutritional status of Rhizophora apiculata seedlings were analyzed across mangrove stands with different sediment composition in The Philippines and Southern Thailand. Plant growth differed 10-fold and the production of new leaves, roots and branches varied between 50- and 100-fold across sites. Most (>60%) of the variance in mangrove growth rate across systems could be accounted for by differences in the nutrient concentration of the leaves, which was in turn related to the interstitial nutrient concentration and the silt plus clay content of the sediments. Nutrient-poor coarse sediments were characteristic of mangroves located in the mouths of rivers draining small watersheds, while sediments at the mouths of large rivers had high silt, clay, and nutrient contents, thus allowing the development of nutrient-sufficient, fast-growing R. apiculata seedlings. The growth of R. apiculata seedlings increased significantly when the plants grew adjacent to rivers draining areas >10 km2. The results provide evidence that growth of R. apiculata seedlings at the edge of the progressing mangrove forests is often nutrient limited, and that the extent of nutrient limitation depends on the delivery of silt and nutrients from the rivers. The coastal zones adjacent to small (<10 km2) drainage areas seem unsuitable to support adequate growth of R. apiculata seedlings, and afforestation programmes should, therefore, target mud flats adjacent to large rivers instead.This research was funded by the STD-111 programme of the Commission of the European Union (project TS3'-CT94-0301). We thank N Agawin for assistance in the field. This is a contribution to the LOICZ program (project no 27), a core project of the IGBP program.Peer reviewe