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 cm(-1) per month, respectively, compared to 1.15 +/- 0.15 and 1.28 +/- 0.09 mm cm(-1) per month in the controls. Annual internode production declined significantly with burial depth in Avicennnia, whereas, it increased in Sonneratia. Although seedling survival of Rhizophora was not significantly affected by different sediment levels, the overall survival of this species was much lower than that of Sonneratia. The results reveal that Sonneratia will be better suited for colonizing or being planted in areas potentially subject to abrupt high sedimentation. (C) 2002 Elsevier Science B.V. All rights reserved.