The environmental variables, crab burrows, vegetation structure and growth form in Avicennia marina, Ceriops tagal and Rhizophora mucronata forest types showing contrasting within-species growth forms were investigated from August to October 1999. Emphasis was placed on the gradient of environmental parameters (interstitial water salinity, pH, temperature and sediment textural properties) in the various forest types, differences in crab burrowing activity and differences in tree morphological growth characteristics between the shorter landward forest type and the taller seaward forest type. The morphological characteristics considered include tree height (HT), diameter D130, the height of the first successful branch (SBH), bulk crown height (BCH), crown volume (CV), diameter at the base of the stem (BD), and the number of prop roots per tree (NR) in R. mucronata. The density and the above ground height of pneumatophores in A. marina and knee roots in C. tagal were also determined. Interstitial water salinity, pH and temperature were lower (p < 0.05), in the seaward forest types of A. marina and C. tagal, while organic matter content was higher (p < 0.05) in the same forest types compared to the landward forest types. Only interstitial water salinity was significantly lower in the seaward forest type of R. mucronata, while there was no difference in pH, temperature and organic matter content between the two forest types. The seaward forest type in A. marina and C. tagal had a higher proportion of silt and clay, whilst the landward forest types had a higher sand proportion. There was no much difference globally in textural classes between the two forest types of R. mucronata. Crab burrow density was significantly higher (p < 0.05) in the seaward forest type of A. marina and C. tagal compared to the landward forest types, while the density was the same in both forest types of R. mucronata. Tree heights were significantly shorter in the landward forest types than in the seaward forest types. The density of adult trees was higher (p < 0.05) in the landward forest types than in the seaward forest types. The correlation coefficients between tree height, and the other tree morphological characters respectively, were almost constantly higher in the taller seaward forest type than in the shorter landward forest type of the three mangrove species. A Principal Component Analysis gave identifiable clusters of the various forest types in A. marina, but no distinct pattern was observable in C. tagal and R. mucronata. The results therefore show a negative correlation between tree height and salinity and a positive correlation between tree height and crab burrow density respectively. The correlation coefficient between tree height and diameter D130 in the landward forest types were consistently low showing that these forest types really represent a dwarf growth form. From the findings of this study therefore, it is safe to conclude that salinity probably exerts unfavourable stress to tree growth and that crab burrowing among other factors is positively correlated with tree height. The shorter landward forest type is really a dwarf form and not a forest consisting of younger trees. |
