Colonization success of common Thai mangrove species as a function of shelter from water movement

Seedling survival and growth of the 3 common SE Asian mangrove species Avicennia alba, Rhizophora mucronata and Sonneratia caseolaris were quantified experimentally along 2 spatial gradients of shelter: (1) between 2 stations, at the inner and outer end of the sheltered Pak Phanang Bay (SW Thailand); and (2) for each station, among plots across a gradient of vegetation density from the mangrove forest edge inwards. Exposure to water movement, quantified as gypsum clod card weight loss, was found to vary more than 5-fold between seasons, which contributed most of the variance accounted for (73%). Variation between plots was higher than that between the 2 stations: clod card loss ranged between 3.0 and 4.6 g d(-1) in the plots, whereas the grand means of the 2 stations were 3.4 and 3.7 g d(-1), respectively. These differences between stations and plots were comparable to the :patterns found for mangrove seedling survival. Survival was high (80 to 93%) in most treatments in R.. mucronata, with the exception of the most exposed plot (30%). In the other 2 species, overall survival was significantly less but was highest in the outermost plots with the lowest tree density. This pattern confirms the successional status of these 3 mangrove species. Seedling growth, expressed as height increase, was significantly reduced with increasing neighboring tree density for A. alba and S. caseolaris, whereas R. mucronata showed an opposite pattern. Internode production of all 3 species was highest in the most exposed plots. Overall, relative growth rate, expressed as height increase, declined with the age of the seedlings.