Periodic Mangrove Growth Induced by Water Salinity Fluctuations Under Tropical Monsoon Climate, Thailand

Abstract

Mangrove tree growth is crucially affected by water salinity in tropical monsoon areas having distinct seasonal rainfall. However, the seasonal growth pattern of mangroves in these areas remains insufficiently understood. We investigated stem growth and leaf phenology of mangroves and inundation water salinity over 2 years in an estuarine mangrove forest in Trat with clear seasonality and abundant rainfall (2938 mm), and in a shoreline fringe forest at Bangpu with moderate rainfall (764 mm) in Thailand. Salinity fluctuations at both sites were regulated by the monthly rainfall and the geographical location. All studied mangroves showed a seasonal stem-growth pattern, with higher growth rates in the wet seasons due to lower water salinity. The varied rates of seasonal stem growth among species at Trat corresponded with their zonation. The leaf emergence rates were also high in the wet season for Rhizophora apiculata at Trat and Avicennia alba at both sites. Salinity fluctuation driven by the seasonal rainfall directly affected leaf emergence of R. apiculata and both leaf emergence and stem growth of A. alba at Trat. However, at Bangpu, leaf phenology of A. alba responded to the temperature and decoupled from stem growth, which is directly influenced by monthly rainfall. Rhizophora apiculata stem growth was associated with the variation in leaf emergence. Thus, seasonal changes in the saline environments induced the periodic growth pattern of mangroves. Interestingly, the growth pattern of A. alba growing under different rainfall intensities responds to different triggers and indicates its high ecophysiological plasticity, enabling it to thrive across tropical mangrove forests.