Effect of hydrology on the structure and function of mangrove ecosystems in the Can Gio Mangrove Biosphere Reserve, Vietnam

Abstract

The influence of hydrology on the mangrove forests of Vietnam has received little scientific attention, even though hydrology is recognized as the primary forcing function in mangrove ecosystems worldwide. The purpose of this dissertation research was to determine the effects of hydrology on specific structural attributes and functional processes within the mangrove forests of Can Gio, a province in southeastern Vietnam. Khe Vinh (KV) and Mui O (MO), two locations within compartment 17 of the Can Gio Mangrove Biosphere Reserve, were chosen as study sites. This research addressed two questions: (1) What are the characteristics of the hydrological regime at the Can Gio mangrove forest? and (2) How does the hydrological regime in the Can Gio mangrove forest affect soil properties, sedimentation, litter decomposition, primary production and species distribution. Tidal effects of the China Sea and the Saigon and Dong Nai Rivers affected the hydrological regime of the Can Gio mangrove forests. Average high tide and low tide were higher in the dry season than in the wet season. The different mangrove vegetation zones had different flooding frequencies at the KV and MO study sites. Zone 1 (nearest to the shoreline) at the KV site had a lower elevation than the other, more inland, mangrove zones at both the KV and MO sites. Overall, flooding frequency and elevation affected various soil properties. Low elevation zones had the highest sedimentation rates and flooding frequency. No sedimentation occurred at the MO site. Litter decomposition at the KV and MO study sites was dependent on the tissue structure of the species and the zones in which they occurred. Species that had thin and soft tissues had a higher decomposition rate than species with thick and hard tissues. The decomposition process was affected by vegetation zone, elevation, and flooding frequency. Flooding frequency and elevation affected primary production and species distribution at the study sites. More species were found in the higher elevation zones, which had dry, compacted soil. However, zones with a single dominant species, such as Rhizophora apiculata or Avicennia alba, had the greatest amount of litter fall.