Name: Suthathip Umnouysin
Supervisor: Sasitorn Poungparn
Climate change has resulted in decreasing area and efficiency of carbon sequestration in mangrove ecosystem. To understand carbon dynamics in mangrove ecosystem, it is necessary to study net ecosystem production (NEP) which is a key to clarify carbon-sink efficiency. A common method of NEP estimation is summation method by balancing between net primary production (NPP) and heterotrophic respiration (HR). NPP is a summation of biomass increment (∆B) and litter production (L). However, previous NEP estimations by this method in the mangrove forests excluded coarse woody debris (CWD) which has been mentioned as an important carbon pool because of low decomposition rate. Therefore, the aim of this study is to estimate carbon budget of CWD for estimation of NEP in a secondary mangrove forest at the Trat River estuary. The CWD distribution was investigated in 1-ha permanent plot showing clear three vagetation zones of dominant tree species from the river fringe to inland as Avicennia, Rhizophora and Xylocarpus zone, respectively. The results showed that CWD mass was the highest in the Xylocarpus zone (6.33 t ha-1), followed by the Rhizophora zone (6.14 t ha-1), and the Avicennia zone (3.55 t ha-1), respectively. Component of CWD mass in each category differed by vegetation zone. Mass of downed wood CWD was the highest in the Rhizophora zone, while mass of CWD in standing dead trees was the highest in the Xylocarpus zone. Then, annual ∆B, L and CWD input were recorded, and showed that the Avicennia zone had the highest ∆B, while the Rhizophora zone had the highest CWD input. Given by the summation of ∆B, L and CWD, the NPP ranged from 5.97 to 11.9 t C ha-1 y-1. The highest contribution of CWD to NPP was in the Rhizophora zone (37%). Moreover, the rates of CWD respiration which is a component of HR was also measured. They were 0.22, 0.52 and 0.28 t C ha-1 y-1 for the Avicennia, Rhizophora and Xylocarpus zones, respectively. The Soil respiration was calculated by using an equation of Poungparn et al. (2009). It was 2.30, 2.20 and 2.12 t C ha-1 y-1 for the Avicennia, Rhizophora and Xylocarpus zones, respectively. The total HR was calculated as 2.52, 2.72, and 2.40 t C ha-1 y-1 for the Avicennia, Rhizophora and Xylocarpus zones, respectively. Finally, the NEP was balanced as 8.88, 9.18, and 3.57 t C ha-1 y-1 for the Avicennia, Rhizophora and Xylocarpus zones, respectively. The results indicated a significant role of the CWD for the NEP estimation in the mangrove forest. The data obtained by this study is a merit for enhancing the accuracy of NEP estimation in the mangrove forest.