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Inventory data on tree weights of 104 individual trees representing 10 mangrove species were collected from mangrove forests in South-East Asia to establish common allometric equations for the trunk, leaf, above-ground and root weight. We used the measurable tree dimensions, such as dbh (trunk diameter at breast height), DR0.3 (trunk diameter at 30 cm above the highest prop root of Rhizophora species), DB (trunk diameter at lowest living branch), and H (tree height) for the independent variable of equations. Among the mangrove species studied, the trunk shape was statistically identical regardless of site and species. However, ρ (wood density of tree trunk) differed significantly among the species. A common allometric equation for trunk weight was derived, when dbh2H or DR0.32H was selected as the independent variable and wood density was taken into account. The common allometric equations for the leaf and the above-ground weight were also derived according to Shinozaki's pipe model and its extended theory. The common allometric relationships for these weights were attained with given ρ of each species, when DB2 or dbh2 or DR0.32 was selected as the independent variable. For the root weight, the common equation was derived from the allometric relationship between root weight and above-ground weight, since these two partial weights significantly correlated with each other. Based on these physical and biological parameters, we have proposed four common allometric equations for estimating the mangrove tree weight of trunk, leaf, above-ground part and root.

Abstract:Carbon dioxide emission through soil respiration is an important component of the carbon balance in forest ecosystems. However, little information is available on the rates of soil respiration in mangrove forests. We studied the rate of soil respiration in a secondary mangrove forest in eastern Thailand on an estuary of the Trat River during both the wet and dry seasons. A study site of 40 × 110 m was established and a series of vegetation zones identified: Sonneratia, Avicennia, Rhizophora and Xylocarpus, in order of increasing elevation inland. Soil respiration was measured during low tide, using an infrared gas analyser connected to a respiratory chamber, by excluding the respiration of above-ground roots from the chamber. At least 19 measurements were performed in each zone for each season. The rate of soil respiration significantly increased with increasing soil temperature. The soil temperature which was usually lower than that of sea water showed a trend that decreased with distance from the river in both wet and dry seasons. The relative land elevation causes different periods of inundation among the vegetation zones. The period was longest in the Sonneratia zone located on the river fringe, and became shorter moving inland. Thus, the elevation and relevant period of inundation are considered to be causal factors warming the soil. Consequently, the difference in soil temperature caused significantly different rates of soil respiration among the vegetation zones in the mangrove forest. Overall, the average rate of soil respiration ranged from 0.456 to 0.876 μmol CO2 m−2 s−1, supporting the view that mangrove forests have lower rates of soil respiration than do upland forests.

The distribution of mangrove forests is limited to the coastal zones of tropical and subtropical regions, and their total area is far smaller than that of upland forests (Spalding et al. 2010). Mangrove forests often show unique patterns of biomass allocation and carbon dynamics because they are periodically submerged by tides (Komiyama et al. 2008). Therefore, the contribution of mangrove forests to the global carbon fixation process should be carefully evaluated even though their distribution area is limited.

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Salinity is a crucial factor regulating mangrove growth. We evaluated seasonal variations in soil water salinity and the water replacement process in a tropical monsoon mangrove forest of eastern Thailand during 2015–2018. Trunk growth of Avicennia alba was monitored monthly using dendrometer bands and was analyzed in relation to water replacement. Soil water salinity showed remarkable seasonal variation that was influenced by the infiltration of inundated water from the river, with a salinity level similar to that of seawater during the middle of the dry season and to that of fresh water during the middle of the rainy season. Patterns of soil water salinity shifted seasonally in both horizontal and vertical distributions, highlighting soil water replacement between the two seasons. In the middle of the rainy season, soil water salinity across the horizontal distribution was nearly fresh at most sampling points on the river side but gradually increased landward along a 120‐m transect. The vertical distribution of soil water salinity showed relatively low salinity at the surface horizon, which gradually increased downward to a 100‐cm depth. In the dry season, the soil salinity gradient in both distributions was opposite to that in the rainy season. This seasonal change in soil water salinity was significantly related to the trunk growth of A. alba, which increased greatly during the rainy season. Seasonal water replacement causes fluctuations in soil salinity and probably nutrient availability. The combination of low soil water salinity and large nutrient influx might enhance trunk growth during the rainy season.

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