Structure and Productivity of a 20-Year-Old Stand of Rhizophora apiculata Bl. Mangrove Forest

Mangroves are dominant interface ecosystems between the land and the sea in the tropics, and are of importance in the economy of many of these regions in terms of mangrove-linked fisheries and forestry. Recently, mangroves have been of particular interest in relation to global change both because of the possible high carbon sequestration as well as being in the 'forefront' of any sea-level change, because of their location. To understand the impact of global change on these ecosystems (considered terrestrial and aquatic at the same time) and vice versa, it is necessary to obtain 'a more comprehensive and realistic picture of the terrestrial carbon cycle', which is one of the aims of the GCTE Programme. We therefore present here some of the results of our long-term study (started in the mid-1970s) on the carbon and nutrient budget of a mangrove ecosystem as a basis for further studies, including the proposed large-scale biogeochemical transects and climate models proposed by GCTE. The tree density of the 20 m × 40 m plot in the 20-year-old stand was equivalent to 2425 stems per hectare (1975 live trees per hectare). Size (girth at breast height) of Rhizophora apiculata trees ranged from 9 to 75.5 cm with a mean at 39 cm. The smallest live tree weighed 10 kg and the biggest weighed 510 kg with a mean biomass of 122 kg. About 70% of the trees were below 100 kg but the 30% of the bigger trees contributed to slightly more than half of the total biomass of the plot. The canopy had an average height of 21 m. The total standing biomass was 114 t C ha-1; 74% of the biomass was in the trunk, 15% in the roots (10% in stilts and 5% below-ground) and 10.6% in the canopy (only 2.6% in leaves). Using allometric regressions, we obtained a net productivity (root turnover and loss through leaching were not measured but only approximated as equal to small litter production) of 17 ± 5 t C ha-1 yr-1. If greater accuracy (than ± 30%) is needed, direct measurements of root turnover and leaching from roots would be needed. Using the gas exchange method and using the mean value for a whole day's net photosynthesis measurements (averaged at 6 μ mol m-2 s-1), 1.5 μ mol m-2 s-1 for leaf respiration, a leaf area index of 4, and assuming respiration of the non-leaf tissues to be the same as for leaves, we estimated net productivity to be 11.35 t ha-1 yr-1, almost at the lower limit of the allometric estimate. Use of leaf to tree to stand models may improve the accuracy of this method.