Carbon sequestration and annual increase of carbon stock in a mangrove forest
Highlights
► Mixed species mangrove biomass regression models have been developed. ► This model can be applied to estimate spatial variation of carbon sequestration. ► Annual increase of carbon stock exhibits faster turn over than the tropical forest. ► Carbon sink in terms of live biomass is several fold greater than that of sediment. ► Resource availability is more important over recovery from a significant disturbance.
Introduction
The increase of atmospheric CO2 is unavoidable but sinks for this carbon are not well understood. During 2000–2009, combination of fossil fuel and land use change lead to an emission of 8.8 ± 0.86 Gt C per year with coefficient of variance (C.V) 9.77%, whereas the atmospheric growth and the uptake by land and ocean together could account 8.8 ± 1.12 Gt C a−1 with C.V 12.75% (Global Carbon Project, 2009). Two large reservoirs: the terrestrial biosphere and the ocean uptake CO2 approximately in equal proportion. Greater coefficient of variance for the uptake by land and ocean indicates considerable annual variability in the estimation of CO2 storage by the ocean and land. This could be due to the weakening of sink strength of the ocean and increasing capacity of forest uptake in response to atmospheric CO2 increase (Spiecker et al., 1996, Lewis et al., 2004, Ciais et al., 2008). These CO2 ‘sinks’ are not stable, in fact, they are highly variable and respond to elevated atmospheric CO2 levels and climatic change. Therefore, it is of interest to know the size of the land and ocean CO2 sinks and their evolution with time.
Information on the spatial variation in carbon sequestration in different types of forest cover in the land could achieve further improvements of accuracy of global sinks. Sixty two percent (62%) to 78% of the global terrestrial C is sequestered in the forests, and about 70% of this C is stored in the soil (Dixon et al., 1994, Schimel, 1995) with slow turnover rate (Guggenberg et al., 1994). Tropical forests process about six times as much carbon as the anthropogenic emission. Changes in carbon dynamics in tropical forest with 50% contribution to global terrestrial gross primary production (GPP) (Grace et al., 2001) could alter the pace of climate change (Adams and Piovesan, 2005). Regional studies of carbon exchange vary in showing disequilibrium state of Tropical forest and in increasing stocks of tree carbon (Phillips et al., 1998, Lewis et al., 2009). Apart from resource availability and pollution stress, succession and global change could have varying importance at different region to produce different spatial and temporal pattern of carbon uptake by trees (Muller-Landau, 2009). Mangrove forest accounts for about 2.4% of tropical forest (www.fao.org/forestry/mangroves, Spaulding et al., 1997) and to improve accuracy of global carbon sink quantification of carbon dynamics is essential in the mangrove swamps (Chmura et al., 2003).
The Indian Sundarbans mangrove forest in the estuarine portion of the River Ganges covers an area of 9630 km2 out of which 4264 km2 is law protected forest. It is the largest delta on the globe (world’s heritage site, www.unesco.org/en/list/452) and covers about 2.84% of the global mangrove area (15 × 104 km2).
Attempts are made to quantify biomass and characterize carbon dynamics in tropical rainforest, yet uncertainty remains, specifically with respect to the mangrove forest (Ometto et al., 2005, Pyle et al., 2008, Lewis et al., 2009, Miller et al., 2004). The objective of this study is to quantify carbon sequestration and to examine the controlling factors for annual increase of carbon stock in the tropical mangrove forest.
Section snippets
Study area, geology, soil and climate
The study sites are located in the Sundarbans (21°32′ and 22°40′ N; 88°05′ and 89°E), a natural mangrove forest, which is part of the estuary associated with the river Ganges, on the northeast coast of the Bay of Bengal, covering a total area of 9630 km2 out of which 4264 km2 comprising intertidal habitat. The area is covered with thick mangroves, which can be treated as forest and aquatic sub ecosystems (1781 km2). In 1985, the Indian Sundarban was included in UNESCO’s list of world heritage
Results
Annual movement of the Inter-tropical Convergence Zone in this part of the world produces significant changes in micrometeorological parameters throughout the year because of differential temperature and pressure in different seasons (Table 1). Mean temperature and humidity are 28.12 ± 1.67 °C and 80.36 ± 9.74%, respectively with annual rain fall 1750 mm. Geologically the area is the result of extensive fluvio-marine deposits of the river Ganges and Bay of Bengal and the character of the
Discussion
Soares and Schaeffer-Novelli (2005) analyzed the above ground biomass of Rhizophora mangle and Laguncularia racemosa in southeast Brazil and recommended the use of models related to structural development such as dbh (diameter of breast height) and height and stressed the need to obtain specific data for each geographical area. Chave et al., 2005 deduced a mixed species tree biomass regression model for converting plot census data into estimates of AGB across a broad range of tropical forest.
Conclusion
- 1)
Regression models developed in this study involving breast height diameter, height and density can be used to estimate the living above ground and below ground biomass of mangroves.
- 2)
Application of the model allows estimating the spatial variation of above ground biomass and sequestration of anthropogenic carbon dioxide in relation to the resource availability and environmental constrain.
- 3)
Estimated carbon stock is lower than in the terrestrial tropical forest and their annual increase exhibits
Acknowledgements
We are thankful to the funding agency, Department of Science and Technology, New Delhi, Thanks are also due to Sundarban Biosphere Reserve and Divisional forest office, South 24 Parganas, Govt. of West Bengal for giving permissions to carry out the study. Authors are grateful to the journal editor Dr. H. B. Singh and two anonymous reviewers for their constructive comments.
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