Unique characteristics of ecosystem CO2 exchange in Sundarban mangrove forest and their relationship with environmental factors

Sundarban mangroves, known to be the largest single block of tidal halophytic mangrove forest globally, covers a significant portion of the world's mangroves and contributes substantially to the global mangrove carbon budget. Nevertheless, there is still a lack of long-term continuous monitoring of CO2 fluxes over this crucial ecosystem and their dynamics in response to climatic variations. This study presents the carbon balance and seasonal dynamics of this oldest and undisturbed mangrove ecosystem using quasi-continuous eddy covariance measurements from April 2012 to March 2016. These tidal forests are found to be net C sink with a mean net primary production (NEP) of 276 g C m(-2) yr(-1) (+/- 35; +/- 1 Standard deviation amongst the four years). The NEP is partitioned into gross primary production (GPP) and ecosystem respiration indicate gross sequestration of 1581 (+/- 152) g C m(-2) yr(-1) with a respiratory loss of 1305 (+/- 184) g C m(-2) yr(-1). We have observed that inter- and intra-annual variations in CO2 fluxes are primarily controlled by environmental factors such as temperature, vapour pressure deficit (VPD), rainfall and photosynthetic active radiation. At the study site, the photosynthetic activity and light use efficiency decreased with an increase in temperature and VPD. Though minimal seasonal fluctuations in CO2 fluxes were observed, a strong linkage has been observed between C fluxes and satellite-derived enhanced vegetation index (MODIS EVI) during the study period. Further, the carbon assimilation rates were found to be higher under diffused sky than normal conditions due to the increased photosynthetic activity by penetration of diffused radiation within the mangrove canopies. Additionally, ecosystem productivity during the study period was higher during the wet periods than dry periods. Our findings show that ecosystem productivity rates are strongly related to temperature, vapour pressure deficit, rainfall, and radiation, implying that global warming could have a significant impact on these environmental factors and other underlying parameters (such as salinity, tidal inundation patterns etc.) and potentially reduce the C sequestration potential of these tropical mangroves of India.

Automated summary

Sundarban mangroves, the largest tidal halophytic mangrove forest in the world, play a significant role in the global mangrove carbon budget. However, there is a lack of long-term continuous monitoring of CO2 fluxes and their response to climatic variations in this ecosystem. This study presents the carbon balance and seasonal dynamics of the Sundarban mangroves using quasi-continuous eddy covariance measurements. The findings show that these mangroves act as a net carbon sink and their CO2 fluxes are influenced by environmental factors such as temperature, vapour pressure deficit, rainfall, and radiation.