Synergistic effects of climate and salinity on radial growth of Excoecaria agallocha L. in the Sundarbans world heritage mangrove ecosystem

The Sundarbans World Heritage ecosystem provides valuable ecosystem goods and services to >7.5 million coastal people in South Asia. This sentinel mangrove ecosystem is threatened by historical anthropogenic dis-turbances, salinity intrusion and long-term environmental changes including sea level rise. Understanding which factors regulate mangroves' growth is crucial for taking effective management decisions under global changes. However, a paucity of long-term mangrove tree growth data has limited our ability to quantify and predict the impacts of global changes on mangrove ecosystems. Using long-term growth data archived from the stem discs, this study, therefore, aims to understand how climatic and habitat variables influence radial growth dynamics in Excoecaria agallocha, a hyperdominant mangrove species in the Bangladesh Sundarbans. We find E. agallocha forms a distinct tree-ring boundary characterized by a narrow (2-4 cells wide) band of radially flattened fibres. The crossdatable tree-rings enabled us to develop two ecological zone-specific (low and high saline zone) chronologies (1972-2020) covering the entire salinity stress gradient present in the Sundarbans. Our analyses show: (I) radial growth rate in E. agallocha is significantly lower in the high saline zone, (II) monsoonal pre-cipitation enhances radial growth, (III) large-scale climate drivers, such as sea surface temperature (SST) of the equatorial Pacific and the Indian Ocean, and the El Nin similar to o-Southern Oscillation (ENSO) influence tree growth through modifying the local precipitation in varying seasons, and (IV) local habitat variables, such as soil salinity and siltation have combined effects on radial growth of trees although soil salinity exhibits stronger negative effect. Our results highlight the importance of integrating both climate and local habitat variables with tree-ring data for a better understanding of tree growth dynamics under changing environment.