Practical applications

Chapter 2 specified the environmental requirements of the threatened mangroves and produced species distribution maps. Chapter 5 uncovered how functional responses of different mangrove species vary along the environmental gradients and generated species- and community-level trait maps under current and future environmental conditions. Together, these species-centric and trait-based findings, can help forest managers selecting suitable species and sites for replanting. For example, a significantly stronger negative response of H. fomes abundance and canopy height to increasing salinity compared to the responses of the generalists (E. agallocha and C. decandra) imply for choosing relatively benign upstream habitats for H. fomes replanting and planting the generalists or salt-tolerant early-successional species (e.g. S. apetala) in the highly degraded barren areas for initial site stabilization.

The process of biotic homogenization has been underway in the Sundarbans resulting in spatial contraction of diverse and distinct mangrove communities in the Sundarbans (Chapter 3). Biodiversity maps revealed that both the historical and contemporary biodiversity hotspots were located at the northern (specifically the Kalabogi region) hyposaline habitats that support the unique association of X.

mekongensis and H. fomes, the two species most at risk of local and global extinction. Therefore, future conservation and protection initiatives should primarily focus on these surviving hotspots because further habitat degradation and exploitation of these threatened species may push them to the brink of extinction.

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Establishing coastal protected areas has been common practice in the tropics to conserve forest resources and to offer social and economic benefits to the forest-dependent communities (de Almeida et al. 2016). A PAN (the largest coastal PAN in the world) is also operational in the Sundarbans to confirm completely undisturbed habitats for plants and animals. My species density maps (Chapter 2), biodiversity maps (Chapters 3 and 4) and the forest productivity maps (Chapter 5) have shown that the established PAN does not cover the species density, biodiversity, and forest productivity hotspots. Therefore, this thesis advocate for bringing these hotspots under protected area management to ensure the long-term conservation of the many threatened species living there.

Historical harvesting had a significant negative effect on the abundance of most of the mangroves (Chapter 2) and appeared responsible for the diminishing distinctness of the mangrove communities (Chapter 4). Many rare and endemic plant species including the climax H. fomes have faced range contractions or local extirpations over the last three decades (Chapter 3). Although logging has been legally prohibited since 1989 (Sarker et al. 2011), opportunistic harvesting of valuable timber-yielding species (mostly X. mekongensis and H. fomes) by poachers is common (Iftekhar & Saenger 2008). The BFD has ratified the

‘Bangladesh Biodiversity Act 2017’ to reduce biodiversity loss and recently initiated the SMART patrol management system to stop this illegal practice in the Sundarbans. My species density and biodiversity maps can guide these valuable protection and monitoring initiatives through tracking individual species populations and community-level diversity changes or predicting changes and recognizing habitats or species that may be affected by future human interventions.

Both mangrove enhancement (reducing biotic and abiotic stresses that caused mangroves’ population decline) and mangrove restoration (restoring specific areas where certain mangrove species previously existed) initiatives are regularly taken in the tropical coastal regions to enhance species resistance and resilience to climate change and to offset predicted losses from climate change impacts (Lewis 2005). However, inadequate understanding of which environmental drivers regulate mangroves abundances, composition and functions have resulted in unsuccessful mangrove enhancement and restoration projects in many countries

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(Lewis 2005) including the Sundarbans (Islam et al. 2014). This thesis determined the key drivers responsible for spatial variations in mangrove distributions, diversity, and functions in the Sundarbans. For example, salinity and historical harvesting predominantly affect H. fomes abundance, siltation, disease, historical harvesting and pH affect the diversity and distinctness of the mangrove communities, and soil salinity, alkalinity, and siltation act simultaneously to limit mangroves’ resource acquisitive traits and overall ecosystem productivity. These novel habitat and functional insights and the spatial maps of species, biodiversity, traits, and forest productivity can guide the future mangrove enhancement and restoration initiatives in the Sundarbans.

Also, my proposed integrated approach (Chapter 5) for quantifying trait-environment relationships and predicting ecosystem properties (such as productivity) is not restricted to mangroves. It can be readily applied to any other forest ecosystems of the world (e.g. boreal, temperate, and other tropical forests) to address various management and conservation issues therein.

Finally, based on the findings in Chapters 2 ─ 5, this thesis recommends to ─

• Replant globally endangered H. fomes and locally threatened X.

mekongensis in less saline upstream habitats (Chapter 2).

• Focus protection actions (e.g. regular patrolling, increase the number of resource protection officers and rangers) on the endangered (e.g., H.

fomes) as well as the rare endemics (e.g. C. ramiflora, C. manghas and A.

cucullata) whose populations and geographic ranges have substantially declined in recent times (Chapter 3).

• Take extreme caution while implementing nutrient enrichment programs because the mangroves of the Sundarbans may suffer from nutrient toxicity in highly silted hypersaline habitats (Chapter 4).

• Focus future conservation and protection initiatives primarily on the surviving biodiversity hotspots located in the northern Sundarbans (Chapters 3 and 4).

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• Extend or establish new protected areas for immediate protection of the remaining species density, biodiversity and forest productivity hotspots in the Sundarbans (Chapters 2, 3, 4 and 5).

• Take initiatives to test the suitability of interventions such as river dredging to increase the freshwater supply in the Sundarbans to reduce the salinity stress on tree growth and productivity of the ecosystem.