Trait-based evidence of salinity-induced functional diversity loss in mangroves: Implications for ecosystem resilience.

Mangrove forests-vital for global carbon storage and coastal protection-are increasingly threatened by salinity intrusion resulting from sea-level rise and alterations in the hydrological regimes. While the functional importance of mangroves is well recognized, the mechanistic pathways through which salinity reorganizes community-level trait composition and compresses functional diversity remain unresolved. This gap is particularly acute in megadeltaic systems like the Sundarbans, where biodiversity and ecosystem service provisioning co-occur with steep salinity gradients. Elucidating how trait syndromes shift and diversity contracts across these gradients is critical to forecasting mangrove ecosystem responses and informing adaptive conservation strategies. This study quantified eight foliar traits (leaf area, specific leaf area, leaf dry matter content, total chlorophyll, stomatal density, leaf shape index, leaf succulence, and leaf carbon content) and four functional diversity indices (Rao's quadratic entropy, functional richness, evenness, and divergence) across a continuous soil salinity gradient using plot-level data from 59 sites in the Sundarbans. Trait-environment relationships were analyzed using linear regressions, spatial mapping, and multivariate ordination (principal components analysis [PCA], non-metric multidimensional scaling [NMDS]), while controlling for biotic factors such as species richness and abundance. Salinity significantly reduced functional diversity, particularly trait dissimilarity (RaoQ), supporting the hypothesis of abiotic filtering that favors functionally similar, salt-tolerant species. These reductions were most pronounced in high-salinity western zones dominated by generalist stress-tolerant species. Foliar traits shifted predictably with salinity, with reductions in leaf area, dry matter content, stomatal density, chlorophyll, and carbon content, and increases in leaf succulence and specific leaf area-indicating trade-offs toward conservative resource-use strategies under osmotic stress. Species abundance strongly influenced functional diversity independent of salinity. High abundance reduced trait dissimilarity and evenness, reinforcing the dominance of a few trait syndromes under stress. By integrating spatially explicit trait, salinity, and abundance data, this study provides novel evidence that abiotic filtering and biotic dominance jointly constrain community-level functional diversity in mangroves. Trait convergence and dissimilarity collapse under salinity stress indicate narrowing ecological strategies with reduced resilience. Conservation strategies should prioritize freshwater inflow and low-salinity habitat restoration. Trait-based indicators offer a predictive framework to sustain mangrove function under accelerating climate stress.