Hydraulic Traits Constrain Salinity-Dependent Niche Segregation in Mangroves

To understand the mechanisms underlying species assemblage along salt gradients in intertidal zones, we measured the xylem hydraulic vulnerability curves (<i>HVC</i>s), leaf water potential (<i>ψ</i>), stomatal conductance (<i>g</i>_s), specific leaf area (<i>SLA</i>), and wood density (<i>WD</i>) for six mangrove species of <i>Avicennia marina</i>, <i>Bruguiera gymnorrhiza</i>, <i>Aegiceras corniculatum</i>, <i>Kandelia obovata</i>, <i>Sonneratia apetala</i>, and <i>Sonneratia caseolaris</i>. We found the following: (1) <i>A. marina</i> and <i>B. gymnorhiza</i> had the most negative <i>P</i>₅₀ (water potential at which 50% of hydraulic conductivity was lost), while <i>S. caseolaris</i> and <i>S. apetala</i> had the least negative <i>P</i>₅₀, indicating different resistance to embolism in xylem; (2) <i>P</i>₅₀ and <i>P</i>₈₈ (water potential at which 88% of hydraulic conductivity was lost) declined with increasing salinity from the onshore to offshore species, as their water regulation strategy meanwhile transitioned from isohydry to anisohydry; (3) <i>B. gymnorhiza</i> had smaller <i>SLA</i> but larger hydraulic safety margin (<i>HSM</i>), implying potentially higher capacity of water retention in leaves and lower risk of hydraulic failure in xylem. These results suggest that hydraulic traits play an important role in shaping the salt-driven niche segregation of mangroves along intertidal zones. Our research contributes to a more comprehensive understanding of the hydraulic physiology of mangroves in salt adaption and may facilitate a general modeling framework for examining and predicting mangrove resilience to a changing climate.