Phylogeography of Halophytes Across Saline Landscapes: A Systematic Review of Coastal–Inland Connectivity and Genetic Differentiation

Saline habitats, which include both coastal and inland, host specialised halophytic communities that can withstand high salinity. Coastal communities are shaped by tidal inundation and marine connectivity, whereas inland saline habitats are driven by evaporation, groundwater salinisation, and greater landscape isolation. These contrasting hydrological and environmental variables likely drive divergent patterns of genetic diversity, influencing local adaptation, connectivity, and evolutionary divergence, which is important for understanding species persistence and guiding conservation of saline ecosystems under environmental change. This systematic review brings together phylogeographic studies on halophytic species from both environments to uncover global drivers of genetic difference. A systematic search of Web of Science, Scopus, and PubMed was done, and 20 studies were identified focusing on key genera including Salicornia, Sarcocornia, Suaeda, and Triglochin. Distinct genetic clades were often associated with habitat type or geographic region, indicating repeated divergence linked to coastal–inland environmental gradients. Genetic differentiation between coastal and inland halophyte populations is primarily driven by habitat fragmentation, restricted gene flow, historical refugia and recolonisation, salinity-mediated adaptation, and differences in dispersal capacity. As a result, coastal populations are generally more genetically connected, whereas inland populations tend to be more isolated, structured, and evolutionarily divergent. The data also showed a strong regional bias; while research is well established in Europe, Asia, and North America, African inland saline ecosystems are critically understudied. We conclude that habitat connectivity and dispersal pathways are the primary determinants of halophyte genetic structure. Future research must integrate ecological niche modelling and landscape genetics to resolve the evolutionary dynamics of these taxa, particularly in under-sampled regions such as southern Africa, to support effective conservation of saline biodiversity. However, the synthesis is constrained by the small number of eligible studies (n = 23) and their uneven geographic distribution, with limited representation from Africa, South America, and Australia, which restricts the ability to draw fully global conclusions. Recognising and protecting these systems is essential for safeguarding global saline biodiversity.