Phylogeographic Structure of Iris ensata Uncovers an Arid Barrier Shaping East Asian Alpine‐Wetland Divergence

ABSTRACT Aim Understanding the evolutionary history and phylogeographic patterns of plants is essential for gaining insights into how climatic and geological processes have influenced current floral biodiversity. An east–west‐oriented arid belt, roughly spanning 30° N–45° N across East Asia, has persisted since the Paleogene and may have acted as a long‐standing dispersal barrier between the northern and southern floras of the region. This study aims to clarify the influence of this arid belt on the disjunct distribution of a hygrophilous wetland plant. Location China, Korea, Japan, and the Russian Far East. Taxa Iris ensata Thunb. (Iridaceae). Methods We integrated ecological niche modelling (ENM), molecular approaches (chloroplast DNA and nuclear microsatellite markers), and demographic inferences to investigate the genetic structure, phylogeographic patterns, and evolutionary dynamics of I. ensata . Results Ecological niche modelling suggested that I. ensata maintained relatively stable habitats during the Last Glacial Maximum, in contrast to the pronounced southward range contractions documented in many temperate East Asian taxa. Analyses of 48 populations covering the species' natural range revealed three well‐defined genetic groups: the Tianmu Mountains (TIM), the Japanese Archipelago and South Korea (JAK), and the Northeast China and Russian Far East (NEC). The strong regional differentiation and the presence of unique cpDNA haplotypes within the TIM and NEC groups suggest that both regions served as glacial refugia. Demographic reconstructions based on nSSRs supported a model of divergence with low gene flow, with lineage splits primarily occurring during the mid‐to‐late Pleistocene epoch. Main Conclusions Collectively, these findings support the hypothesis that the persistent arid zone across northern China has functioned as both a climatic and biogeographic barrier, shaping the north–south lineage divergence in I. ensata . This study provides new evidence for the arid‐belt mediated diversification hypothesis and underscores the complex evolutionary processes governing plant diversification across the humid and semi‐arid landscape of East Asia.