From Calcium tolerance to Magnesium adaptation: divergence of the Karst and Danxia Adiantum ferns

Background Mountains generate and sustain high plant diversity, but the roles of geology, habitat heterogeneity, and climate in driving speciation remain unclear. Here, we used a hierarchical approach to disentangle substrate effects, elemental coupling, and evolutionary constraints, revealing the relative contributions of substrate, climate, and phylogeny to differentiation in Adiantum series Gravesiana , endemic to China’s Karst and Danxia landforms. Results Phylogenetic signals were strong for soil Ca, C and K and for leaf Mg, but not for leaf Ca and soil Mg, revealing an aboveground–belowground decoupling. Despite strong Limestone–Danxia soil contrasts, only leaf Ca and leaf Mg differed distinctly between A. juxtapositum (Danxia) and the limestone-soil lineages. Soil C and Ca correlated only with other soil variables, whereas the leaf Mg variation was better explained by taxonomy and microclimate than by soil Mg. Conclusions These results support a two‑phase diversification model: early branching of A. juxtapositum on Danxia soils was driven by bedrock‑mediated soil differentiation (especially C and Ca), while subsequent diversification on limestone soils was driven by genetic background and microclimate via leaf Mg adaptation. Our study reveals stage-specific roles of geology and climate in montane plant diversification. Importantly, it provides the first experimental evidence for the floristic geo-lithology hypothesis by bridging soil chemistry and biological response.