Germination syndrome divergence among pairs of sympatric sister species along an estuarine salinity gradient

Closely-related species co-occur within heterogeneous habitats by occupying separate ecological niches as the result of divergent selection that maintains adaptive differentiation. Soil salinity is one of the most important factors influencing vegetation distribution in salt marshes, where germination is a critical stage in the life cycle of many halophytes. We characterized phenotypic divergence among two pairs of sympatric sister species of sea lavenders (Plumbaginaceae), by evaluating their germination traits and distribution patterns across a wide salinity range in both controlled experimental conditions, and in salt marsh study sites. The four species segregate into two germination syndromes, each with a polyploid Limonium species and diploid Myriolimon species pair. L. narbonense and M. ferulaceum have broad geographical distributions and their abundance within salt marshes was independent of soil salinity levels. Their germination syndrome was characterized by high germination percentages (c. 95 %) in freshwater, constant and high germination speed independent of salinity concentration, and reduced total germination (lower than control) after high salinity exposure. In contrast, the syndrome of L. algarvense and M. diffusum, two regionally endemic species, features moderate germination percentages (c. 30-80 %) over a wide salinity range (0-200 mM) with a reduction of germination speed at higher salinities. Furthermore, their capacity for recovery germination after high salinity exposure supports their abundance in salt marshes. These results suggest intertidal salinity influences on variation in germination traits among pairs of sympatric sea lavenders underlies their abundance, niche segregation, and phenotypic divergence in salt marshes.