Selection pressures associated with strong environmental gradients on salt marshes provide a mechanism for the genetic subdivision of species. We investigated the importance of differentiated habitat niches in explaining the distributions of 2 forms of the halophyte Sarcocornia perennis: ssp. perennis in lower/middle marshes and the Iberian endemic ssp. alpini on more elevated salines. The fundamental niches of the 2 subspecies were investigated at the Odiel marshes, SW Spain with an experimental transplantation to 4 points along an elevation gradient. Subspecies perennis survived at the 3 lower elevations, from +2.26 to +2.84 m relative to Spanish Hydrographic Zero (SHZ) and flowered only at the highest of these locations. Subspecies (ssp.) alpini survived only at the 2 higher elevations (+2.84 and +3.65 m SHZ), flowering only at the highest location. As expected, sediment redox potential increased at transplant sites up the marsh, with decreasing tidal submergence; sediment salinity also increased and its water content decreased with increasing elevation. Sediment pH also declined with increasing elevation. Air conditions in the Mediterranean summer become cooler and moister with proximity to the sea. The failure of ssp. alpini to survive below its limit was attributed to its intolerance of increasingly hypoxic, reducing sediments. The failure of ssp. perennis to survive above its limit was associated with hypersalinity and water stress in summer. The fundamental niches of the 2 subspecies showed limited overlap (i.e. one transplantation site): that of ssp. perennis approximates the realized niche observed in the field but the realized niche of ssp. alpini appears to be more restricted than its fundamental niche.