We experimentally examined plant zonation in a previously unstudied Chilean salt marsh system to test the generality of mechanisms generating zonation of plants across intertidal stress gradients. Vertical zonation in this system is striking. The low-lying clonal succulent, Sarcocornia fruticosa, dominates the daily flooded low marsh, while intermediate elevations are dominated by the much taller Spartina densiflora. Irregularly flooded higher elevations are dominated by Schoenoplectus californicus, with the small forb, Selliera radicans, found associated with Schoenoplectus at its base. Transplant studies of all four species into each zone both with and without competition revealed the mechanisms driving these striking patterns in plant segregation. In the regularly flooded low marsh, Sarcocornia and Spartina grow in the zone that they normally dominate and are displaced when reciprocally transplanted between zones with neighbors, but without neighbors they grow well in each other's zone. Thus, interspecific competition alone generates low marsh zonation as in some mediterranean marshes, but differently than most of the Californian marshes where physical stress is the dominant factor. In contrast, mechanisms generating high marsh patterns are similar to New England marshes. Schoenoplectus dies when transplanted to lower elevations with or without neighbors and thus is limited from the low marsh by physical stress, while Selliera grows best associated with Schoenoplectus, which shades and ameliorates potentially limiting desiccation stress. These results reveal that mechanisms driving community organization across environmental stress gradients, while generally similar among systems, cannot be directly extrapolated to unstudied systems. This finding has important implications for ecosystem conservation because it suggests that the mechanistic understanding of pattern generation necessary to manage and restore specific communities in novel habitats cannot rely exclusively on results from similar systems, and it identifies a critical role for experimental ecology in the management and conservation of natural systems and the services they provide.