Plant competition dominates grass species interactions in a migrating marsh-forest ecotone

• The tidal marsh-forest ecotone consists of countervailing environmental gradients in salinity stress and light availability. • Salt marsh grasses were more salt tolerant, but not more light-demanding, than an upland grass. • Growth of all species was equally suppressed by shade and competition with neighbors. • A field collection of high marsh grass biomass in a forest clearcut showed a positive effect of competitive release. Ecotone communities are areas of high species turnover and steep environmental gradients. We investigated how environmental gradients shape species interactions between upland and tidal wetland grass species to shed light on zonation in these species’ distribution patterns, in the context of understanding how their distributions may change with sea level rise and forest dieback. Across the coastal marsh-forest ecotone, there are stark shifts in two vital conditions for plants: salinity and light availability. We measured these abiotic conditions and plant productivity at sites spanning the ecotone and within a forest clear-cut, where canopy shading had been experimentally removed. In a greenhouse manipulation of plant species neighbor, salinity, and light availability, we hypothesized that plant interactions would become more facilitative in stressful conditions and that more salt tolerant species would also be more light-demanding. Species salt tolerance followed an expected ranking based on species zonation in the field, but all species responded similarly to reductions in light. Plant interactions in all treatments were negative or neutral, never facilitative. Despite low resource availability of light and elevated levels of salinity within the marsh-forest ecotone, plant interactions there reflect a benign stress environment, in which competition is predominant and where resource limitation, stress, and competition have additive, negative effects on plant performance.