Competition rather than facilitation affects plant performance across an abiotic stress gradient in a restored California salt marsh

The Stress Gradient Hypothesis predicts facilitation will become more important than competition where abiotic stress is high, and the framework successfully positive interactions between species in many systems. Fewer studies have focused on intraspecific facilitation, to our knowledge none examine Pacific Coast salt marshes of North America. We used two that tend occur large, monospecific patches test for a restored California marsh, tides evaporation during summer create moisture salinity gradients across elevation. tested performance Frankenia salina or Jaumea carnosa restoration plots using treatments: clustered plantings promote widely spaced limit interaction individuals. characterized soil water potential gradient measured plant survival, cover, physiology, susceptibility herbivory elevation planting treatments. Soil declined sharply with elevation, suggesting should be higher upslope. However, tissue was unaffected by survival high—suggesting growing conditions remained benign. A seawater addition treatment did not alter response plants gradient. At 19 months, prevailed entire both species, less transplant cover plantings. fared better transient period heavy rabbit herbivory. Aside from this temporary benefit, clustering reduce strongly suppressed growth—suggesting designs high marsh minimize competition.