The Stress Gradient Hypothesis predicts that plant-plant interactions will shift from negative under benign conditions to positive under stringent conditions. Despite broad support for predictions of this framework from natural systems, typical restoration practices neglect the potential for facilitation to enhance restoration outcomes in stressful habitat. In coastal salt marsh, tidal inundation drives salinity and moisture gradients affecting plant performance, and these natural stressors can be exacerbated by coastal recontouring projects intended to buffer sea level rise. Studies clustering transplants together have demonstrated increases in seedling survival and growth at salt marsh restoration sites over the short term, but clustered plantings may experience stronger competition from neighbors as plants grow. Furthermore, planting patterns may affect non-native recruitment in ways that impact native cover, the most common metric of restoration success. In a fully randomized experiment, we tested effects of clustering on two native species in the marsh-upland ecotone, Frankenia salina and Jaumea carnosa. Tight and loose neighbor arrangements spanned salinity and moisture gradients in the ecotone, where we tracked transplant survival, growth, and physiology, as well as native and non-native cover over time. We ask: 1) whether effects of planting pattern shift across elevation as predicted by the Stress Gradient Hypothesis; 2) whether patterns are consistent across species; and 3) whether pattern affects the ability of natives to hold space against invaders.
Results/Conclusions
At our restoration site in central California, competitive interactions dominated in the marsh-upland ecotone. Neither salinity nor moisture gradients led to a shift from competition to facilitation for either species in this year of relatively mesic conditions. We found higher stem counts and greater percent cover of transplanted species in loose vs. tight neighbor plots, with more bare ground in tight neighbor plots. We also saw little difference in survival or tissue water potential between neighbor treatments. Taken together, these results suggest that competition prevails at our site. Thus, rather than fostering positive interactions, tight clustering of plants enhanced competition and decreased restoration success.