Coastal dune vegetation plays a key role in dune formation and stabilization through sediment trapping and erosion control. To restore degraded dunes, revegetation of dune building species is critical. Planting density has been found to effect growth of marsh species, with closer plantings alleviating stress through facilitation. As coastal dunes are high stress environments, it is expected that dune species may also exhibit facilitative interactions based on the Stress Gradient Hypothesis. Therefore, planting grasses in clumped configurations may lead to more successful dune revegetation. The objective of this research was to determine how planting density affects the growth of two dominant dune grasses along the US Atlantic coast, Ammophila breviligulata and Uniola paniculata, through field surveys of natural distribution, density, and a manipulation study of planting densities. Natural distribution differed between the two species with A. breviligulata occurring at lower dunes and U. paniculata occurring at higher dunes. Ammophila breviligulata occurred more densely than U. paniculata. Planting density of U. paniculata had an effect on growth parameters (shoot length, stem number, and ramet number) but not survival with dispersed plantings (50 cm apart) having higher growth than clumped plantings. The effect of density planting on growth parameters may impact dune building processes. Sparsely planted U. paniculata may have greater sediment capture compared to densely planted U. paniculata due to greater stem number and biomass resulting in taller, steeper dunes.

Dune grasses are integral to biogeomorphic feedbacks that create and alter foredunes and barrier island stability. In a glasshouse study, Ammophila breviligulata Fern. and Uniola paniculata L. were planted together and subjected to sand burial to quantify morphological and physiological response. Ammophila breviligulata physiological and morphological performance declined when planted with U. paniculata but U. paniculata was not affected when planted with A. breviligulata. Burial had a positive effect on A. breviligulata and U. paniculata as indicated by electron transport rate and total biomass at the end of the experiment. Due to their different growth strategies, A. breviligulata and U. paniculata form continuous versus hummocky dunes, respectively. As global temperatures rise and U. paniculata migrates into A. breviligulata dominated habitat, A. breviligulata performance may diminish, and changes in dune form could result in altered island stability via increased overwash. Foredune community structure could also change due to the shift in dominant species which could alter dune succession.