Coastal dunes are valuable systems that are often deteriorated due to human impact, climate change, and species invasion. Revegetation efforts are critical to dune restoration as dune vegetation affects sediment deposition and influences dune morphology. Revegetation planting designs focus on large spacings (50cm) to reduce competitive interactions between plants. However, based on the stress gradient hypothesis, dune grasses may exhibit greater facilitative rather than competitive interactions due to the high stress nature of coastal dune systems (burial, salinity, drought, and wave stress). Dune grasses may facilitate each other through shading, reduced impact of sand weight from burial, or soil-moisture redistribution. My study objectives were 1) investigate how planting configurations (clumped or spaced) affect plant growth and physiology of coastal grasses and 2) understand the influence of interspecific and intraspecific interactions on plant growth and physiology. Six experimental plots (6 X 4m) were established at equal elevations atop the primary dunes at Back Bay National Wildlife Refuge, VA. Each plot was divided into six 2 X 2 m plots with three different treatments: species (Ammophila breviligulata or Uniola paniculata), planting design (clumped or spaced), and mixture level (monoculture or mixture of both species). Monthly measurements were collected for two growing seasons including; survival, Electron transport rate (ETR), leaf length, leaf number, stem number, ramet number, specific leaf area (SLA), and expansion rate.
Results/Conclusions
Percent survival was affected by mixture level, but not by planting design. The mixed plantings had the lowest percent survival per plot. ETR was significantly higher in the monoculture plantings where there were no interspecific interactions, but there was no effect by planting design. Shoot length was not affected by mixture level or planting design. Leaf number was affected by mixture level and planting design where monoculture and spaced plantings had greater leaf number. Stem number and ramet number were both affected by planting design, with spaced plantings showing greater stem and ramet number. These findings indicate greater competitive over facilitative interactions in clumped and mixed plantings of A. breviligulata and U. paniculata. Although dunes are a high stress environment, the competitive interactions outweigh the facilitative interactions in these dune grasses. My research indicates that in east coast dune systems, re-vegetation efforts should continue planting designs of 50cm spacings between plants with single species. With the range shifts of U. paniculata into A. breviligulata habitat enhanced by climate change, my findings indicate competitive interactions between the species.