Functional traits explain ecosystem engineering in dune building grasses

Background/Question/Methods

Stability of coastal systems is threatened by oceanic and atmospheric drivers of climate change.  Sea-level rise and increase frequency and intensity of coastal storms emphasize the importance of dune building processes to protect interior landforms.  Recovery and growth of dune building grasses following sand deposition drives topographic structure of coastlines, but functional role of species is poorly understood.  Recent research suggests that certain grasses maintain lower elevations, while others increase topography through rooting strategy.  Our goal was to examine aboveground and belowground functional traits of three dune building grasses, Ammophila breviligulata, Uniola paniculata, and Spartina patens in response to different levels of sand burial.  Ammophila breviligulata creates tall dune ridges while U. paniculata tends to maintain lower, hummocky dunes.  In contrast, Spartina patensmaintains flat, low elevation topography, creating a feedback with overwash of sediments.  We exposed each species to four levels of sand burial: 0, 5, 10, and 25 cm over five applications (~every 2 weeks).  Plant height, and stem/leaf count were monitored every two weeks.  At the end of experiment we measured biomass, root length, belowground stems, and specific leaf area (SLA). 

Results/Conclusions

As burial level increased, A. breviligulata decreased aboveground and increased belowground biomass, maintaining root length across treatments.  Uniola paniculata increased both above and belowground biomass, but rooting length declined with burial level.  Highest aboveground growth for U. paniculata occurred at the 5 cm burial level.  Spartina patens consistently exhibited higher aboveground biomass all burial levels with root length decreasing with increased burial level.  For all species, SLA decreased as burial level increased, but A. breviligulata maintained higher SLA compared to U. paniculata and S. patens.  These functional traits explain how A. breviligulata creates high dune ridges at varying rates of sand deposition by maintaining high growth rates and rooting depth, which allow access to freshwater in sandy, moisture limited environments.  In contrast, at higher burial rates U. paniculata access to fresh groundwater is limited by reduced root length.  Spartina patens responds quickly to sand burial, but maintains lower elevations by allocating growth aboveground (into number of leaves) and reducing rooting length and belowground biomass.  These results can help inform sustainable coastal management including dune restoration and interior coastal protection.