Seasonal Variations in the Biomechanical Properties of American Beachgrass in Cold-Weather Climates

American beachgrass (Ammophila breviligulata) is one of the most common grasses found on dunes along the U.S. East Coast. This beachgrass plays a vital role in protecting beaches from erosion; its densely growing stems dissipate wave energy, and its intricate root systems hold sediment in place. The flow resistance (i.e. plant flexibility or stiffness) provided by the beachgrass is a key component when modeling how flows such as dune overwash and waves are modified by coastal vegetation, and how coastal vegetation affects sedimentation. As a perennial plant, American beachgrass undergoes seasonal senescence and regrowth, and in the Northeast, is subject to freezing temperatures. To fully understand how this coastal grass can protect from flooding, waves and erosion, we ask: How does the elastic modulus of American beachgrass vary on a seasonal time scale? Sampling locations were chosen at sites along the New Hampshire Seacoast where sediment overwash had been observed and a bi-monthly sampling procedure was established. For each sample, the elastic modulus of the lower stem was determined with a three-point bend test, and other plant properties (e.g. stem height, diameter) were measured. It was found that the elastic modulus of beachgrass in New Hampshire may vary by up to a factor of 2 throughout the year, indicating that plant flexibility changes significantly with the seasons. Thus, parameterizations of hydraulic resistance provided by coastal plants may need to incorporate seasonal variation, particularly in locations where a range of climatic conditions are observed.