INFLUENCE OF GRAZING ON BARRIER ISLAND VEGETATION AND GEOMORPHOLOGY, COASTAL NORTH CAROLINA

Shackleford Banks forms the southern end of Cape Lookout National Seashore on North Carolina’s Outer Banks. Although none of the Outer Banks is truly undeveloped, the southern barrier islands escaped the large-scale dune modification of the 1930’s and subsequent island-narrowing that occurred on the northern banks. In the past the Outer Banks were widely used for domestic animal grazing, but during the 1950’s animals were removed or contained in pens. The exception is the presently free-ranging herd of 100+ horses on Shackleford Banks. Shackleford’s SW wave and wind exposure affects its morphology, making geomorphic comparisons with the now-ungrazed, E-facing barriers inappropriate. Therefore, the impact of horse grazing on Shackleford Banks is difficult to discern, and leads to debate over management of the horse herd.

In an effort to shed light on grazing effects on barrier islands, we took advantage of an ongoing (albeit inadvertent) experiment at the northeast end of Cedar Island, North Carolina where two prograding barrier spits extend into southern Pamlico Sound. Overwash has produced active barrier migration here, implying that the spits are equivalent to scaled-down barrier beaches, complete with dune-colonizing grasses. The northwestern and southeastern spits have been separated for 45 years by a well-maintained fence. The southeastern spit is used as pasture for cattle and horses. Offshore profiles of the two spits are similar, but grazing has produced clear differences in vegetation and onshore morphologies. On the grazed spit, Ammophila and Spartina grasses are short (<25 cm), dunes are low (<1.5 m) or absent and washover fans are broad. The ungrazed spit comprises lush, tall dune and marsh grasses, dunes up to 3.5 m high and narrow, localized washover channels. Cedar Island demonstrates that grazing impacts the stability of barrier dunes and marshgrass shorelines. These impacts are relevant to Park Service management of barrier islands.