HORSE GRAZING EFFECTS ON AMERICAN BEACHGRASS ANDGEOMORPHOLOGICAL CHANGE AT ASSATEAGUE ISLAND NATIONAL SEASHORE,U.S.A

摘要

Horse grazing on American beachgrass and dune development are examined in six pairsof experimental plots using 1997, 1999, and 2000 NASA lidar surveys, 1993 and 1998 colorinfraredphotography, and 2001 field work. The study site, Assateague Island, is a 57-km-longbarrier island located along the Atlantic coast of Maryland and Virginia. During the seventeenthcentury, non-native horses were introduced onto the island and have remained free-roaming onAssateague Island National Seashore (ASIS) since 1965. A principal staple in the ASIS feralhorse diet is American beachgrass, a hardy pioneer dune-building plant which is vital to naturaldune development at ASIS.Changes in topography and vegetation cover within experimental plot pairs areinvestigated using NASA ATM lidar to map topographic change and color-infrared photographyand field work to compare vegetation cover. Each plot pair consists of identical 15- x 20-m plots:an unfenced plot permitting horse grazing and a fenced plot restricting horse grazing. The sixplot pairs were established in areas of similar vegetation cover and topography along the ASIScoast in 1994. Preliminary examination of the color-infrared photography and field data indicatessignificant vegetation cover within the fenced experimental plots. Elevation profiles of the plotpairs created from lidar surveys display a 1- to 2-m topographic variance between fenced andunfenced plots. Preliminary examination of these data indicates dune development occurs withinthe exclosures due to the increased vegetation cover and resulting dune stabilization, suggestingthe disturbance of American beachgrass by feral horse grazing disrupts natural dune formationand stabilization.