IMPACT OF COW GRAZING ON BARRIER ISLAND GEOMORPHOLOGY: VEGETATION AND TOPOGRAPHY SURVEYS OF THE NORTH BAY BARRIER, NORTH CAROLINA

The North Bay barrier is a 125 m-wide, microtidal barrier spit/island complex at the south end of Pamlico Sound. Shallow bays and marshes separate the 10 km-long barrier from the Cedar Island uplands. Many of its features suggest that the barrier is a scaled-down analogue of open-ocean barrier islands. A number of human alterations, including jetties, sand mining and beach buggy traffic have affected the North Bay barrier since installment of the Hwy 12 ferry terminal in the mid-1960’s. The present study examines the effects of cow grazing (25-80 head) on one limb of the barrier. The North Bay barrier is ideal for studying the large-scale geomorphic impacts of grazing because a well-maintained, shore-normal fence has bisected the barrier since the ferry service began. Whereas other human activities affect the entire barrier, the fence segregates the cows to the southeastern half, leaving the northwestern half as a ‘control’ site.

We measured topographic transects in both areas using a laser EDM-equipped total surveying station. We also measured vegetation heights, analyzed weights of aboveground biomass per unit area, and collected samples for grain-size analysis. Ungrazed vegetation heights range from 1 – 118 cm, mean = 31.1 +/- 30.5 cm, n = 44 (all standard deviations are 1-sigma; trees and unvegetated sites are excluded here); grazed vegetation heights are 1 – 45 cm, mean = 7.8 +/- 7.8 cm (n=51). Ungrazed aboveground biomass weights range from 0.05 – 301.20 g/m^2, mean = 100.26 +/- 124.35 g/m^2 (n = 13), vs. grazed biomass values from 0.40 – 50.40 g/m^2, mean = 15.00 +/- 12.93 g/m^2 (n = 13). The lack of tall dune-colonizing grasses in grazed areas likely accounts for measured differences in subaerial topography: ungrazed mean elevation above water is 0.99 +/-0.40 m, ranging from 0.43 – 2.29 m, n = 102 points; vs. mean grazed elevation = 0.99 +/- 0.25 m, range 0.41 – 1.56 m, n = 115. Although the mean above-water elevations are virtually the same, there is more overall relief in the ungrazed area. The differences in relief due to grazing alter the style of storm overwash, such that in the grazed area evidence of washovers occurs in broad swaths, whereas narrow inter-dune channels confine overwash in the ungrazed area.