Analysis of tidal marsh vegetation patterns in two Georgia estuaries using aerial photography and GIS |
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Authors: | Carrie B Higinbotham Merryl Alber Alice G Chalmers |
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Institution: | (1) Department of Biology and Biochemistry, University of Houston, Houston, TX 77204, USA;(2) School of Public and Environmental Affairs, Indiana University, Bloomington, IN 47405, USA; |
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Abstract: | Aerial photographs and GIS analysis were used to map the distribution of tidal marsh vegetation along the salinity gradients
of the estuaries of the Altamaha and Satilla Rivers in coastal Georgia. Vegetation maps were constructed from 1993 U.S. Geological
Survey Digital Orthophoto Quarter Quads, 1∶77,000-scale color infrared photographs taken in 1974 and 1∶24,000-scale black
and white photographs taken in 1953, Changes between years were identified using a GIS overlay analysis. Four vegetation classifications
were identified and groundtruthed with field surveys: salt marsh (areas containing primarilySpartina alterniflora), brackish marsh (Spartina cynosuroides andS. alterniflora), Juncus (Juncus roemerianus), and fresh marsh (Zizania aquatica, Zizaniopsis miliacae, and others). There was no evidence for an upstream shift in marsh vegetation along the longitudinal axis of either estuary
over the time frame of this analysis, which implies there has not been a long-term increase in salinity. Although the inland
extent of each marsh zone was further upstream in the Satilla than the Altamaha, they corresponded to similar average high
tide salinities in each estuary: areas classified as salt marsh occurred from the mouth up to where average high tide salinity
in the water was approximately 15 psu;Juncus ranged from 21 to 1 psu; brackish marsh ranged from 15 to 1 psu; and fresh marsh was upstream of 1 psu. Approximately 63%
of the 6,786 ha of tidal marsh vegetation mapped in the Altamaha and 75% of the 10,220 ha mapped in the Satilla remained the
same in all 3 yr.Juncus was the dominant classification in the intermediate regions of both estuaries, and shifts between areas classified asJuncus and either brackish or salt marsh constituted the primary vegetation change between 1953 and 1993 (87% of the changes observed
in the Altamaha and 95% of those in the Satilla). This analysis suggests that the broad distribution of tidal marsh vegetation
along these two estuaries is driven by salinity, but that at the local scale these are dynamic systems with a larger number
of factors affecting the frequently changing borders of vegetation patches. |
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