For each transect, we measured elevation and distance from a benchmark using a rod and 

 level. Elevations were surveyed wherever there was a noticeable break in slope or change in 

 species. The average distance between points was about 7.5 m (25 ft). Along each transect we 

 collected and tagged samples of species for laboratory typing and verification, noting such 

 information as the elevation of the boundaries between different species. By measuring the 

 length of the transect that a species covered and dividing it by the transect 's total length, we 

 computed percentages for the distribution of each species along a transect. 



Results of Individual Transects 



Table 2-1 (see page 44) summarizes the results of the twelve transects. 2 It presents the 

 principal species observed along each transect, their "modal"— or most common— elevations, the 

 percentage of each transect they covered, and the length of each transect. For example, in 

 transect number 6, Borrichia frutescens was found at a modal elevation of 118 cm (3.86 ft) and 

 covered 40 percent of the transect, or about 37 m (120 ft). 



Because species often overlapped, the sums of the percentages exceed 100. In addition, to 

 omit any marginal plants that exist at transition zones, a modal elevation differs slightly from the 

 arithmetic or weighted mean. 



Composite Transect 



To model the scenarios of future sea level rise, we had to develop a composite transect from 

 the data in Table 2-1. Thus, for each species, one modal elevation was estimated from the various 

 elevations in Table 2-1. Similarly, the percent of each transect covered by an individual species 

 was used to estimate an average percent coverage for all transects (Table 2-2, p. 45). 



This information allowed us to choose for our composite the five species that dominated the 

 high and low marshes in all the transects: Spartina alterniflora, Salicomia virginica, Limonium 

 carolinianum, Distichlis spicata, and Borrichia frutescens. We call these the indicator species. 

 Figure 24 shows the modal elevations for these five species, for two other salt-tolerant plants 

 found in the transects {/uncus roemerianus and Spartina patens), and for a species found in tidal 

 flats and under water (Crassostrea virginica). The primary zone where each species occurs is 

 indicated by the shaded area; occasional species occurrence outside the primary zone is 

 indicated by the unshaded, dashed-line boxes. Figure 24 also outlines the boundaries for the six 

 habitats and indicates the estimated percentage of the study area that each covers. 



FIGURE 2-4 



COMPOSITE TRANSECT— CHARLESTON. S.C. 



Highland 477< 



O 



< 

 > 



Tidal Flat 6% 



Water 27% 



- 10-YR STORM 



-PEAK YEARLY TIDE 



- SPRING HIGH WATER 



- MEAN HIGH WATER 

 -NEAP HIGH WATER 



-MEAN SEA LEVEL 



- MEAN LOW WATER 

 ^SPRING LOW WATER 



Spartina Alterniflora (2.4) 



♦ 10 



♦8 



♦6 



- +4 



♦ 2 







-2 

 -4 



■ -6 



1000 



2000 3000 



TYPICAL DISTANCE (FT ) 



4000 



5000 



Composite wetlands transect for Charleston illustrating the approximate percent occurrence 

 and modal elevation for key indicator species or habitats based on results of 12 surveyed 

 transects. Minor species have been omitted. Elevations are with respect to NGVD, which is 

 about 15 cm lower than current sea level. Current tidal ranges are shown at right. 



43 



