Two major aspects of tidal inundation and exposure, i.e., frequency and duration, were examined in relation to the abrupt demarcation between adjacent monotypic zones of Spartina alterniflora and Juncus roemerianus of salt marsh in Davis Bay, Mississippi. The marsh surface
in the zone of S. alterniflora has a range in elevation from 0.24 m below mean low water (MLW) to 0.54 m above MLW. The J. roemerianus zone ranges from 0.54 m to 0.75 m above MLW. Spartina alterniflora occupies a seaward area on the tidal plane that spans an average of
0.76 m, an elevational spread three times that occupied by J. roemerianus (0.21 m). Analysis of annual tidal cycles for 1975 indicated that the entire zone of S. alterniflora is flooded much more frequently (139 times) than the entire J. roemerianus zone (16 times). Partial
flooding occurs more often. Most (78%–98%) of the high tides occur within the lower three-fourths of the S. alterniflora zone. Correspondingly, the lower three-fourths of the S. alterniflora zone was flooded from 35–87% (hours innundated: 3,045 to 7,645) of annual
total (8,750) in comparison to 10% (855 h) for the upper one-fourth. The zone of J. roemerianus is flooded from 0.8 to 5% (75 to 475 h) of the annual total, clearly indicating that this habitat is exposed most of the time. While major portions of the two monotypic stands have extremely
different tidal relationships, frequency and duration of tidal flooding could not be related to the line of abrupt delineation between the plant zones, suggesting that factors other than the tide are involved.
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