for TBD and 0.14 m for BID). Thus, in order for tidal waters to wet the downstream marshes, 

 the tidal waters must be close to or already inundating the upstream marshes where no natural 

 levees exist or are poorly developed. TBD marshes are flooded more frequently than BID marshes 

 because the rate of tidal water rising is much greater in the TB system. The ingressing tide is 

 delayed in the BI system by the weir. Tidal lag becomes more apparent the further upstream in 

 the watershed where bayous become smaller, more sinuous, and more impeded by vegetation and 

 slow tidal egress. This normal tidal lag explains the differences observed between (TBU) and 

 (TBD). For example, difference in tidal peaks between Fearman Bayou tidal station and the 

 Chenier au Tigre tidal station (located near the end of the Mcllhenny Canal) varies from 2 to 4 

 hours, depending upon tidal strength. On the other hand, differences in tidal lows vary from 6 to 

 12 hours depending upon how much marsh inundation occurred-the more inundation, the longer 

 the difference. 



Plant composition and distribution 



The most abundant marsh type at the Rainey Sanctuary is the brackish marsh as defined by the 

 most abundant plant species, Spartina patens and Scirpus olneyi (Chabreck 1972). During cycles 

 of wet years, intermediate salinity marsh plants such as cattail (Typha sp.) and bull tongue 

 {Sagittaria lancifolia) become much more abundant. Marsh vegetation exhibits both gradational and 

 distinct lateral zonation depending upon elevation and site history. The most distinct zonation 

 occurs across well-developed natural levees along both Toms and Big Island Bayous. Roseau cane 

 (Phragmites phragmites) with mangrove (Iva frutescens) dominate the highest portions of the levee 

 adjacent to the bayou. Hog cane {Spartina cynosuroides) mixes with and eventually replaces the 

 Phragmites marshward as marsh elevation drops. Spartina patens mixes with and replaces the 

 Spartina cynosuroides as elevation approaches average marsh interior elevation. Juncus roemerianus 

 and Fimbristylis castanea are also locally abundant in this zone. These three vegetative zones are 

 subject to great variability in widths, depending upon how well- developed the bayou levee is. In 

 upstream areas without levees, very little zonation is apparent and Juncus is absent. 



In the marsh interior not influenced by natural levees, a mixed plant community of Spartina 

 patens and Scirpus olneyi is found. Species dominance in any one spot is dependent on relative 

 elevation and site history. Dominance of species other than Spartina patens and Scirpus olneyi is 

 unusual and usually limited to local topographic highs (Meeder 1987). Elevation across the marsh 

 is very uniform (difference usually less than 9 cm) but increases to 15 cm in areas of recent snow 

 goose or muskrat eatouts. Scirpus olneyi was almost always most common along "ridges" (+5 cm 

 above average), whereas Spartina patens was almost always most abundant in "swales" (-5 cm below 

 average elevation), along lake and bayou margins of interior marshes, and in areas unburned for 

 2 or more years. In the lowest spots, marsh soil continuity is often lost and Spartina patens is the 

 only plant found. The loss of the spreading root system of Scirpus olneyi, which binds soil, may 

 be a major factor in marsh surface degradation. In these cases Spartina forms very elevated clumps 

 (with roots exposed) which become progressively higher as organics are lost from between clumps. 

 These observations are based on more than 12.6 km of marsh survey and numerous site visits in 

 surrounding marshes. Relative elevation is believed to exert control over plant distribution because 

 relative elevation determines the hydroperiod, which has a manyfold relationship with plant 

 ecophysiology. 



In addition to elevation, Scirpus olneyi distribution was related to site history. Important aspects 

 of site history that greatly affect plant species composition, percentage of cover, and structure are 

 the frequency of burning, the type and intensity of grazing, and the grazing recovery state. For 

 example, adjacent marsh plots will have very different species composition after only 1 year of 

 burned and unburned treatments. Scirpus olneyi is favored under burned conditions (O'Neil 1949). 



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