date but were dried up by June 2000). In general, 1999 

 was a relatively wet year, and the moisture might have 

 attributed to the survival of the mats. The persistence 

 of the mats for an extended time could explain the 

 unusually high soil NH^^ levels. Unfortunately, the 

 high levels were not seen until the end of the study 

 period, and any subsequent positive effects on the 

 vegetation would hav^e taken several months to 

 mamfest. 



Vegetation 



The most significant vegetation changes seen as a 

 result of hydrographic events through the channels 

 were the germination and establishment of the annual 

 succulent Salicomia bigelovii. The relative success of this 

 plant appears to provide relevant information 

 regarding the timing and quantity of fresh water 

 needed to promote sexual (seed) coloni2ation in 

 hypersahne salt marshes. Because the species is an 

 armual and reproduces primarily by seeds, successful 

 establishment can only occur if soil salinity 

 concentrations are reduced to a level that alleviates the 

 osmotically induced seed dormancy. 



In salt marsh habitats, the ability of halophytes to 

 reproduce sexually (seeds) is critical for the success of 

 plant populations (Ungar 1991 and Allison 1996). 

 While vegetative growth can expand plant cover in a 

 particular area, seeds are necessary for establishment at 

 locations distant from neighbors. Seeds also allow for 

 reestabUshment following disturbance events like 

 flooding, drought or burial that oftentimes result in 

 adult plant mortality. Quick recovery is only possible 

 through seeds, which can withstand the disturbance by 

 remaining dormant and then germinating afterwards. 



In this study, the timing of soil sahnity reduction was 

 found to be critical. Hydrographic events which 

 lowered soil salinity values during the late fall/early 

 winter allowed the successful germination and 

 establishment oi Salicomia bigelovii during the spring and 

 summer, independent of whether or not the 

 intermediate seasons were wet or dry. The degree of 

 survival was most likely a fiinction of air temperatures 

 and evaporation rates, which were relatively low in the 



winter compared to summer and did not increase as 

 rapidly. VCTien summer approached and soil salinity 

 increased, the seedlings were large enough to withstand 

 the increasing salinity concentrations. If an event 

 occurred in the late spring or summer, the seeds might 

 have germinated, but unless the soU salinity was kept 

 suppressed by sequential diluting events, the seedlings 

 would not sunave and establishment likely failed. 



Flooding events through the Rincon Overflow 

 Channel occurring in the late fall significandy increased 

 the number of established plants seen the following 

 spring compared to the number seen after only a 

 precipitation event. In June 1999, Salicomia bigelovii 

 cover at Station II (52%) was 26% greater than that 

 seen at the Reference Station (26%), while cover in 

 June 1998 was approximately the same at the two 

 stations (11 to 13%). Heavy rains occurred during fall 



1997 with no flow event, hence the approximately 

 equal cover at the two stations. While m fall 1998, 

 heavy rains and a flow event occurred through the 

 Rincon Overflow Channel, possibly leading to a 

 doubling in cover at Station II compared to the 

 Reference Station. The flooding of Station II was 

 reflected in the soil salinity levels, which were 6 to 

 20 ppt. These values are 1 to 11 ppt lower than the 

 Reference Station and Station III, respectively, 

 immediately following the flow event. 



The lack of a significant difference between Salicomia 

 bigelovii covet at Station III between June 1998 and 

 June 1999 indicated that diverted fresh water in the 

 channels might not have affected more than just the 

 lower (adjacent) portions of the transect at Station III. 

 Most likely, during a major event such as the October 



1998 event, most flow diverted through the Nueces 

 Overflow Channel backed up at the intersection of the 

 upper and central segments of Rincon Bayou by the 

 private road crossing, and a majority of this volume 

 likely passed through the Rincon Overflow Channel. 

 Some amount of fresh water reached Station III, as 

 open water salinity was 8 to 1 1 ppt lower than the 

 other two stations following the October 1998 event, 

 but this water likely remained channelized. However, 

 open water salinity values were 8 to 9 ppt and 5 to 



6 ppt higher following the July 1997 and September 



1999 events, respectively. The higher salinity values. 



6-46 



Vegetation Communities 



