flooding, fresh water drained slowly. This effect was 

 magnified at Station II after an event activating the 

 Rincon Overflow Channel, as a larger amount of water 

 affected this station as compared to the other stations. 

 It is important to note that B. maritima cover began to 

 increase following the composite hydrographic event 

 during the summer of 1999, as the winter months were 

 dry and the waterlogged conditions potentially 

 alleviated. At Station III, the vegetation zone began 

 above the mean water line and so was not flooded as 

 easily by channel water. Furthermore, water from 

 precipitation did not produce standing puddles that 

 could waterlog the soils. 



The decrease in cover oi Monanthocloe littoralis at the 

 Reference Station and Station II following the summer 

 1999 composite hydrographic event was most likely 

 due to flooding of the transects. During the time of 

 the floods, the transect soils were covered by thick, 

 green cyanobacterial mats. When the waters flooded 

 the transects, the mats were lifted from the soils; when 

 the waters retreated, the mats were left lying on top of 

 most of the vegetation. The thick mats most likely 

 caused decreases in plant cover by passively shading 

 the vegetation, making interception of light necessary 

 for photosynthesis almost impossible. Fortunately, 

 cover of both Batis maritima and M. littoralis began to 

 quickly recover after the event. In this instance, 

 although flooding initially served as a disturbance 

 resulting in decreases in adult plant cover, the 

 UkeHhood of long-term successful establishment might 

 not have been affected by the event. However, as 

 noted by AUison (1996), the recovery may depend 

 upon the presence of occasional freshwater inxmdation. 



Biomass values and R:S ratios did not show obvious 

 freshwater mediated responses. Previous studies have 



suggested that plants living in hypersaline soils invest 

 more energy into below-groimd tissues in order to 

 cover a larger area of soil to obtain water (BrugnoU and 

 Bjorkman 1992; Kuhn and Zedler 1997). However, 

 the physiological capacities of the plants that control 

 their salinity tolerance can vary with each species, and 

 some species may allocate different amounts of 

 biomass to the below-ground tissues depending on 

 their individual ability. Additionally, waterlogging of 

 soils may cause below-ground material to die in some 

 plants and not in others, thereby decreasing the 

 R:S ratio. The analyses were further complicated as a 

 need for increased nutrient uptake might also have 

 resulted in the plants increasing their below- ground 

 tissues, especially when increases in nutrients occurred 

 after precipitation or flooding events. 



SUMMARY 



The increase of fresh water into the channels and tidal 

 flats of the upper Nueces Delta positively impacted the 

 emergent vegetation by decreasing salinity, allowing for 

 annual seed reproduction, decreasing bare area and 

 increasing vegetative expansion of plant cover. 

 Although similar responses were seen at the Reference 

 Station following heavy precipitation, the effects were 

 accentuated at Station II, which was direcdy impacted 

 by flow through the Rincon Overflow Channel. The 

 project demonstrated the sensitivity of vegetation to 

 salinity levels and indicated that periodic freshwater 

 inundation could alleviate the stressful condition 

 imposed by hypersalinity. Without fresh water 

 flooding, soil salinity concentrations at several 

 locations in the study area would have likely increased 

 to toxic levels and resulted in plant mortality. 



6-48 



Vegetation Communities 



