Vegetation changes in a large estuarine wetland subsequent to construction of floodgates: Hexham Swamp in the Lower Hunter Valley, New South Wales

Floodgates were constructed in 1971 on the main creek draining Hexham Swamp, a large wetland on the floodplain of the lower Hunter River, New South Wales. Substantial changes in vegetation have occurred in Hexham Swamp subsequent to the construction of the floodgates. Previous areas of mangroves and saltmarsh have been reduced (180ha to 11ha, and 681ha to 58ha, respectively), and Phragmites australis has expanded (170ha to 1005ha). Much of the mangrove loss (ca. 130ha) was a result of clearing, and the remainder has gradually died off. The factors contributing to the dieback are likely to be a combination of drying of the soil, root competition and, at times, waterlogging. Field sampling as well as microcosm and reciprocal transplant experiments involving key species, Sarcocornia quinqueflora, Sporobolus virginicus, Paspalum vaginatum and Phragmites australis, suggest that a reduction in soil salinity has been an important factor in initiating successional change from saltmarsh to Phragmites reedswamp. The data also suggest that increased waterlogging has been an important factor in initiating vegetation change. This apparently paradoxical result (floodgates and associated drainage generally result in drying of wetlands) is likely to have resulted from occlusion of drainage lines (by sediment and reeds) and is, therefore, likely to be a condition that developed gradually. That is, the initial effect of the floodgates is expected to have been a drying of the swamp, followed over time by an increasing wetness. An examination of vegetation changes after removal of cattle from part of Hexham Swamp, suggests that grazing had little effect on species composition of vegetation or rate of expansion of Phragmites australis. However, grazing does affect vegetation structure (height and density), possibly favours some coloniser species (e.g.;Sarcocornia quinqueflora) in particular environmental conditions, and possibly inhibits establishment of Phragmites australis.