Understanding the characteristics of a groundwater-dependent ecosystem: Eucalyptus parramattensis Woodland at Porters Creek Wetland, NSW

Little is known about groundwater-dependent ecosystems in south-eastern Australia and few studies have examined the impact of landclearing upslope of such ecosystems. The eastern edge of Porters Creek wetland, near Warnervale on the Central Coast of New South Wales, supports a Eucalyptus parramattensis subsp. parramattensis (Parramatta Red Gum) population. The population has been listed as an Endangered Population (EP) and was showing signs of dieback during the 2002 to mid-2007 drought. Water table depth, soil moisture tension, soil temperature and groundwater salinity beneath this woodland were measured to investigate whether the woodland is associated with a shallow aquifer and/or high soil moisture conditions, and whether an upslope construction development would impact on the ecosystem. The composition, structure and health of the Eucalyptus parramattensis Woodland was also examined. Monitoring over 5 years demonstrated that Eucalyptus parramattensis is associated with a shallow, slightly saline to saline (8–12 ppt) water table, 1.4–3.1 m beneath the surface of a drainage depression wetland. During the drought water table drawdown rates were quite rapid, and rainfall events took up to a month to replenish water table levels andclearing for an upslope development significantly decreased the average water table level by 0.3–0.5 m during the construction phase, while peaks that occurred at the base of the slope after large rainfall events reversed the normal direction of the water table gradient. The water table re-stabilised to pre-construction levels after construction was completed and the area was landscaped and revegetated with grass. The short-term alteration of groundwater conditions did not appear to impact on tree health; the poor tree health observed prior to construction was most likely due to the lowering of the water table (and/or an increase in groundwater/soil salinity) during the drought years. Our results suggest that developments that revegetate slopes above wetlands as quickly as possible after clearing, or undertake clearing in stages, will minimise effects on water table levels. Pervious detention basins designed to capture runoff and allow infiltration into the soil help replenish aquifers, but also require weed maintenance. This study highlights the significance of maintaining shallow aquifer and catchment processes for the conservation and rehabilitation of coastal wetlands, many of which are listed as Endangered Populations or Endangered Ecological Communities.