Ecology and Conservation Significance of Alkaline Wetlands in LUTRUWITA/Tasmania, Australia

Abstract

Groundwater-dependent wetlands with alkaline water chemistry are typically associated with carbonate rocks such as limestone and dolomite and often display distinctive landforms, soils and biota. A form of such a wetland type occurs in association with alkaline springs and seepages of inland superhumid, maritime southwest Lutruwita/Tasmania. Unlike commonly described calcareous and alkaline wetland types, the Tasmanian examples are characterised by low biomass and minimal organic soil development, despite occurring within landscapes otherwise dominated by ombrogenous blanket peatland. These peat-bound karstic wetlands are rare, include distinctive geomorphic features and processes and support both a geographically restricted endemic vascular plant flora and living freshwater stromatolites. Collectively these characteristics contribute to the natural values of the Tasmanian Wilderness World Heritage Area. Knowledge about peat-bound karstic wetlands and their vulnerability to changing fire regimes and climate remains limited. The ecosystem processes that inhibit peat accumulation and prevent woody plant invasion are poorly known but essential for the persistence of this rare ecosystem. Here we examine the distribution and morphology of peat-bound karstic wetlands and present results of nine years of vascular plant monitoring. We found peat-bound karstic wetlands are restricted to locations where calcium rich alkaline groundwater discharge is sufficient to prevent neutralisation by acidic surface water. They are lotic wetlands that have a frequent but intermittent (day-month) shallow flow and comprise poorly developed stream beds, associated alluvial deposits and pools. Vegetation along permanent transects showed either little change or minor increases in the abundance of acidic peatland species. Ad hoc observations and aerial imagery suggest that these wetlands are subject to infilling and colonisation by species from adjacent vegetation over longer time scales. Fire frequency may restrict plant growth and woody plant colonisation, while dissolution and flushing of organic matter by flow of alkaline groundwater may be important in preventing peat accumulation.