Abstract
Tuart (Eucalyptus gomphocephala) is a woodland tree native to the Swan Coastal Plain of Western Australia supporting habitat for vulnerable fauna (e.g. western ringtail opossum) and threatened flora (e.g. Acacia benthamii). Stands have experienced a reduction in size from ~ 112,000 hectares in 1996 to ~ 30,000 hectares at present. The decline has raised concerns ranging from biodiversity and ecosystem functioning to cultural and economic values. The cause of the decline remains unknown, but factors currently under investigation include hydrology and water quality, fire and competition, pests and pathogens, environmental correlates, and nutrition. Research was conducted to investigate possible hydrologic causes of Tuart decline within Yalgorup National Park on the basis of three observed processes: a 15-20% reduction in winter rainfall since the 1970s, salinity rise in Lake Clifton (a lake within the park) beginning in the 1990s, and the 1993 construction of a channel linking the estuary to the Indian Ocean. ArcGIS was used to construct maps of the study area, which was composed of 24 Tuart sites characterized as healthy, intermediate, or unhealthy. State Department of Water (DoW) well bores and Australian Bureau of Meteorology (BoM) weather stations located near the study area were identified. Bore and precipitation data were then obtained from each organization’s online database. These data were used to identify trends in precipitation and bore-water levels to investigate the effects of the shift to a lower rainfall regime, channel construction, and salinity increase in Lake Clifton. This led to a proposed mechanism of salinity increase in Lake Clifton and the surrounding groundwater, which may or may not be linked to the observed decline in the Tuart population.