Open Access

Peter Smith, Judy Smith

• As part of a wider study of floodplain vegetation along the River Murray, we carried out a field survey in 1987–1988 involving collection of floristic and vegetation condition data from 335 sample plots (each 400 m2 in area), between Hume Dam and Lake Alexandrina (including the Edward-Wakool anabranch system). The floodplain vegetation is dominated by just two tree species, River Red Gum (Eucalyptus camaldulensis) and Black Box (Eucalyptus largiflorens), but the composition of the understorey shows much greater variation, both along the river and across the floodplain. A total of 499 plant species, subspecies and varieties were recorded from the survey plots, of which 316 (63%) were native and 183 (37%) were exotic. From analysis of the floristic data we identified 37 vegetation communities, not including the vegetation of permanent wetlands and cleared areas; 21 communities were distinguished in the River Red Gum zone, 12 communities in the Black Box zone, and 4 communities on rises within the floodplain. The main floristic division among the River Red Gum communities was between Riverine Plain/ Headwaters Zone communities of the upper Murray, and Mallee Zone communities of the lower Murray. Among the Black Box communities, the main floristic division was between inner floodplain communities and outer floodplain communities, with a further division between South Australian communities and New South Wales/Victorian communities. Major factors influencing the floristic patterns included flooding frequency/duration and soil salinity. Eucalypt health declined steadily downstream and was poorest in the lower reaches of the river below the Darling Junction, where 60% of the trees were healthy, 18% unhealthy (at least 40% of the canopy dead) and 22% dead. By comparison, at the upper end of the river, above Tocumwal, 84% of the trees were healthy, 14% unhealthy and only 2% dead. Overall, the condition of Black Box trees (44% unhealthy or dead) was worse than the condition of River Red Gum trees (29% unhealthy or dead). Eucaypt regeneration was also poorest below the Darling Junction, with regenerants present in 77% of plots upstream of the Darling but only 35% of plots downstream. The findings of poor tree health and sparse regeneration below the Darling coincide with the most heavily regulated part of the Murray, where the reduction in flooding due to upstream storages and water extraction, mainly for irrigation, has been greatest. Black Box regeneration was much sparser overall than River Red Gum regeneration (regenerants present in 69% of River Red Gum plots but only 29% of Black Box plots). The poor condition of the Black Box trees, coupled with their poor regeneration, suggests that the long-term future of this species along the Murray, particularly below the Darling Junction, is tenuous, even though it is a dominant component of the vegetation. The integrity of floodplain vegetation along the Murray has been severely compromised by weed invasion. Weeds were common throughout the survey area, but were most prevalent in the climatically wetter sections of the river, at both the downstream and upstream ends (below Mannum and above Tocumwal). The median number of exotic species per plot equalled or exceeded the number of native species in these sections of the river, whereas native species outnumbered exotic species in the other river sections. Communities of the River Red Gum zone and the rises were generally weedier than those of the Black Box zone. Exotic species strongly influenced the community classification. They were the dominant overstorey species in two communities (Salix species – willows) and outnumbered native species in the understorey of another eight communities. At the lower end of the river, below Mannum, the River Red Gums that originally fringed the river had been mostly replaced by dense thickets of the exotic Weeping Willow, Salix babylonica. Other factors that have impacted on the floodplain vegetation at the plant community level have been river regulation and soil salinisation. Stabilization of water levels in the lower Murray by construction of a series of weirs and barrages has favoured the spread of some communities at the expense of others. The favoured communities, which are characterised by stands of Common Reed, Phragmites australis, along the water’s edge, appear to be artefacts of river regulation. Salinisation has resulted in death of eucalypts and replacement of eucalypt communities by shrub communities dominated by samphires, Tecticornia species. The samphire community characteristic of the most saline sites is one of the most species-poor communities on the Murray floodplain. Logging along the Murray in New South Wales and Victoria has resulted in extensive replacement of old growth River Red Gum forests and woodlands by more even-aged stands of straight young trees. Following the recent conversion of many areas of State Forest along the Murray in both New South Wales and Victoria to National Park or Regional Park, and thus the cessation of logging in these areas, they should now revert gradually to mature forest and woodland. This study is the first to describe broad scale floristic patterns in the floodplain vegetation of the Murray covering most of the length of the river. It also provides data on the vegetation condition in the 1980s, and provides a benchmark of conditions before the prolonged Millenium Drought in south-eastern Australia from 1997 to 2010. More recent surveys of vegetation condition have reported a severe decline in tree health during the drought. The results from our 1987–88 survey are important because they show that the deteriorating condition of the vegetation was already evident in the 1980s and although exacerbated by the subsequent drought, it is not just a consequence of that drought. The results are consistent with the conclusion that the primary cause of the decline has been river regulation and water extraction for irrigation. The rate of deterioration has increased rapidly since the 1980s because of the drought. There has been some improvement since the breaking of the drought, but the poor condition of the River Murray floodplain vegetation, an Australian icon, remains a major conservation and management issue. The impacts of climate change – higher temperatures and reduced rainfall – have compounded the problem and will continue to do so at an increasing rate. The results of the study support listing of the floodplain vegetation of the lower reaches of the river as a critically endangered ecological community.