Vegetation

The pattern and distribution of individual plant species and plant associations on the Cooper floodplain is primarily influenced by hydrology. This was acknowledged by Mollenmans et al. (1984) who correlated their defined flood classes (above) with vegetation types represented on the floodplain. They produced the following very general description:

 Tall woodland occurs near major channels that are annually filled by average flows;

 Woodland and tall shrubland occurs in less frequently inundated areas (Level 1 Flood);

 Tall to low shrubland (with or without emergent stunted Coolibah) occurs in even less frequently inundated areas (Level 2 Flood); and finally

 Forbland/herbland is found in least frequently inundated areas (as influenced, although perhaps insignificantly, by Level 3 Floods).

This very simplified zonal description effectively acknowledges the influence of a definite moisture gradient from the more mesic channel to xeric outer floodplain conditions. However the authors also recognized that an interplay of additional factors added further complexity to the over-simplified descriptions above. The interplay of hydrology, soils and floodplain micro- topography creates a far more complex picture. The end result is a distinct spatial heterogeneity expressed as a mosaic of vegetation communities of differing patch size and shape across the landscape.

Thus, distribution of the range of perennial species encountered upon the Cooper Creek floodplain may be determined by permutations of hydrology, soils and micro-topography interacting laterally and longitudinally within the region. These permutations result in a myriad of niches that may suit one or a mix of species. The more reliably watered parts of the system contain the most structurally and floristically diverse expressions of the region‟s flora. Along channel levee banks and particularly adjacent to more permanent waters, such as the „perennial‟ waterholes of the Cooper, are extensive stands of Eucalyptus camaldulensis var obtusa / E. coolabah woodland, often associated with an understorey of Melaleuca trichostachya,Acacia stenophylla, A. salicina and Bauhinia gilva. This riverine association occurs on the deeper coarser sands and silty clay loams of channel levee banks. In places the channel becomes discontinuous. This may create either a braided channel system of swampy conditions, usually dominated by Muehlenbeckia florulenta shrubland, or result in a broadening and shallowing creating a vast floodout of Sporobolus mitchellii grassland before reconsolidating downstream into a clearly defined channel.

Progressing longitudinally down the Cooper into South Australia, as channels receive less reliable floods and saline conditions increase, there is a gradual loss of those species requiring

more mesic conditions. A typical sequence of species loss is revealed as the Cooper flows from the Queensland border to the Coongie Lakes system in South Australia. Initially Melaleuca trichostachya drops out, to be gradually and sequentially followed by the loss of Eucalyptus camaldulensis and then Bauhinia gilva to the north of the lake system. This longitudinal shift is also mirrored by a lateral shift in species across the floodplain from well watered channel to rarely watered floodplain margin (Plate 2-10. A typical lateral sequence may involve a shift from Eucalyptus camaldulensis / E. coolabah riverine fringe, through a band of E. coolabah

woodland with an understorey of Muehlenbeckia florulenta, down through a swampy depression of Chenopodium auricomum, rising through a fringe of Atriplex nummularia to end in a shrubland of halophytic Tecticornia indica at the outer margin of the floodplain. In the south of the study area as the main channel loses definition and floodwaters expand across extended floodouts the Eucalyptus coolabah riverine fringe, previously confined to channel banks, expands across the floodouts creating extended elongated woodlands. These woodlands may be accompanied by a dense Muehlenbeckia florulenta understorey, given appropriate flooding regimes.

Muehlenbeckia florulenta can also occur as vast monospecific stands on the deep heavy cracking grey clays of the Cooper floodplain in frequently or regularly flooded regions of the floodplain system. The best expressions are found in the major swamp systems of the region in some of the major topographical depressions of the region such a Tirrawarra and Embarka swamps in South Australia.

Monospecific stands of Atriplex nummularia can also be found widely scattered throughout the system but most particularly on low sandy rises at the periphery of the floodplain subject to rare flooding events.

Chenopodium auricomum also occurs widely as monospecific stands in low lying depressions across the floodplain on deep cracking grey clays subject to periodic waterlogging.

Sporobolus mitchellii low open to low sparse tussock grassland is widespread throughout the floodplain system in South Australia. It occurs most typically as a band on the upper margins of lakes on a slight sandy veneer over heavy cracking grey clays, and as extensive monospecific grasslands across the beds of shallow lakes. Sporobolus can also occur as part of a mix on lakes shores in association with Cyperus gymnocaulos and Tecticornia indica in the less regularly flooded areas of the floodplain. In the rarely flooded regions, Sporobolus may grade abruptly into a monospecific stand of Cressa cretica (rosinweed) across shallow lake beds.

The perennial vegetation encountered around many lake fringes is particularly interesting. In these situations a definite zonation is evident. This is expressed clearly in the more intermittently filled lakes within the study area. When dry, these reveal a marked sequential shift in species from lake bed to surrounding lake fringe. Along this gently rising gradient soil varies from self mulching fine textured clays within the lake bed through to a sandy veneer on

clay on the lake margin to deeper coarser sands at the base of surrounding dunes. The vegetation from lake bed to dune base typically follows the sequence; Cressa cretica, Sporobolus mitchellii, Cyperus gymnocaulos to Eucalyptus coolabah.

The productivity of the floodplain environment of the study area is clearly evident in the floristic diversity recorded from the region. Research within the Coongie lakes district alone has recorded approximately 350 plant species, highly significant for a district flora in the arid zone (Mollenmans et al., 1984, Gillen and Drewien, 1993).

Plate 2-1 Aerial view of the North West Branch of Cooper Creek