Flow Variability and Vegetation Dynamics in a Large Arid Floodplain: Cooper Creek, Australia

Abstract

Throughout arid and semi-arid inland Australia, many extensive floodplains occur in association with rivers which are amongst the most hydrologically variable in the world. As rainfall in these areas is characteristically low and patchy, conditions in Australia's 'dryland' floodplains fluctuate unpredictably between extended periods of drought and huge floods that transform vast areas into wetlands, often for months at a time. Vegetation in these floodplains is commonly dominated by short grass and forb associations and patches of open succulent shrubland which are attributed with high ecological and socio-economic values due to their provision of habitat to a diverse array of terrestrial and aquatic fauna and their productive native pasture growth. In temperate and tropical floodplains, a substantial number of studies have shown that plant community composition and structure is determined primarily by flow and alterations to flow in these areas, through water extraction or river regulation, have resulted in many changes to the vegetation including loss of biodiversity and mass invasions of exotic species. Despite increasing pressure for water resource development in 'dryland' regions, relatively little is known regarding the effects of highly variable flows on the vegetation dynamics of arid floodplains, particularly in Australia. This thesis addresses this knowledge gap by examining the role of flow in the vegetation dynamics of a large arid floodplain in central Australia: the Cooper Creek floodplain. The effects of flow on plant community dynamics, from an organism level to that of the landscape, are examined across a range of spatial and temporal scales. Results are presented from a two year temporal vegetation survey during which time two flood pulse events of differing sizes occurred. A large-scale spatial survey was also conducted to determine the effects of flood history on spatial variation in plant community composition and structure. The composition of the soil seed bank and its contribution to vegetation dynamics were additionally investigated through a series of germination trials. Amongst common arid floodplain plants, life history traits that enable persistence under variable hydrological conditions were also considered via several experiments aimed at determining the effects of flow on the outcomes of various life history stages including germination, growth and dispersal. Throughout the study, results are presented for plant groups that were predefined on the basis of life form, life span and taxonomic divisions within these categories. Plant community composition and structure in the Cooper Creek floodplain exhibits significant shifts both temporally, in response to flood pulse wetting and drying, and spatially, in response to flood history. Flood pulse inundation has the potential to influence each life history stage across the range of plant groups present and the outcomes of these appear to be determined by hydrological attributes such as flood pulse timing, duration and rate of drawdown. Vegetation consequently exhibits gradual zonation on a gradient of flood frequency along which plant groups occur at predictable locations depending on their life history traits and recent hydrological conditions. A substantial proportion of species display ruderal life history traits including large, persistent soil seed banks and rapid life cycles which enable escape in time from the stresses associated with flooding and drought. These species, mostly comprising annual monocots and forbs, are widespread throughout the landscape and their presence in the extant vegetation is related primarily to the time since the last flood pulse event and the hydrological attributes of this. Perennial species, particularly shrubs, do not appear to rely similarly on the soil seed bank for recruitment and their distribution in the floodplain vegetation is likely to be determined more by their ability to tolerate either flooding or drought. Overall, this study demonstrates that flow, despite its variability, has an overriding influence on vegetation dynamics in the arid floodplain of the Cooper Creek. The spatial and temporal variability of flow maintains a heterogeneous mosaic of plant communities of differing composition and structure. Given this close relationship between flow and vegetation dynamics, anthropogenic alterations to flow are likely to result in changes to the vegetation including homogenisation of plant communities across the floodplain landscape and eventual loss of biodiversity.