The importance of seasonal flow timing for riparian vegetation dynamics

© 2012 Dr. Joe GreetNaturally variable river flows are considered integral to sustaining the ecological integrity of river systems including riparian vegetation. However, many of the world’s rivers do no flow naturally; instead their flows are regulated to meet human needs. Flow regulation for the purpose of providing water for irrigation typically results in changes to seasonal flow patterns. Changes to the natural flow regime will affect riparian vegetation. However, the importance of seasonal flow timing for riparian vegetation dynamics is not well known, this thesis investigates its importance. I surveyed the extant riparian vegetation, soil seed banks and water-borne seed dispersal (hydrochory) along five rivers in south-eastern Australia, subject to varying degrees of altered seasonal flow timing (regulation). Soil seed banks and sods were also collected from the riparian zones of these rivers and subject to inundation experiments to assess the effects of timing of inundation on germination, growth and reproduction. Following a large spring flood in the study area, the extant riparian vegetation of the rivers was resurveyed. Altered flow timing was associated with more exotic taxa and fewer native taxa in the extant vegetation. In particular, greater numbers of short-lived exotic terrestrial taxa and exotic grass taxa, and fewer native woody taxa, were associated with increasing level of regulation. Evidence for mechanisms explaining these vegetation patterns were provided by my experiments. Exotic grass taxa were favoured by conditions that mimicked the flow patterns of the regulated rivers: they germinated more frequently in autumn than in spring, and flowered in greater numbers when inundation was delayed from spring to summer. In contrast, native amphibious forbs (particularly annuals) were observed to germinate more frequently in spring and to be favoured by inundation in early spring, conditions which mimic the natural flow regimes of the study area. Hydrochory surveys suggested that natural flow peaks are important for the recruitment of native woody taxa, and the scarcity of such taxa in the regulated systems is likely to be a result of the altered flow regimes not providing for their recruitment. Post-flood vegetation surveys highlighted the importance of spring flow peaks for reducing the extent of exotic taxa within the riparian zone. The studies in this thesis provide evidence that natural seasonal flow timing is important for maintaining native riverbank vegetation and for curbing the extent of exotic taxa within the riparian zone. More specifically, changes to the timing of flow peaks promote or disadvantage plant species depending on their life-history with natural timing of flow peaks most important for native riparian plant species adapted to natural flow regimes. In temperate zones, winter/spring flow peaks are particularly important for sustaining native plant species with life-histories synchronized with seasonal flow events. This thesis contributes to our understanding of the importance of flow timing for riparian vegetation dynamics, knowledge that is amenable to the improved management and restoration of regulated river systems