Vegetation resilience to mega-drought along a typical floodplain gradient of the southern Murray-Darling Basin, Australia

Abstract

'Questions': How diverse and resilient is vegetation following a decade of extreme drought along a typical floodplain gradient of semi-arid south-eastern Australia? How do mechanisms of resilience (i.e. persistence and soil seed banks) vary between major plant groups and spatially with respect to habitat type and position along a flood frequency gradient? 'Location': Southern Murray-Darling Basin, Australia. 'Methods': We surveyed understorey vegetation and conducted germination trials to examine responses to re-wetting from soil seed banks of seven major habitat types along a typical floodplain gradient of the southern Murray-Darling Basin. We assessed abundance, species richness, functional diversity and composition, including exotic species, within and between extant and germinating assemblages. 'Results': Understorey vegetation was surprisingly diverse, although low in cover, following a decade of extreme drought, with considerable numbers of plant species (61) and functional plant groups represented. Historically drier habitats, towards floodplain margins, had higher species richness and cover overall and for exotic species. Plant assemblages exhibited high heterogeneity between habitats. Soil seed banks were very dissimilar from extant vegetation, comprising mainly amphibious and damp taxa as well as some terrestrial herbs, mostly annuals. Seed banks were most abundant and diverse in intermediate floodplain habitats, and their composition was very distinct between habitat types. 'Conclusions': Semi-arid floodplain vegetation is likely to be highly resilient to prolonged drought. Plants persisting under dry conditions do not appear to rely on local soil seed banks for regeneration and may either tolerate drying in situ or arrive from neighbouring ecosystems. Soil seed banks allow understorey vegetation to respond to re-wetting, especially in intermediate floodplain habitats. Lake bed assemblages and aquatic/amphibious species lacking soil seed banks appear most vulnerable to drought. Vegetation resilience is promoted by landscape heterogeneity.