Recruitment and demographic structure of floodplain tree species in the Queensland Murray-Darling basin, Australia

Globally, floodplain and riparian forests are rapidly declining ecosystems due to anthropogenic influences coupled with a changing climate. In the Murray-Darling Basin, Australia, little is known about the demographic structural status and recruitment of floodplain and riparian tree species. The aim of this study is to provide a baseline assessment of the current demographic structure of the ecosystem-defining tree species: River Red Gum (Eucalyptus camaldulensis), Coolibah (Eucalyptus coolabah), Black Box (Eucalyptus largiflorens) and River Cooba (Acacia stenophylla) within the basin in Queensland. Tree size data were collected from 131 plots of 0.25 ha located along major waterways and the abundance of individuals at the seedling, young tree, mature and senescent life cycle stages was determined. The diameter size distribution for the four species approximated a negative exponential distribution (reverse J-curve). Low tree abundance in the < 10-cm diameter class in all the species relative to the reverse J-curve suggested low recruitment and possible early evidence of a decline in the sustainability of the populations according to the Law of de Liocourt. This low abundance of young trees was more pronounced in Black Box and extended to the < 20-cm diameter class, but this species was not well replicated, and more data are required. In addition, the low abundance of recruitment trees occurred concurrently with the observed below average rainfall and erratic rainfall distribution in the last two decades relative to long-term totals. These results suggest a need for management interventions to foster seedling recruitment such as grazing management and tree planting programmes to ensure species establishment and survival along basin waterways.