Using stable isotopes to identify soil moisture sources of key species in riparian woodlands of the northern Murray-Darling Basin

Natural abundance stable isotopes have been used to investigate tree-water relations and interactions between trees and groundcover vegetation in seasonally water-stressed environments. This approach will be implemented to identify water sources utilised by the phreatic dominant canopy species Eucalyptus camaldulensis (river red gum) and the invasive alien groundcover species Phyla canescens (lippia) under low ambient soil moisture conditions in riparian woodlands of the northern Murray-Darling Basin. Lippia is a low-growing clonal species, readily dispersed by flooding and capable of dominating groundcover vegetation under mesic conditions. Under drought conditions in southern Queensland, it persists in drought refugia beneath mature E. camaldulensis tree canopies. Trees with access to shallow groundwater resources play a critical role in mediating drought impacts on groundcover vegetation in this landscape; however, these trees also facilitate lippia abundance and reproduction, contributing to increased ecological risk. High lippia cover under trees may also exacerbate significant dieback evident in these eucalypts in response to land use intensification, signalling complex degradation processes in these woodlands. This research asks about the role of interactions between these two key structurally and functionally dominant species in determining access to scarce water resources in this landscape. The study will help to clarify the extent of variation in the isotopic signatures of source waters in this landscape, and the role of shallow groundwater resources in ecosystems which are subject to seasonal and long-term drought, and potentially at risk from high levels of groundwater extraction and/or predicted climate change.