Native ecosystems in production landscapes are subject to altered disturbance regimes and degradation pressures which potentially impact their composition, structure, ecological function and resilience to future disturbance. This is particularly true of remnant Eucalyptus camaldulensis/E. tereticornis riparian woodlands in irrigated cropping areas on the Upper Condamine floodplain, at the headwaters of the Murray-Darling Basin. The Upper Condamine floodplain forms part of the Darling Downs agricultural landscape in which major land and water use change has occurred since colonisation. Riparian woodlands on the floodplain exhibit poor tree condition and significant populations of the invasive groundcover weed, lippia (Phyla canescens). This study investigated links between landscape pattern, hydrology and management, and the condition of trees and composition of riparian woodland ecosystems to identify key patterns and drivers of ecological condition and function and better understand the role of altered disturbance regimes in these systems.
We developed a resilience-based state-and-transition model that provides insight into the potential for persistence of riparian E. camaldulensis/E. tereticornis woodland ecosystems on the floodplain. Significant transitions identified include the historical shift from a pre-invasion lippia-free landscape to one in which lippia is a dominant and functional component of native ecosystems on the floodplain, altering ecological responses to flooding and grazing. A second critical transition, driven by chronic groundwater decline associated with the development of irrigated agriculture, is currently being realised in the poor condition of the dominant and characteristic phreatophytic canopy species; this is likely to have been exacerbated by drought conditions, indicating a loss of resilience within these communities. Loss of this important functional species complex points to a future transition to Acacia stenophylla-dominant riparian woodlands in which eucalypts play a limited functional role. Further research is required to determine the value of such alternative system states in terms of ecosystem service provision.
The potential to redress chronic groundwater decline in this system is limited under current water resource allocation arrangements. This study suggests that sites in which groundwater levels remain accessible to E. camaldulensis/E. tereticornis should be prioritised in terms of management to maintain riparian function and strategies defined to stabilise/supplement alluvial groundwater levels.