Groundwater thresholds for drought resilience in floodplain woodlands: a case study from the northern Murray-Darling Basin

In ephemeral river systems, canopy condition in dominant riparian and floodplain tree species may depend on access to shallow groundwater resources, particularly during drought. However, unsustainable groundwater extraction and chronic groundwater decline, evident in many agricultural landscapes worldwide, effectively decouples tree roots from deep soil moisture resources, increasing the susceptibility of trees to changes in precipitation. In such regions, drought may trigger loss of canopy condition and have long term consequences for the function and survival of trees and the composition, structure and function of ecosystems they dominate. However, critical groundwater depth thresholds have been difficult to identify. This study used a novel approach including boosted regression trees, quantile regression and threshold analysis to explore the relationship between groundwater depth and tree condition for two dominant tree species, Eucalyptus camaldulensis (river red gum) and E. populnea (poplar box); both species occur on the Upper Condamine floodplain, a region experiencing groundwater depth declines of 25+m in the northern Murray-­‐Darling Basin, southern Queensland. Distinct non-­‐linear responses were apparent, with minimum groundwater depth thresholds identified at 12.1m for E. camaldulensis and 12.6m for E.populnea, beyond which canopy condition declined significantly. This approach represents a repeatable method of quantifying ecological response thresholds along groundwater depth gradients. Its application may enable safe operating limits for groundwater resource management to be identified, supporting improved decision making to support resilient floodplain ecosystems. This will be particularly important in regions where groundwater decline driven by increasing water demand and drying climates is predicted