Triple La Niña 2020–2022 revival: river red gum and coolibah recruitment in the Queensland Murray–Darling Basin floodplain, Australia

Michael R. Ngugi; Jaye Lobegeiger; Janice L. Kerr; Victor J. Neldner; Jessica McCabe; Glenn B. McGregor; Luke Hogan

doi:10.1071/RJ25017

RESEARCH ARTICLE (Open Access)

Triple La Niña 2020–2022 revival: river red gum and coolibah recruitment in the Queensland Murray–Darling Basin floodplain, Australia

Michael R. Ngugi ^A ^* , Jaye Lobegeiger ^B , Janice L. Kerr ^C , Victor J. Neldner ^A , Jessica McCabe ^B , Glenn B. McGregor ^B and Luke Hogan ^A

+ Author Affiliations

- Author Affiliations

^A Queensland Herbarium and Biodiversity Science, Department of the Environment, Tourism, Science and Innovation (DETSI), Brisbane Botanic Gardens, Mt Coot-tha Road, Toowong, Qld 4066, Australia.

^B Water Planning Ecology, Department of the Environment, Tourism, Science and Innovation (DETSI), Ecoscience Precinct, Block C, 41 Boggo Road, Dutton Park, Qld 4102, Australia.

^C Water Planning, Department of Local Government, Water and Volunteers (DLGWV), Ground, 203 Tor Street, Toowoomba, Qld 4350, Australia.

^* Correspondence to: michael.ngugi@qld.gov.au

The Rangeland Journal 47, RJ25017 https://doi.org/10.1071/RJ25017

Submitted: 7 May 2025 Accepted: 3 September 2025 Published: 9 October 2025

© 2025 The Author(s) (or their employer(s)). Published by CSIRO Publishing on behalf of the Australian Rangeland Society. This is an open access article distributed under the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License (CC BY-NC-ND)

Abstract

Floodplain forests in the Queensland Murray–Darling Basin (QMDB) have experienced significant recruitment challenges following multiple droughts, including the Millennium Drought (1997–2009). This study investigated the impact of the rare triple-dip La Niña event (2020–2022) on floodplain tree recruitment patterns for river red gum (Eucalyptus camaldulensis Dehnh.) and coolibah (Eucalyptus coolabah Blakely & Jacobs). Field assessments across QMDB’s three water catchment areas showed recruitment at 34% of plots. Days since last flood (P = 0.04) was significantly associated with recruitment density, whereas bankfull exceedance floods showed a marginally positive effect on recruitment density (P = 0.080), and total days of exceedance had a marginally negative influence on recruitment density (P = 0.06). Open woodland and woodland areas with low canopy cover exhibited significantly higher recruitment rates (P = 0.001 and P = 0.021) compared to forests. Low native fauna grazing pressure showed strong association with increased recruitment (P = 0.007); during wet conditions, abundant grass cover diverted browsing pressure on seedlings. Demographic analysis demonstrated improved reverse J-curve distributions for both species compared with previous assessments, suggesting that this triple-dip event recruitment episode may help restore juvenile cohorts. Findings suggest that although flooding appears to contribute to recruitment through site preparation and seed dispersal, sustained rainfall appears crucial for seedling survival. These results have important implications for environmental water management, highlighting that infrequent but significant recruitment episodes may be sufficient for maintaining sustainable populations over the extended lifespans of these species, while demonstrating the limits of direct management interventions.

Keywords: climate variability, demographic recovery, environmental flows, episodic recruitment, floodplain forests, floodplain restoration, rangeland ecology, tree demographics.

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