Junction of the Warrego and Darling rivers Selected Area 2021-22 - Appendix F Vegetation contingency monitoring

The Warrego River Western Floodplain is dominated by two flood-dependant vegetation types; coolibah woodland wetlands and lignum shrublands, adapted to both long dry periods and prolonged periods of inundation. The presence, distribution and condition of these vegetation types is driven by differences in hydrological regimes and further influenced by increased inundation patterns due to long-term water management structures on Toorale National Park. Compared with communities in other Northern Basin catchments, vegetation on the Western Floodplain is considered to be in relatively good condition.

In autumn 2020 a widespread inundation event flooded the Western Floodplain. This was the largest inundation event observed since LTIM/Flow-MER vegetation monitoring began in 2015 and the resulting inundation reached areas of the Western Floodplain that had not received flows since 2012. As a result, a contingency monitoring program was established to capture vegetation response and investigate whether differences in hydrological regime explained present vegetation response patterns. Specifically, did sites that had received short, infrequent inundation regimes respond similarly to those sites that had received longer and more frequent inundation following a single widespread event?

All sites across both vegetation types responded positively to the autumn 2020 inundation event and although this event triggered initial response convergence across the floodplain, site level differences in time since connection and inundation duration promoted response divergence, indicating that vegetation responses were also connected to water regimes. For instance, community condition findings reflected inundation regime timeframes recommended to achieve ‘Good’ vegetation condition in both lignum and coolibah communities. While some vegetation responses, such as community condition in lignum shrubland, were consistent with observed inundation regime patterns along a north-south floodplain gradient, this was not always the case, with variation in inundation duration from the 2020 event driving differences in species richness, vegetation cover and species diversity in coolibah.

Long-term monitoring programs contribute to our understanding of watering regime requirements of flood-dependant vegetation communities, enabling up-to-date information that can be used to assist decisions surrounding adaptive management and prioritisation of water management options and delivery. In this study, most observed vegetation responses could be explained by and were linked to time since connection, inundation duration, past watering regime, or a combination of these inundation metrics. The continuation of contingency site monitoring into the future would build on the current Flow-MER core monitoring dataset of the Western Floodplain and allow greater insight into the long-term effects of different inundation regimes in two dominant flood-dependent vegetation communities.