Flood resilience: Response of an Australian sub-tropical riparian rainforest to catastrophic flooding

Riparian taxa are subject to dynamic, intense, flood-related stressors, and have evolved traits to persist in this environment. Climate change-induced intensification of flood regimes pose a threat to these ecosystems, and little is understood about their resilience to this intensification. Following two consecutive major floods (the first flood record-breaking) along the subtropical coast of eastern Australia in March 2022, we used methods based on persistence (resprouting, seedling recruitment), and floristic structure (height, DBH) and assemblage, to assess the resilience of an old growth riparian rainforest to severe flooding in the 12-month post-flooding window. Smallerwoody plants (stems <10 m tall, <30 cm DBH) were the most impacted and were significantly impacted by flooding. Native species richness and plant density (plants/m⁻²) significantly decreased between before and 3 months post-flooding, after which they continued to significantly increase to surpass pre-flood values. Overall, ~35% of taxa exhibited resprouting, ~28% of taxa exhibited seedling recruitment, and ~11% exhibited both resprouting and recruitment. An additional ~21% native taxa were introduced to the site via seedling recruitment, along with 65 invasive species. Model-based multivariate analysis showed flooding significantly altered community floristics (p = 0.026) at 3 months. At 12 months post-flooding the community was recovering, becoming more floristically similar to its pre-flood composition. The riparian rainforest exhibited high resilience to intense flooding. Impact, persistence, and resilience varied amongplots, and the community took 12 months to move into recovery. We found that persistence, and floristic structure and composition weighted against impact were effective measures of ecosystem resilience. In the absence of further consecutive events, mature-phase riparian rainforests are likely to be structurally and floristically resilient to climate change-induced amplification of flood regimes. Further studies should build on this framework to include invasive weed species impacts, for a more accurate assessment of impacts.