Two negatives make an affirmative: can extreme flooding reduce the expansion of invasive submerged macrophyte in a large river?

Plant invasion and extreme climate event are both important ecological issues under the background of global climate change. However, how these two incidents interact with each other is still debatable in different ecosystems. In this study we investigated the interaction between the extreme flooding event during the autumn of 2021 in the Han River and the invasion of Elodea nuttallii based on a long-term field survey from 2020 to 2023 and two indoor controlled experiments (propagule bank experiment and decomposition experiment). We hypothesized that two negatives (extreme flooding event and invasive submerged macrophytes) can make an affirmative (macrophyte community consisting of native species). The field survey found that the extreme flooding caused a critical change of transparency until seven months later the water quality turned into the initial condition, and the maximum biomass of E. nuttallii decreased significantly in 2022 after the flooding. Abundant propagule bank of native macrophytes in the sediment contributed to the strong resilience of macrophyte community responding to the extreme flooding; the maximum total biomass of macrophyte community in 2022 did not differ from that in the two years prior to the flooding. Additionally, more species of native macrophyte was found in the field survey after the extreme flooding. Decomposition rates of E. nuttallii fragments was large as 0.69 d, and long-time high turbidity lead to a very fast run-out of the only reproduction tissue (fragments) of this alien species in the river, which resulted in the slowing of its recovery. Inspired by this study, we further proposed a cost-effective methodology to control the invasive species E. nuttallii, i.e., the combination of propagule bank of native macrophytes in the sediment and artificially manipulated pulse flooding.