Plant Roots Steer Resilience to Perturbation of River Floodplains

Freshwater ecosystems along river floodplains host among the greatest biodiversity on Earth and are known to respond to anthropic pressure. For water impounded systems, resilience to changes in the natural flow regime is believed to be bidirectional. Whether such resilience prevents the system from returning to pristine conditions after the flow regime changes reverse is as yet unclear, though widely documented. In this work, we show that temporal irreversibility of river floodplains to recover their status may be explained by the dynamics of riparian water-tolerant plant roots. Our model is a quantitative tool that will benefit scientists and practitioners in predicting the impact of changing flow regimes on long-term river floodplain dynamics. Plain Language Summary Catchment impoundment and the withdrawal of flowing water from mountain torrents and rivers for human needs are practices that modify the mean discharge and variability of natural streams. The long-term impact includes changes to floodplain morphology and the compositions of terrestrial and aquatic biodiversity. Vegetation encroachment is then widely observed on floodplains where water is not a limiting factor for plants to grow. The extent to which such alterations are reversible is an important object of this study, and has important implications for water management strategies when hydraulic structures reach the end of either their physical life or their economic benefit. We develop a comprehensive theoretical model that reveals the important role of plant roots in these processes. The model is applied to impoundment of the River Maggia in Switzerland. It is found that natural conditions before dam construction might not be fully restored by simply removing the dam. Our approach offers an important step toward improving natural water management schemes and optimal dam regulation strategies in the face of human and climatic hydrological changes. Key Points We develop a mechanistic model for river floodplain equilibrium states and their response to changing flow regime Temporal irreversibility to reversible conditions is shown to originate from plant root adaptation to the new regime We use a worldwide common example of water impoundment to quantify long-term floodplain dynamics