Assessing the importance of channel-floodplain connectivity for the resilience of floodplain ecosystem trajectories

Rivers and their floodplains in drylands provide critical ecosystem services, support biodiversity, and serve as hotspots for biomass production. Riverscapes are characterized by biogeomorphic succession trajectories, which depend on periodic flood disturbances. Studying undisturbed free-flowing rivers, which are increasingly rare and globally threatened, enhances our understanding of natural river behavior and can inform restoration and management of regulated systems. Key threats affecting an increasing number of rivers worldwide are the fragmentation of longitudinal connectivity due to dams, reduced lateral connectivity from flood protection measures such as dykes and the withdrawal of water for irrigation. This raises the question how rivers and their floodplains naturally maintain resilience over time and space and how they might respond to anthropogenic modifications as well as climate-induced changes in hydrologic connectivity and water availability.
Our research assesses the role of hydrologic connectivity in distinct phases of the fluvial biogeomorphic succession concept, which describes the interrelationship between hydrogeomorphic and vegetation dynamics and how they change over time. Interpreting the different biogeomorphic succession stages through the lens of ecosystem resilience is a promising approach towards quantifying resilience. To address the lack of long-term, large-scale monitoring, we utilized satellite time series analysis complemented by field data. We conducted remote sensing analysis at the river corridor scale, integrating digital geomorphometry and Sentinel-2 imagery for detailed habitat type mapping. Landsat time series analysis, using the LandTrendr segmentation algorithm, provided insights into the spatio-temporal dynamics of vegetation as well as hydromorphology and thus the functional channel-floodplain connectivity. Field data collected from 44 floodplain forest plots along the Naryn River in Kyrgyzstan across topographic gradients complemented the remote sensing-derived findings with detailed ecological information and provided insights into the vertical dimension of hydrologic connectivity.
The results show a significant influence of the lateral and vertical distance on the vegetation development over time and space as well as the species composition and density. These findings underscore the importance of lateral and vertical hydrological connectivity for semi-arid floodplain ecosystem succession. Interpreting these findings using the ecological resilience framework, we applied the ball-and-cup model across spatial and temporal scales, from individual river elements to entire river corridors. Notably, the biogeomorphic stage emerged as a critical switching point in the succession trajectory. These findings emphasize the importance of maintaining small units and the vertical connectivity for reach scale resilience and therefore have implications for the design of moving targets for the process-based conservation and restoration of rivers.