Channel-floodplain connectivity drives vegetation dynamics in semiarid floodplains: a remote sensing analysis of the Naryn river corridor in Kyrgyzstan, Central Asia

In the semiarid climate of Central Asia the rivers and their associated floodplain ecosystems have a high relevance as regional hotspots of biodiversity and for the provision of ecosystem services. One of these rivers is the Naryn River in Kyrgyzstan which is part of the headwaters of the Aral Sea basin. Upstream of the Toktogul Reservoir, which is the first barrier in the river course, the Naryn is still in a nearly natural state over a length of 600 km. The valuable floodplain habitats directly depend on the natural dynamics of this river. In particular rejuvenation and thus community structure of floodplain forests depends on the interaction of flow regime and hydromorphology controlled by longitudinal and lateral connectivity of water and sediment. Despite the ecological relevance, riparian ecosystems along the Naryn have not been investigated yet in detail. Especially the relative importance of the ecological and physical processes as well as anthropogenic effects on floodplain forest succession trajectories are not yet understood. This is a crucial issue for biodiversity conservation and environmental management as ongoing plans for additional dam construction are likely to heavily modify hydrologic and sediment connectivity. This is likely to have significant downstream impacts such as channel incision caused by sediment deficit or modification of the hydrological regime. The modifications therefore impact the current natural riparian ecosystem structure and functioning. In this study, we contribute to fill this knowledge gap and develop a conceptual succession model for this semiarid floodplain forest explaining the recent shape and distribution of habitat patterns. For this purpose, we use detailed ecological information derived from field data collected on 44 plots in the floodplain forest of the central Naryn basin together with river corridor scale remote sensing analysis using Sentinel-2 for detailed habitat derivation. These assessments are complemented by Landsat time series analysis with the LandTrendr segmentation algorithm to gain insights on large-scale spatio-temporal dynamics of vegetation and hydromorphology. Based on the results, we were able to determine the relevance of local morphology for longitudinal habitat patterns and to quantify the relationship between lateral connectivity and floodplain forest community structure at the scale of an entire river corridor. The conceptual model derived from the results explains the complex longitudinal and lateral pattern as well as the succession trajectories of floodplain forest communities along the Naryn River. It helps to better understand the natural processes and potential effects of anthropogenic activities to support forest and biodiversity conservation. In addition, it provides a basis for further research in other semiarid floodplain ecosystems.