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Riparian zones are biodiversity hotspots in montane ecosystems and are of critical conservation concern. However, studies on longitudinal diversity patterns and environmental drivers have been restricted to aquatic fauna, while the animals that rely on both river and riparian resources have been of much less concern. Here, we examined the multifaceted diversity distribution of riparian birds along longitudinal gradients and analyzed the importance of environmental factors in shaping these patterns in the Changbai Mountains. Hump‐shaped relationships between elevation and taxonomic, phylogenetic, and functional diversity, as well as with the conservation value index, were evident along each of the classic submontane rivers. Forest cover, vegetation height variation, and land cover patches positively affected the taxonomic diversity indices. In addition to the species richness, fluvial geomorphology variables (river sinuosity and gravel bar) were significantly related to the phylogenetic diversity. However, there was no statistical evidence for a relationship between functional diversity and the environmental variables examined. This study emphasized the necessity of including multiple diversity measures beyond taxonomic diversity and demonstrated the importance of both terrestrial and aquatic components in shaping the multifaceted biodiversity pattern of riparian organisms living in riparian zones. The results suggested that conservation priority should be given to both rivers and banks in the middle reaches and that riparian birds could be good candidate indicators of environmental change in the submontane river‐forest ecotone.

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Summary The Scaly-sided Merganser Mergus squamatus is a globally ‘Endangered’ species breeding in north-east Asia. Limited by information on the geographic distribution of suitable habitat, the conservation management programme has not been comprehensive or spatially explicit for the breeding population. This study combines potentially important environmental variables with extensive data on species occurrence to create the first species distribution model for the breeding Scaly-sided Merganser, followed by a GAP analysis to highlight the unprotected areas containing suitable habitat. The predictive map showing the most suitable breeding habitat for the Scaly-sided Merganser covered broad-leaved deciduous forest distributed in six provincial regions in south-east Russia, north-east China, and North Korea. The conservation GAP, i.e. 90% (38,813 km2) of highly suitable habitat, is mainly concentrated in the Sikhote-Alin and Changbai mountain ranges. This study suggests that prioritizing conservation of unprotected broad-leaved deciduous riverine forests within the above two mountainous regions should be included in international conservation planning, and the remaining suitable patches need to be preserved to allow range expansion in future. This predictive map improves the expert global assessment of breeding Scaly-sided Merganser distribution and provides a basic reference for establishing conservation areas or implementing conservation actions for the breeding Scaly-sided Merganser in north-east Asia.