Insects Associated with Declining Riparian Black Alder (Alnus glutinosa) Stands: Assemblage Structure, Within-Season Patterns, and Distance–Zone Patterns in the Utrata and Łutownia River Valleys

Insect assemblages associated with declining riparian black alder (Alnus glutinosa) stands were compared at the community level between two lowland river valleys in Poland that differ in anthropogenic transformation: the near-natural Łutownia River valley within the Białowieża Forest and the urbanised Utrata River valley in the Warsaw metropolitan area. Insects were sampled using active hand sampling and Barber pitfall trapping along a short-distance gradient from the channel during repeated monthly field campaigns in summer and early autumn. The near-natural valley supported a richer assemblage, with a stronger representation of forest-associated, saproxylic and aquatic taxa, whereas assemblages in Utrata contained a higher proportion of widespread disturbance-tolerant taxa. In both valleys, diversity was highest at an intermediate distance from the channel margin, and the between-valley contrast was more pronounced than the within-season shifts visible in the campaign summaries. The results identify a clear assemblage contrast between the two valleys, a consistent intermediate-distance diversity maximum, and only modest within-season variation in pooled campaign totals. These findings indicate that structurally complex riparian alder forests with retained dead wood are associated with richer and more taxonomically varied insect assemblages; microhabitat-related interpretations are retained as cautious sampling-point-scale associations because tree-level attributes and individual microhabitat categories were not quantified separately.