Unveiling patterns and drivers of multifaceted diversity in dam-affected riparian forests

Abstract Riparian forests are among the most diverse ecosystems, yet their diversity is threatened by hydrological alterations. This study examines patterns of vascular plant diversity and structural characteristics across riparian forests in the Irati Basin, a transition zone between temperate and Mediterranean climates in northern Spain that includes a river section affected by a dam. Furthermore, it identifies the main environmental and structural drivers shaping this diversity. We sampled the composition of vascular plants and the structure of 54 forest plots and calculated their taxonomic, functional and phylogenetic diversity. We compared diversity indices and environmental and structural variables among two control sections (temperate and Mediterranean) and the dam-affected Mediterranean section. A total of 424 species were recorded across the study with an exceptional mean richness of 76.7 species per 200 m 2 . Plots in the temperate section consistently showed higher diversity and were located at higher elevations, with cooler temperatures, higher rainfall and significantly greater bryophyte cover. The Mediterranean sections (control and dam-affected) were more similar to each other, although the dam-affected section exhibited greater summer river flow, soil organic matter and electrical conductivity, as well as higher volumes of terminal and dead trees, likely associated with the high abundance of beavers in this section. Additionally, the dam-affected plots hosted wet-habitat taxa missing in the control sections. These differences in the dam-affected section, particularly in summer river flow and the occurrence of wet-habitat taxa, are consistent with patterns that may reflect hydrological regulation, although alternative environmental and spatial factors cannot be excluded. Environmental and structural variables influenced overall diversity patterns, with each of the analysed diversity dimensions responding to different drivers, although the increase in mean annual temperature negatively influenced most of them. Overall, our findings highlight the need to consider multiple diversity dimensions and environmental drivers when assessing patterns of riparian forest diversity to fully understand ecosystem complexity and therefore effectively manage and conserve riparian ecosystems.