Challenges for regeneration and stability of riparian forests in a dam-regulated basin: Potential lag effects on tree community assembly

Riparian forests are highly dynamic and diverse ecosystems whose biodiversity, functional integrity, and structure are threatened by pressures such as river regulation by dams. However, studies simultaneously assessing riparian woody plant regeneration and forest structure under dam influence remain scarce. This study investigates the effects of a dam on the structure and regeneration patterns of riparian forests, by assessing their compositional and functional diversity of the woody component. The research was conducted in Irati River basin (northern Iberian Peninsula), where 54 plots were selected across dam-affected and non-affected sections. In each plot we sampled woody species regeneration (seedlings and saplings) and population structure of established trees. We subsequently compared taxonomic and functional diversity between seedlings and established trees, and modelled Fraxinus spp. regeneration. The dam-affected section exhibited reduced taxonomic and functional diversity among tree seedlings, with pioneer riparian species declining. Regeneration in the dam-affected section was dominated by Hedera hibernica , a fast-growing liana potentially favoured by the altered flow regime. Even Fraxinus angustifolia , the dominant non-pioneer riparian tree species, experienced regeneration limitations in the dam-affected section, with a marked reduction in individuals from the seedling to sapling stage. Our findings suggest that flow regulation leads to a compositional and functional mismatch across life stages and a disruption in natural riparian woody species regeneration, with lag effects threatening the long-term integrity of riparian ecosystems. We propose implementing river-specific ecological flow regimes that mimic natural flows and maintain natural recession periods in regulated rivers, essential for key ecological functions such as regeneration. • River regulation reduces taxonomic and functional diversity in regenerating trees. • Upland non-pioneers replaced riparian pioneers, especially in regulated areas. • Regulated flow aids Hedera hibernica , limiting space for other tree seedlings. • Ash regeneration faces early life-stage losses likely driven by flow regulation. • Dam affects tree regeneration, with lag effects threatening future forest structure.