Temporal changes in riparian forest vegetation: Unexpected responses to streamflow alteration under climate change

Riparian forests are highly dynamic and species-rich ecosystems, although they face multiple pressures. One of the most significant is river regulation, which has mixed effects on riparian forests. We resurveyed historical plots to assess the ecological impact of a dam on the Irati River in northern Spain, located in a temperate-Mediterranean ecotone. This dam regulated the Irati River by altering flow peaks, reducing winter flow and increasing summer average discharge. We investigated how flow alteration and climate change affected riparian vegetation composition between Old (1997–2003) and New (2022−2023) plots, with a particular focus on moisture-demanding, alien and ruderal species. The plots were divided into three sections: (1) the temperate control, (2) the Mediterranean control, and (3) the Mediterranean dam-affected. A shift in species composition was observed in the studied forests, each section displaying a distinct pattern of change. We associated the changes in plant species composition to climate change and river regulation. Both control sections exhibited increases in thermophilous species and a decline in moisture-demanding riparian species. Conversely, the dam appears to support riparian species by maintaining summer flows, mitigating climate-induced drought impacts. Additionally, ruderal species increased in both Mediterranean sections, suggesting that their spread was more likely facilitated by industrial and agricultural development rather than by the dam. We conclude that dam-induced summer flows may partially buffer climate change effects on riparian forests, providing guidance for flow management prescriptions and highlighting the need for long-term, multifactorial assessments of river regulation.