Complex growth responses of riparian tree species to dam-related hydrological alterations at their climatic niche limits

Artificial dams disrupt natural river flows, endangering riparian trees and their ecosystems. We studied the impact of a dam located in the transition from temperate to Mediterranean climate in SW Europe. We used dendrochronological analysis on 250 individuals of three widespread riparian tree species ( Alnus glutinosa , Fraxinus angustifolia , F. excelsior ) to evaluate their growth responses to climatic and dam-related hydrological changes. Growth patterns differed among species, with Alnus glutinosa showing a stable tendency, while both Fraxinus species exhibited greater variability. Temperature was the most limiting factor for the three species, but climate-growth relationship was species-specific. The increase in the average annual temperature favoured Alnus glutinosa and F. angustifolia but limited F. excelsior growth, revealing the vulnerability to rising temperatures of F. excelsior rear-edge populations in the warmest margins of the species climatic niche. Additionally, rising summer temperatures reduced the growth of Alnus glutinosa and Fraxinus angustifolia in the sub-Mediterranean zone. The dam altered the river’s hydrology, changing a unimodal annual flow pattern to a bimodal one with higher summer water discharge. This shift favoured the annual growth of dam-affected Alnus glutinosa and Fraxinus angustifolia trees. However, the resilience and resistance of the latter to cope with dry years significantly decreased after the dam’s construction compared with the control sections. This may place a long-term risk for key structural trees in these riparian ecosystems, especially under climate change scenarios with increasing droughts, underscoring the importance of implementing ecological flow regimes as a mitigation strategy.