Sanz‐Zubizarreta et al. (2025) Complex growth responses of riparian tree species to dam-related hydrological alterations at their climatic niche limits

Identification

• Journal: Dendrochronologia
• Year: 2025
• Date: 2025-10-04
• Authors: Irati Sanz‐Zubizarreta, Idoia Biurrun, Nere Amaia Laskurain, Patricia María Rodríguez‐González
• DOI: 10.1016/j.dendro.2025.126425

Research Groups

• Department of Plant Biology and Ecology, Faculty of Science and Technology, University of the Basque Country UPV/EHU, Bilbao, Spain
• Bergara, Spain
• Forest Research Centre, Associate Laboratory TERRA, School of Agriculture, University of Lisbon, Lisbon, Portugal

Short Summary

This study used dendrochronological analysis on three riparian tree species in SW Europe to assess their growth responses to climatic and dam-related hydrological changes at their climatic niche limits. It found species-specific growth patterns, with dam-induced increased summer flow benefiting Alnus glutinosa and Fraxinus angustifolia growth, but significantly reducing the latter's resilience and resistance to drought, highlighting long-term risks under climate change.

Objective

• To assess the effects of climate and dam-related hydrological alterations on the growth of dominant riparian trees.
• To identify which climatic variables affect the growth of each tree species.
• To determine if species sensitivity to climate varies among climatic zones.
• To evaluate if tree growth patterns changed between pre- and post-dam periods.
• To investigate if dam construction affected tree resilience and resistance to drought events.

Study Configuration

• Spatial Scale: Irati sub-basin, Ebro Basin, northern Iberian Peninsula, SW Europe. The study area spans an altitudinal range between 402 meters and 894 meters above sea level, covering an area of 1860 square kilometers. Seven river sections were defined, categorized into temperate and sub-Mediterranean climatic zones.
• Temporal Scale: Dendrochronological analysis covered the period 1950–2022. The pre-dam period was defined as 1950–2003 (or 1975–2003 for dry year analysis), and the post-dam period as 2005–2022. The Itoitz Dam was completed in 2004. Field sampling was conducted at the end of autumn 2022.

Methodology and Data

• Models used: Linear mixed effects models (LMEs) were fitted using the lme function from the R package nlme. An autoregressive correlation structure (AR3) was applied to account for autocorrelation in tree growth series. Model selection was based on AICc and BIC, with a stepwise approach using the stepAIC function from the MASS package for some models.
• Data sources:
  • Tree-ring data: 246 living trees (Alnus glutinosa, Fraxinus angustifolia, Fraxinus excelsior) were sampled from seven river sections. Two cores per tree were taken at breast height. Ring-width was measured with a LINTAB measurement table (0.01 mm resolution) and J2X software, cross-checked with COFECHA, and converted to Basal Area Increment (BAI).
  • Climatic data: Monthly mean temperature (TG in °C) and monthly total precipitation (RR in mm) were obtained from the E-OBS dataset ver. 28.0 (1950–2022) at a 0.1º grid resolution. The Standardized Precipitation Evapotranspiration Index (SPEI-9) was calculated using the SPEI package in R.
  • Hydrological data: Monthly river discharge (FLOW in m³/s) was obtained from seven hydrological stations (CEDEX, Meteo Navarra). Missing data were filled using regressions with nearby correlated stations.
  • Dendrometric variables: Height, diameter at breast height (DBH), inclination, and number of live/dead stems were recorded for each sampled tree.

Main Results

• Species-specific growth patterns and climate responses:
  • Alnus glutinosa showed a stable growth tendency with higher basal area increment (BAI) and ring width, while Fraxinus species exhibited greater variability.
  • Temperature was a key limiting factor for all three species, but climate-growth relationships were species-specific.
  • Increased average annual temperature favored Alnus glutinosa and Fraxinus angustifolia growth but negatively impacted Fraxinus excelsior, indicating vulnerability of its rear-edge populations to rising temperatures.
  • Rising summer temperatures reduced growth for Alnus glutinosa and Fraxinus angustifolia in the sub-Mediterranean zone.
  • Increased summer precipitation favored Fraxinus excelsior growth, and spring precipitation positively correlated with growth for both Fraxinus species.
• Dam-related hydrological alterations and tree growth:
  • The Itoitz Dam altered the Irati River's hydrology from a unimodal to a bimodal annual flow pattern, characterized by decreased winter flow and increased summer water discharge (average summer flow increased from 5 m³/s pre-dam to 15–17 m³/s post-dam).
  • This shift positively affected the annual growth of dam-affected Alnus glutinosa (maintaining stable growth compared to a decline in control areas) and Fraxinus angustifolia (showing increased growth).
• Resilience and resistance to drought events:
  • Alnus glutinosa resilience and resistance to drought were not significantly influenced by the dam or summer river flow.
  • Fraxinus angustifolia trees in dam-affected areas showed significantly decreased resilience (estimate of -1.07) and resistance (estimate of -0.22) to drought during the post-dam period, despite overall higher summer flows. This is attributed to increased water extraction from the Canal de Navarra during dry years.

Contributions

• This study provides a unique, integrated assessment of climate change and dam-related river regulation impacts on riparian tree growth, resilience, and resistance in a critical temperate-Mediterranean climatic transition zone.
• It reveals complex, species-specific responses, highlighting the vulnerability of rear-edge populations (Fraxinus excelsior) to warming and the long-term risks for foundation species (Fraxinus angustifolia) despite short-term growth benefits from altered flow regimes.
• The findings underscore the importance of considering both short-term growth responses and long-term resilience/resistance in dam management, advocating for the implementation of ecological flow regimes to mitigate adverse impacts on riparian ecosystems.

Funding

• University of the Basque Country (grant number PIF21/255)
• Basque Government (grant number IT1487–22)
• Portuguese Foundation for Science and Technology (grant numbers UIDB/00239/2020, LA/P/0092/2020)

Citation

@article{SanzZubizarreta2025Complex,
  author = {Sanz‐Zubizarreta, Irati and Biurrun, Idoia and Laskurain, Nere Amaia and Rodríguez‐González, Patricia María},
  title = {Complex growth responses of riparian tree species to dam-related hydrological alterations at their climatic niche limits},
  journal = {Dendrochronologia},
  year = {2025},
  doi = {10.1016/j.dendro.2025.126425},
  url = {https://doi.org/10.1016/j.dendro.2025.126425}
}