ENSO Signals Recorded by Ash Tree Rings in Iberian Riparian Forests

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Climate–atmospheric patterns affect ecological processes. The El Niño Southern Oscillation (ENSO) represents the strongest global source of climate variability at annual scales, but its impacts on Mediterranean forests are still understudied. Here, ENSO signals recorded by river flow and radial growth series of Mediterranean riparian forests are uncovered in the middle Ebro basin, northeastern Spain. A chronology or mean series of tree ring widths (period 1970–2018, 27 trees) was built for narrow-leaved ash (Fraxinus angustifolia). Growth indices increased as the river flow did during the prior winter and from May to September. Ash growth indices and river flow of the hydrological year were positively related (r = 0.54, p < 0.001), suggesting that a higher soil moisture enhances growth. This correlation remained significant after taking out the influence of precipitation. Ash growth indices (r = −0.46, p = 0.001) and river flow during the hydrological year (r = −0.30, p = 0.03) were negatively associated with the January Southern Oscillation Index (SOI). These relationships suggest that high SOI values corresponding to La Niña events lead to reduced river flow, thus decreasing ash radial growth. Further approaches could be used to better understand how the ENSO impacts drought-prone riparian forests subjected to increasing aridity.