Climate, drought and hydrology drive narrow-leaved ash growth dynamics in southern European riparian forests

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Mediterranean riparian forests are among the most threatened ecosystems in Europe. These ecosystems are exposed to land-use changes threatening their reduced habitat and by global warming, which is already triggering aridification processes. To assess the impact of these major threats, we studied the radial-growth responses to climate and drought in the narrow-leaved ash (Fraxinus angustifolia). This riparian tree species presents a relatively large ecological spectrum in its habitat preference in the Mediterranean Basin. We studied five sites arranged across a wide geographical range from Iberia to Italy, subjected to contrasting climatic conditions and located in hydrographic basins with different sizes and water regimes. We found diverse growth responses to climate and drought across the Mediterranean distribution range of the narrow-leaved ash at the individual and site levels. The growth of this species increased in response to wet and cool conditions in the prior winter and spring. The response to summer conditions was only observed in the coldest and wettest site (Ticino). Growth responded negatively to 2–14 month droughts that occurred from previous winter up to summer, particularly in the warmest-driest sites. Growth responses to drought peaked in the warmest-driest sites in terms of climate water balance (Odelouca, Doñana), but not in the driest sites in terms of annual precipitation (Tudela, Zaragoza). Hydrological conditions also affected the narrow-leaved ash with high discharges in the prior winter and early spring enhancing wood production. Considering projected aridification and increased hydrological alteration, implying limited water supply in the Mediterranean region, climate warming will negatively impact productivity of narrow-leaved ash riparian forests. Further research should combine analyses of growth responses to climate and hydrology from tree to basin scales to disentangle their relative roles as drivers of productivity under different scenarios of climate and hydrological changes, in order to aid adaptive management of these key ecosystems.