Sensitivity of black alder (Alnus glutinosa [L.] Gaertn.) growth to hydrological changes in wetland forests at the rear edge of the species distribution

Functional responses of riparian species can be used as surrogates for their vulnerability to climate-driven changes. In particular, black alder (Alnus glutinosa [L.] Gaertn.) might be highly susceptible to changes in habitat at the southern limit of its biogeographical distribution in the Mediterranean Basin. In this study, the long-term environmental signal of black alder on a river system in southern Portugal was determined on trees located in different geomorphological position across the floodplain. For all sampled trees, we related radial growth to monthly precipitation, temperature and streamflow. Tree growth responded to some degree to climatic variables, while a marked reduction of tree-ring growth was observed after extreme hydrologic events leading to mechanical damage and uprooting, especially when located near and downstream on the active channel. Similarly, during the first years of life, tree-ring width tended to be narrower near to the active channel, whereas trees at far and less disturbed sites grew faster and, therefore, showed wider rings. Our results confirm the potential of black alder growth as a proxy for hydrologic conditions in a semi-arid basin, and the possibility of using the response of this species to track environmental changes. Given the ecological and economic importance of black alder, and the potential for rapid changes in its distribution, the identification of those areas the most at risk of flood damage, and the adequate management of high priority alder populations will become progressively more important over coming years.