Drought-induced dieback of riparian black alder as revealed by tree rings and oxygen isotopes

Progressive death of twigs and branches (i.e. dieback) may happen in response to biotic and abiotic agents thereby reducing tree growth and eventually death. Drought-induced dieback has been seldom studied in riparian habitats. We used retrospective tree-ring and oxygen isotope analyses to determine whether growth patterns, sensitivity to climate and hydrology, as well as access to deep subsurface water and microhabitat river variables, are related to Alnus glutinosa [L.] Gaertn. decline. Tree-ring sampling was conducted on A. glutinosa individuals showing dieback 'declining' (defoliated) and compared with paired 'non-declining' (not defoliated) individuals in one slow-running stream. Radial growth of declining trees responded to the rate of precipitation-evapotranspiration from February to July more than non-declining. In contrast, the growth of non-declining trees positively correlated with the October river discharge of the year preceding tree-ring formation. After the severe 1998 drought, the growth of declining trees decreased in comparison to non-declining trees, showing, since then, early warning signals of dieback. Since 1998, resilience decreased as drought events accumulate in declining trees, but not in non-declining trees. Also, trees situated near to the active river channel recover better from drought. In the 1998 tree ring, we found differences in delta O-18 between vigour classes suggesting that non-declining trees had access to deeper water pools in drought years. Our findings provide new information that could be used to forecast changes in black alder dynamics under the current climate change scenario, especially at the species' xeric range edges, and assist managers in designing riparian forest adaptation strategies.