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Key message Extremely high fine root mortalities were observed under drought in 2018, increasing fine root mortality in young trees could be explained by differences in root distribution compared to old trees. Juvenile trees in floodplains are prone to high mortality leading to a low success in reforestation in these habitats. One of the reasons contributing to high mortality of juvenile trees could be limited water and nutrient uptake due to a high level of fine root mortality, especially during summer droughts on higher terraces of floodplains. Strategies of different tree species in hardwood floodplain forests (HFFs) on keeping fine roots alive are still poorly understood. During the record-breaking summer drought of 2018, we examined the relationship between tree age and fine root dynamics of  Quercus robur  and  Ulmus laevis  along the Elbe River in Germany. Root area index (RAI), live root density (LRD) and relative root mortality (RRM) of young and old  Q. robur  and  U. laevis  were analysed by taking soil cores three times during the progression of a summer drought. Old oaks had a lower RAI in the upper soil than young oaks, while RAI of elms did not differ between young and old trees. RRM was very high during summer reaching more than 100% on average. RRM of young trees of both species increased with increasing drought during summer, while RRM of old trees did not change. We argue that differences in the response of RAI between oaks and elms reflect the sink competition between growing deep roots and shallow roots, which is pronounced in oaks due to their characteristic tap root system. Differences in root distribution patterns and the ability to perform hydraulic redistribution may explain the differences in RRM between trees of different species and ages.

Fine root mortality under severe drought reflects different root distribution of Quercus robur and Ulmus laevis trees in hardwood floodplain forests