Impact of extreme precipitation events on summer deep seepage below temperate forests in the Northeastern Germany lowlands

Abstract Key Message Stands stocked with European beech ( Fagus sylvatica L.), sessile oak ( Quercus petraea (Matt.) Liebl.), and Scots pine ( Pinus sylvestris L.) show distinct deep seepage patterns. An increasing importance of extreme summer precipitation contributing to deep seepage in the northeastern German lowlands was detected. Extreme summer precipitation events contributed 71% (pine), 22% (young oak), and 15% (beech) of the annual deep seepage. Adapted forest management may promote deep seepage caused by extreme summer precipitation and by precipitation during the winter half-year. Context To date, deep seepage and groundwater recharge in temperate lowland forests occured mainly during the winter half year, the only period in which precipitation exceeds potential evapotranspiration. The increasing occurrence of extreme summer precipitation events, however, has the potential to promote deep seepage during summer. Aims This study aims to quantify the deep seepage feed by extreme summer precipitation events, utilising three large-scale lysimeters below canopies of beech ( Fagus sylvatica L.), young oak ( Quercus petraea (Matt.) Liebl.), and pine ( Pinus sylvestris L.), respectively. Methods Using a seepage hydrograph separation method, we were able to identify two major types of deep seepage: slow deep seepage due to winter precipitation and rapid deep seepage due to extreme summer precipitation events. Results Our measurements attributed substantial portions of deep seepage to extreme summer precipitation events, with distinct differences among lysimeters related to tree species and stand structure. The highest ratio of deep seepage by extreme summer precipitation to annual deep seepage occurred below pine, whereas the highest quantities of deep seepage by extreme summer precipitation were found under young oak. Conclusion Rapid deep seepage due to an increase in extreme summer precipitation events could be the most important mechanism for recharging near-surface groundwater aquifers under pine forests in the northeastern Germany lowlands. Deep seepage may be influenced by the choice of tree species and stand structure.