Climate changes modulate deep seepage under forests – a long-term observation at a north-eastern German lysimeter site

Groundwater recharge is an important ecosystem function of forests. In regions with a negative climatic water balance it shows a distinct annual cycle: in summer, evapotranspiration exceeds the input by precipitation, in the winter, low evapotranspiration allows a rewetting of soils. In times with sufficient water supply, the water moves below the root zone as deep seepage and later becomes groundwater recharge. On our intensive forest monitoring station “Britz” (Germany) we measure deep seepage for various tree species and compositions with large scale lysimeters. Installed in the 1970s they each span a forested area of 100 m² and have a depth of 5 m. Over the last years extreme weather changed the deep seepage to previously unseen patterns. A drought, beginning in 2018, had such an impact that in 2019, for the first time, no annual deep infiltration was detected under the beech site, a site generally promoting deep infiltration (Fig. 1). In contrast to the drought, extreme rainfall occurred from 2018 to 2021 each summer . While this led to flooding in many regions of Germany, in the sandy lowlands it sometimes led to a considerable proportion of the total annual deep seepage under pine stands, which generally have little to no deep seepage. These unseen dynamics influence our understanding of deep seepage formation and have an impact on groundwater recharge in the north-eastern lowlands of Germany.