Long-term deep seepage monitoring in temperate forests: lysimeter evidence of climate‑driven hydrological shifts in northeastern Germany

Groundwater recharge is an ecosystem function provided by forests. At our intensive forest monitoring station “Britz” (Germany) we measure deep seepage, which later becomes ground water recharge, for various tree species and compositions with large scale lysimeters (each covering 100 m² of forest and a depth of 5 m) operating since the 1970s.

Those long-term observations show clear differences between species and forest compositions. With deep seepage being the difference between precipitation reduced by evapotranspiration, evergreen species show little seepage in most years. In dry years our Scots pine stand actually showed no deep seepage at all.

This changed in recent years, with extreme weather events leading to unprecedented seepage patterns. The continent‑wide, multi-year drought that began in 2018 had severe impact.  For the first time in 2019, the beech plot—normally a strong contributor to deep infiltration—recorded no measurable annual deep seepage. Conversely, extreme summer precipitation events occurred each year from 2018 to 2021. While this led to flooding in many regions of Germany, in Britz it led to a considerable proportion of the total annual deep seepage under pine stands.

These novel dynamics reshape our understanding of how deep seepage is generated and highlight the sensitivity of groundwater recharge in the north‑eastern German lowlands to extreme climatic fluctuations.