Dynamics of stable isotopes in precipitation, soil water and groundwater at a Norway spruce and a European beech site at Solling, Germany.

Ongoing changes in climate alter the role of forests in the hydrologic cycle, influencing water transmission to springs and aquifers. Here we compared two forests dominated by either beech or spruce on broadly similar soils (Dystric Cambisols); we monitored the passage of natural-abundance stable isotope signals through the upper meter of soil and onward to springs. The isotopic data were similar between the sites at every time step and at every stage of transit, except at 90-100 cm depth, where the isotopic signal of the beech forest was delayed by approximately 1 month. The data were used in a lumped parameter dispersion model so that physical parameters describing transport could be determined and compared. Modeled residence times were similar between the two forests (123 (sd = 32) vs. 152 (25) days), with high precision to depths of 40 cm. According to the model, rainfall reached 1 meter depth in 200 (8) days under the spruce stand, but required 228 (37) days in the beech. The measurements below the rooting zone (90-100 cm) play a critical role in detecting site/species differences and in prediction of residence times.