All data are collected at the Leibniz Institute of Freshwater Ecology and Inland Fisheries at the Müggelseedamm 310, 12587 Berlin, Germany

Stem size variation was measured with radius dendrometers (DR Radius Dendrometer, Ecomatik, Dachau, Germany) at 95 cm above ground level at the Northern side of the stem with an interval of 15 min and merged to daily results. The time series ranges from 14.05. - 10.10.2020.

Sap flow was measured with ICT sap flow meters (SFM1 instrument, ICT International, Australia) in sap flow mode for Northern Willow (not presented) and temperature mode for Southern Willow at the Northern side of the tree stem at breast height with an interval of 15 min. Daily Flow was calculated with the Sap Flow Tool from ICT. The time series ranges from 18.05. - 31.10.2020.

Precipitation was sampled with an autosampler (ISCO 3700 Teledyne Isco, Lincoln, USA) at an interval of 4h, analysed (Picarro L2130-i, Picarro Inc., Santa Clara, CA, USA) for d18O and d2H respectively and merged to daily results. The time series ranges from 25.05. - 24.09.2020.

In situ soil and tree (= xylem) water isotopic composition was measured in vapor via CRDS (Picarro L2130-i, Picarro Inc., Santa Clara, CA, USA) at an interval of ~2h at soil pits A and B in 3 depths (10, 40, and 100 cm) respectively (only A 10 cm presented). Xylem water was measured at 30 cm (lower) and 170 cm (upper) of the stem in Southern and Northern Willow (only Southern Willow upper presented). Each sample was measured for 10 min with the first 5 min and the last 30 seconds being discarded. The soil time series ranges from 20.05. - 11.10.2020 and for xylem from 27.06. - 02.10.2020.

]]> stable isotopes sap flow stem size Müggelsee 13.652 52.437 In Situ Site 13.648 52.448 Hauke Dämpfling 0000-0002-0554-757X Jessica Landgraf