Figure 1: Location of the “Waldlabor” study site in Zurich, Switzerland (a) , scheme of the tree locations as well as the locations of mobile (SWmobile ) and bulk (SWbulk ) soil water sampling. Precipitation for isotope analysis was sampled outside the forest perimeter at the weather station, at approximately 150 m distance from the site.
Major climate parameters are recorded outside the forest with a compact all-in-one weather station (Atmos41, METER Group, Inc.) at 10-minute resolution. We sampled mobile soil water (the fraction of soil water that has no direct surface contact with the soil, thus is held cohesively and can move freely) and bulk soil water (including the fraction of soil water that is stored in hydration spheres of clay minerals, or held tightly inside the capillary spaces). We sampled mobile soil water (SWmobile ) at 10, 20, 40 and 80 cm depths at two sites (Figure 1) with suction lysimeters (Slim Tube Soil Water Sampler, Soil Moisture Equipment Corp). We applied a suction of 0.7 bar on Mondays and Thursdays and emptied the samplers twice a week on the following Thursdays and Mondays. In addition, we sampled bulk soil (SWbulk ) at two locations (Figure 1) at 10, 20, 40 and 80 cm depths with a 2 cm wide auger every three weeks, and extracted the bulk soil water cryogenically. However, we began sampling mobile and bulk soil water at 80 cm only in June 2021. On 19 sampling dates, roughly every three weeks from July 2020 through the end of October 2021, we sampled beech (two branches each of three to four trees, resulting in n = 119 samples), spruce (one to two branches of two trees, resulting in n = 57) and young spruce (one to two branches of two trees, resulting in n = 70), by cutting branches for cryogenic water extraction. Samples were collected around midday. Immediately after cutting the branches, bark and phloem were peeled off and the remaining wood was placed in exetainers (12 mL Exetainer, Labco Ltd., Ceredigion, UK). Both the xylem and bulk soil samples were stored in those containers at -18 °C until extraction. Cryogenic vacuum distillation was performed in the Institute of Agricultural Sciences stable isotope lab at ETH Zurich, using the equipment and protocol described in Sun et al.(2022).
The 18O and 2H isotopic composition of xylem water isotopes was analyzed with a high-temperature-conversion elemental analyzer (TC/EA) connected to a Delta Plus XP isotope ratio
mass spectrometer via a Conflo III interface (Thermo Fisher Scientific, Bremen, Germany). All other samples (precipitation, soil waters) were analyzed with a triple isotope water analyzer (Los Gatos – TIWA-45-EP). Both instruments are reported to have precisions of 1 ‰ for δ2H and 0:2 ‰ for δ18O.
Isotope data evaluation and the seasonality index
We present the data in timeseries in δ-notation in per mil units (‰), relative to V-SMOW (Vienna Standard Mean Ocean Water). We focus on δ2H, although figures using δ18O data can be found in the supplement and in dual isotope plots. Please note that the regression lines in the dual isotope plot are calculated by reduced major axis regression (described in Harper, 2016) instead of linear regression. This approach is used because classic linear regression assumes that the x-axis has no error/uncertainty, but in the case of a dual isotope plot, there is uncertainty on both axes.
To assess the seasonality of waters we use the seasonal origin index (SOI), introduced and described in Allen et al. (2019):