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):