Trees in seasonal climates may use water originating from both winter and summer precipitation. However, the seasonal origins of water used by trees have not been systematically studied. We used stable isotopes of water to compare the seasonal origins of water found in three common tree species across 24 Swiss forest sites sampled in two different years. The data set provides information on the sites (e.g., latitude/longitude, site name), site characteristics (e.g., weather/climate), tree species studied (beech, spruce and oak), and corresponding observations of stable isotopes of hydrogen and oxygen in tree xylem water. 

Field sampling: Our study is carried out in the context of a long-term forest health monitoring program that now includes nearly 200 sites (Braun et al., 2017, 2020). Each site contains at least one of three key tree species: beech (Fagus sylvatica L.), spruce (Picea abies (L.) H. Karst.), and oak (Quercus robur L.).

We collected samples from 8 individuals of each species between 22 July and 3 August 2019 from 24 sites (n = 258 trees in total, one site has 10 individuals of a species). Fully sunlit branches were collected using pole pruners by a technician suspended below a helicopter. Samples were returned to the ground, immediately enclosed in a black plastic trash bag to suppress transpiration, and left in a refrigerated cooler until further processing (< 24 h). Sites were selected to be broadly representative of the ranges of temperature, precipitation, elevation, and longitude included in the monitoring network as a whole. We compare our observations in 2019 to observations in the same 24 sites from our original sampling in 2015, where we collected samples using the same methodology from 3-8 of the same individuals of each species (n = 147 trees in total); this sampling was conducted between 27 July and 10 August 2015. A smaller subset of these sites was sampled in 2018 (n = 42 trees in total), which is discussed in the supporting information.

Sample processing: For each sample, the bark and vascular cambium were removed from a fully suberized piece of the branch, the remaining xylem sealed in a glass vial, and the vial frozen until water extraction. Water extraction was carried out via cryogenic vacuum distillation at the Paul Scherrer Institute (2015) and at ETH Zurich (2019) on manifolds with an identical design as described in (Orlowski et al., 2013). Branch samples were extracted under a 0.03 hPa vacuum for 2 h at 80 ºC and the evaporated water frozen in a liquid nitrogen trap. Samples were then analyzed for δ¹⁸O and δ²H by TC/EA-IRMS in the Siegwolf Lab at the Paul Scherrer Institute (2015) and in the Kirchner Lab at ETH Zurich (2019). All values are presented per mil (‰) relative to V-SMOW. The standard deviation of an independent quality control standard used for analysis at the Paul Scherrer Institute was ≤ 0.2 ‰ δ¹⁸O and ≤ 0.4 ‰ δ²H. The standard deviation of an independent quality control standard used for analysis at ETH Zurich was 0.17 ‰ δ¹⁸O and 0.62 ‰ δ²H.

Analytical approach: To determine the seasonal origin of the precipitation in trees, we used an index of the isotopic signature of plant xylem water relative to seasonal cycles of isotopes in precipitation (Allen et al., 2019b). This seasonal origin index (SOI) can be estimated by knowing the fractionation-compensated δ²H isotopic signature of xylem water, and the δ²H isotopic signatures of winter, summer, and volume-weighted annual precipitation at each study site. The compensation for fractionation introduced by evaporation involves projecting dual-isotope measurements of xylem water to local meteoric water lines along theoretical evaporation-line slopes, using the method described in detail in Allen et al. (2019b). Isotopic signatures of precipitation were estimated using data from the two years prior to sampling (i.e., August 2013-July 2015 for the 2015 xylem samples, and August 2017-July 2019 for the 2019 xylem samples), as inputs to a previously described and validated model (Allen et al., 2018). SOI provides a measure of the overrepresentation of a season’s precipitation in xylem relative to the representation of that season’s precipitation in the annual precipitation amount. Negative values of SOI suggest an overrepresentation of winter precipitation in xylem and positive values suggest an overrepresentation of summer precipitation in xylem. The SOI will be near −1.0 for soil and plant water samples derived entirely from winter precipitation and near 1.0 for samples derived entirely from summer precipitation.

Literature Cited:

Allen ST, Freyberg J von, Weiler M, Goldsmith GR, Kirchner JW. 2019a. The Seasonal Origins of Streamwater in Switzerland. Geophysical Research Letters 46: 10425–10434.

Allen ST, Kirchner JW, Braun S, Siegwolf RTW, Goldsmith GR. 2019b. Seasonal origins of soil water used by trees. Hydrology and Earth System Sciences 23: 1199–1210.

Allen ST, Kirchner JW, Goldsmith GR. 2018. Predicting Spatial Patterns in Precipitation Isotope ( δ 2 H and δ 18 O) Seasonality Using Sinusoidal Isoscapes. Geophysical Research Letters 45: 4859–4868.

Braun S, Schindler C, Rihm B. 2017. Growth trends of beech and Norway spruce in Switzerland: The role of nitrogen deposition, ozone, mineral nutrition and climate. Science of The Total Environment 599–600: 637–646.

Braun S, Schindler C, Rihm B. 2020. Foliar Nutrient Concentrations of European Beech in Switzerland: Relations With Nitrogen Deposition, Ozone, Climate and Soil Chemistry. Frontiers in Forests and Global Change 3.

Orlowski N, Frede H-G, Brüggemann N, Breuer L. 2013. Validation and application of a cryogenic vacuum extraction system for soil and plant water extraction for isotope analysis. Journal of Sensors and Sensor Systems 2: 179–193.

We provide a read me file (.txt), data file (.csv), a metadata file (.csv), and an ecological markup language file (EML) with the same metadata.