DOI: 10.5061/dryad.4j0zpc8dg
title: Data from: Climatic influences on winter precipitation use by trees in summer

description: 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. 
description: <em><strong>Field sampling:</strong></em>
 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
 (<em>Fagus sylvatica </em>L.), spruce (<em>Picea
 abies</em> (L.) H. Karst.), and oak (<em>Quercus
 robur</em> 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
 (&lt; 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.
 <em><strong>Sample
 processing:</strong></em> 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
 <em>et al.</em>, 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
 δ<sup>18</sup>O and δ<sup>2</sup>H 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 ‰ δ<sup>18</sup>O and ≤ 0.4 ‰ δ<sup>2</sup>H.
 The standard deviation of an independent quality control standard used for
 analysis at ETH Zurich was 0.17 ‰ δ<sup>18</sup>O and 0.62 ‰
 δ<sup>2</sup>H.
 <em><strong>Analytical
 approach:</strong></em> 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 <em>et al.</em>, 2019b). This seasonal origin index
 (SOI) can be estimated by knowing the fractionation-compensated
 δ<sup>2</sup>H isotopic signature of xylem water, and the
 δ<sup>2</sup>H 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 <em>et al.</em>, 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. <em><strong>Literature
 Cited:</strong></em> 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.
description: 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.