Plant water stable isotopes had not been used widely to study plant water use across Northern environments. Doerthe Tetzlaff's EU funded VeWa project addressed this research gap and an inter-site comparison of five involved sites has now been published in Hydrological Processes (open access). We show how the sampled xylem isotopes compare to precipitation, groundwater, and soil water and discuss why there is often a mismatch between the plant water and their potential sources.

We compiled stable isotope data (2H and 18O) of bulk soil water at five long-term experimental catchments across a hydro-meteorological gradient in the northern latitudes (Figure 1). The comparison of the extensive data set, covering different landscape units at each catchment, showed that vegetation, topography and elevation affect the isotopic composition over time and soil depth. Soil water beneath conifers is more enriched in heavy isotopes than beneath heather or oak vegetation. Sampling sites closer to the stream are generally less variable in their stable isotopic composition than sites at gentle hillslopes. Isotopic fractionation due to soil evaporation was observed mainly in the top 10 to 30 cm and was mainly controlled by the precipitation amount while temperature (as proxy for evaporation) was less important and soil moisture did not influence the isotopic fractionation, as revealed by multiple linear regression. 
The study will be published (here) in a Special Issue on "Water in the Critical Zone" in Hydrological Processes. The work was funded by the ERC grant VeWa and you can find a presentation by Doerthe Tetzlaff about that project here.

Tetzlaff, Doerthe, Ala-Aho, P; Buttle, J; Carey, SK; Kohn, M; Kuppel, S; Laudon, H; McDonnell, J; McNamara, JP; Spence, C; Sprenger, M; Smith, A; Soulsby, C :"Using stable isotopes to understand vegetation‐water linkages across northern landscapes (VeWa)"
Sprenger, M; Tetzlaff, D; Ala-Aho, P; Buttle, J; Laudon, H; Mitchell, C; Snelgrove, J; Weiler, M; Soulsby, C: Keynote on "Water fluxes, transport and transit times" including recent work on "Mobile and tightly bound soil water fluxes in northern environments"
See the program of the conference here.

Our study on the influence of forest and shrub canopies on precipitation partitioning and isotopic signatures is out in Hydrological Processes and available on Researchgate.