Do Trees Truly Take Up Winter Precipitation During Summer?

Terrestrial ecosystems’ response to atmospheric changes (e.g., CO₂ levels, vapor pressure deficit) and the redistribution of water on land has drawn significant attention in recent years. Understanding the sources of water that trees utilize is critical for elucidating their adaptation strategies and resilience as precipitation patterns and seasonality shift in a changing climate.

Recent studies suggest that trees primarily rely on winter precipitation during summer (e.g., Allen et al., 2019; Goldsmith et al., 2022; Floriancic et al., 2024). However, the intercomparison of xylem water extraction methods for stable isotope analysis reveals substantial isotopic variation depending on the method employed.

In this study, we used cryogenic vacuum distillation (CVD), the Scholander pressure bomb (SPB), and in situ vapor equilibrium methods to determine the stable isotopic composition (²H and ¹⁸O) of xylem water in Scots pine trees. Our research was conducted under a long-term (20-year) irrigation experiment at the Pfynwald, Switzerland. Sampling included plots with trees growing under naturally dry conditions (control), irrigated conditions (since 2003), and previously irrigated conditions (irrigation ceased in 2014 after 10 years).

Our analysis demonstrates that SPB measurements align closely with in situ vapor equilibrium measurements, while the CVD method exhibits a significant offset in ²H and ¹⁸O isotopic values. Furthermore, we show that conclusions regarding the seasonal origin of xylem water—whether winter or summer precipitation—are highly dependent on the extraction method used. If the choice of extraction method significantly influences conclusions about the seasonal orientation of tree water uptake, our predictions of tree responses to future shifts in precipitation patterns could be fundamentally flawed. These findings highlight the urgent need for methodological standardization to enhance the reliability of isotopic interpretations in tree water uptake studies.