Figure 6: Dual-isotope plot for tree xylem (beech, spruce, and young spruce shown by orange, dark green, and light green, respectively) and summer vs. winter precipitation (light and dark blue, respectively). Boxplots show distributions of isotopic signals in precipitation and xylem waters for winter (November through April) and summer (May through October). The xylem water signatures in both winter and summer are more consistent with winter precipitation. Winter xylem isotopes are not shown for beech because beech does not actively transpire during winter.
We calculated the seasonal origin index SOI (Allen et al. 2019) for all xylem samples (beech, spruce and young spruce) and the mobile and bulk soil water samples. Figure 7 shows the distribution of SOI for the summer half of the year (May through October, in colors) and the winter half of the year from (November through April, in gray). Most xylem isotope samples (Figure 7a-c) had an SOI < 0, indicating that they are mixtures dominated by winter precipitation. This was especially evident in spruce and young spruce xylem, and less obvious in beech, where samples seem to be quite evenly distributed around a SOI of 0. Surprisingly, the few samples of beech xylem water from the winter months were dominated by SOI > 0, suggesting that they were dominated by summer precipitation (or because there is no transpiration in winter months, that they were evaporatively enriched while stored in the branches).
SOI values for mobile soil water showed distinct variation across the different depths: whereas at 10 cm, 20 cm and 40 cm depth most summer soil water had SOI > 0 (indicating a predominantly summer source) winter soil water was well mixed between summer and winter. The fraction of samples with SOI < 0 increased with depth, with winter soil waters at 40 cm mostly having SOI < 0. At 80 cm, mobile soil waters sampled in both winter and summer were well mixed between winter and summer precipitation (based on less than one year of samples because sampling at this depth only started in June 2021).
Bulk soil waters in summer were almost evenly distributed around SOI = 0, with slightly larger fractions of winter precipitation with increasing depth. However, in the winter half of the year (and for both winter and summer soil water samples at 80 cm depth) most bulk soil water samples were predominantly composed of winter precipitation (with the most samples with SOI > 0 at 10 cm depth).