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).