Figure 8: Frequency distributions of the seasonal origin index
(SOI) for the summer half of the year (May through October) for beech(a) , spruce (b) and young spruce xylem (c)(upper row), mobile soil waters in 10 cm (d) , 20 cm(e) and 40cm (f) (middle row) and bulk soil waters in
10 cm (g) , 20 cm (h) , 40cm (i) (bottom row).
The colored and gray bars indicate the distributions for campaigns with
low and high antecedent precipitation, respectively, in the 30-day
period prior sampling. The asterisk indicates that there are significant
differences (p<0.05, t-test) between SOI in dry and wet
antecedent conditions.
Our results suggest that although mean annual precipitation has impact
on the lag between precipitation seasonality and xylem seasonality
(Goldsmith et al. , 2022; Guo et al. , 2018; Williams and
Ehleringer, 2000), antecedent wetness (precipitation in 30 days prior to
the sampling dates) appears to have no significant effect (Figure 8).
Thus, we hypothesize that overall root structures of the trees at a
specific site, which are probably adapted to the average precipitation
that a location receives, are dominantly affecting the lag between
precipitation seasonality and xylem seasonality. Short-term (i.e., last
30 days) moisture availability had little effect on the seasonal signals
in the xylem, thus we infer no shifts in the accessed water pools and
uptake depths at our site for these two non-drought years. However, this
might not hold for exceptionally dry years, as previous studies showed
that trees might shift their water uptake depths during water
limitations (e.g., Brinkmann et al. , 2019; Gartner et al. ,
2009; Meißner et al. , 2012).
Methodological limitations of xylem and soil water signal
interpretation
Although we took regular bulk soil samples with an auger, we did not see
a change in the bulk soil signal (see Figure 2) resulting from the
artificial creation of preferential flow paths through drilling (von
Freyberg et al. , 2020). Recent studies have pointed to potential
extraction bias when using cryogenic vacuum extraction (e.g., Chenet al. , 2020). Whereas for bulk soil water extractions, the bias
is potentially small and thus negligible (Newberry et al. , 2017),
Chen et al. (2020) clearly documented δ2H
offsets in xylem waters. These offsets were attributed to the exchange
of 2H in the wood tissue with waters in the xylem.
However, in a more recent study, Diao et al. (2022) showed that
these offsets are potentially small when large amounts (i.e.,
> 600 µl) were extracted. This was true for most of the
xylem samples (>95%) we presented in the study, where we
typically extracted > 1 ml of xylem water. Another effect
on the xylem signal can originate from water stored in xylem not
actively contributing to transpiration. In a recent study, Barbetaet al. (2022) used a cavitron centrifuge at specific spinning
rates to specifically extract sap water from xylem and intra-cellular
water stored in the xylem tissue. Barbeta et al. (2022) found
that the sap xylem water matched the irrigation water (with no effects
of isotopic fractionation during root water uptake), but the water
extracted from xylem tissue storage was always depleted in
δ2H; this depletion could potentially also influence
our results. However, we calculated the effect of the potential offset
by δ2H depletion, i.e., a bias of -6.1 ‰; Allen and
Kirchner, 2022), and found that our major conclusion derived from
Figure 6 does not change and forest trees (at our site) were indeed
containing a mixture of water dominated by winter precipitation
throughout the entire year (see Supplementary Material – Figure S3).
Therefore, our main conclusions are robust against the potential biases
introduced by water extraction that have been described elsewhere.