4.2 Ecohydrological dynamics
Throughfall (> 1 mm; not shown) only occurred during three
major precipitation events and varied between the sampling points
underneath the canopy as follows: 2.8 - 5.5 mm on 15.09., 9‑15 mm on
29.09 and 2.5-4.8 mm on 24.10. Soil moisture in the upper soils rapidly
responded to rainfall at both sites, though quickly dried until more
persistent rewetting towards November (Fig 4b). The tree site soil was
generally more responsive to wetting particularly following heavy
rainfall on September 29th. The average groundwater
level (Fig 4b) was around 2.3 m b.g.l. and varied only by 2cm as it was
primarily controlled by the water levels in the lake.
Daily mean sap velocity ranged from 0-9.2 cm h-1(0-0.92 m h-1; Fig. 4c-d). The southern side of the
tree showed higher values but similar dynamics until October. Towards
the end of the growing period, the northern side had slightly higher sap
velocities. Cumulative increments of stem size showed progressive growth
between the end of August and November. Normalised for mean sap
velocity, sapvelocitynorm showed the same ranges as mean
ETnorm (i.e. also normalized against the mean) from
August until mid-October, implying there was no limit on the trees
meeting atmospheric moisture demand. As leaf senescence and fall
progresses in the middle of October, ETnorm exceeded
sapvelocitynorm.
Stable water isotope dynamics
Figure 5 shows that stable water isotopes in precipitation were highly
variable and showing more negative values (i.e. high depletion in
heavier isotopes) for events in late August and early November.
δv at the tree and grassland sites (both exemplary shown
for 2 m height) was influenced by depleted rainfall inputs. For example,
δv at the grassland sites (prior to tree monitoring
commencing) showed particularly high depletion in response to depleted
rainfall at the beginning of September (Fig 5a). Summary statistics of
measured stable water isotope values and ranges of precipitation,
groundwater and atmospheric vapour δv (liquid values) are given in Table
1.
After testing Spearman´s rank correlation between δv and
soil moisture at both sites for different heights, only the grassland
site indicated a significant positive correlation of soil moisture with
δ2H of δv (0.3 at 0.15 m; 0.24 at 2 m;
0.22 at 10 m), but none for δ18O. The tree site showed
no significant correlations between δv and soil moisture
for either isotope.
Lc‑excess in precipitation increased from late summer to late autumn,
though variability remained high especially in October. The lc‑excess
was generally negative at both sites until mid-September (Fig. 5b),
reflecting high energy for evaporation. From Mid-September until
November, lc‑excess of δv was generally positive but
more variable. Spearman rank correlation coefficients between
precipitation and δv were 0.55 for δ²H and 0.43 for
lc-excess indicating positive correlations.
The amount-weighted LMWL of the sampling period (July – November 2021)
(Fig. 6) was δ2H = 7.71 ± 0.11 *
δ18O + 7.42 ± 1.12 (R² = 0.987). The dual
isotope plot (Fig 6) clearly shows that precipitation isotopes were
characterized by large ranges (δ2H -145.2 ‰ to -10.2
‰; δ18O ‑19.3 ‰ to -1.5 ‰), groundwater in comparison
showed little variation with mean isotopic signatures (Tab. 1). Beneath
the tree canopy, δv was more homogenous across the
elevation profile than above grass. Grassland showed a higher variance
of ambient vapour within the elevation profile, with a tendency of
near-surface air (0.15 m height) to be more enriched in heavier isotopes
in comparison to the tree-site surface air, but attenuating with height
(Tab. 1). Overall, the boxplots and median values of grassland
δv showed a slightly higher range compared to the tree
site (Fig 6, Tab 1). The Kruskal-Wallis-Test showed significant
differences of δv (p-values < 0.05) between
ground-level (0.15 m) and higher elevations (2 m, 10 m) at the
grassland, while there were no significant differences (p-values
> 0.05) between different heights underneath the tree
canopy and also no significant differences between both sites. The
Wilcoxon signed-rank test indicated a p-value of 0.0507 (α=0.05) between
the sites for 0.15 m height showing they were significantly different.
We also investigated higher resolution dynamics of δvduring precipitation events, and we display here the event on the
afternoon of 15.09.2021 (where 6.6 mm fell between 15:00 – 18:30; Fig.
7). Both sites showed enriched δv values at night
corresponding to the signature of precipitation, with uniform
distribution at different heights. The next day, δvabove grassland reflected clear evaporative losses by more enriched
values just above the surface, where windspeed and soil moisture was
higher, which was also observed during the other events. In the tree
canopy, no differences with height occurred. During the event, soil
water content underneath the tree was 4.2 % and increased to 5.6 % 24
h after the rainfall; whereas the grassland was wetter increasing from
5.0 % to 8.7 % (see also Fig. 4). A common aspect for all the
precipitation events was a close link between precipitation isotopic
signature and δv.