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 ( = 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.