Figure 7: Frequency distribution of the seasonal origin index (SOI) for beech (a) , spruce (b) and young spruce xylem(c) (upper row), mobile soil waters at 10 cm (d) , 20 cm (e) , 40cm (f) and 80 cm (g) (middle row) and bulk soil waters at 10 cm (h) , 20 cm (i) , 40cm(j) and 80 cm (k) (bottom row). The colored bars indicate the distribution for the summer half of the year (May through October), the gray bars indicate the distribution for the winter half of the year (November through April). Asterisks indicate that there are significant (p<0.05, t-test) differences between the SOI in summer and winter.
Looking at the overall seasonality signals in xylem waters (Figures 6) we found temporal disconnections between the water in xylem and precipitation, i.e., the beech and spruce forest trees contained winter precipitation throughout the year. This has been shown across Switzerland in previous work by Allen et al. (2019) and Goldsmithet al. (2022), but those studies were based on snapshot sampling dates in summer, whereas our study shows that these observations may also hold across the year. Although our site typically receives more precipitation during the summer months than during the winter months (ratio of approximately 67% to 33% for May through October and November through April, respectively for our two-year observation period) soils were typically drier during the summer months. This presumably reflects a greater fraction of summer precipitation being evaporated back to the atmosphere, and thus never reaching the deeper soil layers where it would be available for forest trees (i.e., deeper than 10 cm). Also in summer, water in the top layer might be consumed by forest floor vegetation (i.e., shrubs and grasses) that roots in the upper layers of the soil.
A second potential explanation for the relative scarcity of summer precipitation in summer xylem samples might be interception processes in the forest canopy and the forest‑floor litter layer. The fraction of summer precipitation that is available for trees for water uptake may be very small due to canopy and forest floor interception, which may be reducing soil water recharge by ~40% (Floriancicet al. , 2022; Gerrits et al. , 2010) compared to what it would have otherwise been; much of the remainder may also be evaporated from soils or taken up by understory plants before reaching tree roots. Thus, the net precipitation remaining accessible to tree roots following other evaporation processes may explain why winter precipitation dominates tree water sources even if it accounts for less than half of annual precipitation. Indeed, Figure 2d shows that bulk soils deeper than 10-20 cm rarely show summer-like precipitation signatures, even in summer; however, this evidence does not clarify whether the first or second potential reasons is more relevant.
Some previous (in-situ) isotope studies have documented trees taking up recent precipitation (i.e., summer precipitation in summer months). For example, Gessler et al. , 2022 found that a beech tree took up most of its xylem water from the topsoil (filled with recent precipitation), and did not shift water uptake to deeper (most likely older) water pools during dry periods. However, the experiment was carried out during the severe 2018 drought at a single tree outside of a forest, which might not reflect the competition for water that trees experience in a dense forest stand. However, these and similar findings point to the importance of looking at both the seasonal signals in the bulk xylem isotopes and the relationships between precipitation and xylem isotopes during individual sampling dates.
Another reason for the seasonal disconnection between isotope signals in precipitation and xylem lies in the aggregation of the data (i.e., looking at clusters of summer vs. winter isotopic signatures). We saw a seasonal cycle in precipitation isotopes, as evident from Figure 2a, with a significant difference (p < 0.05 ) in both δ18O and δ2H between summer and winter. Distributions of winter and summer precipitation isotopes were still fairly symmetrical (Pearson median skewness between -0.3 and 0.3). However, individual precipitation events occurring during the winter half of the year were isotopically heavier than typical winter precipitation, and individual summer precipitation events were isotopically lighter than typical summer precipitation. This results in partial overlaps between the distributions of summer and winter precipitation isotopes (see Figures 2a and overlap of the boxplots in Figure 6).
Seasonality patterns in wet growing seasons
While the summer of 2020 received normal amounts of precipitation, the summer half of the year 2021 was unusually wet i.e., 607 mm and 752 mm for the summer half precipitation in 2020 and 2021, respectively. Previous studies (Goldsmith et al. , 2022; Guo et al. , 2018; Williams and Ehleringer, 2000) have reported that discrepancies between summer precipitation and summer xylem waters are smaller in wet years; thus a wet summer like the one observed in 2021 should have led to more summer precipitation transpired by trees in summer, as a result of greater input of recent precipitation to the soil. We tested this by replotting SOI as shown in Figure 7 for the summer half of the year (May through October) for the wettest half of sampling dates (in colors) and the driest half of sampling dates (in shades), splitting the dataset by precipitation sums in the 30 days prior to xylem and soil sampling (Figure 8). We observed that the differences between the driest and wettest halves of the sampling dates were rather small in xylem water signatures, with mean SOI varying from 0.00 to ‑0.07, ‑0.15 to ‑0.04, and ‑0.24 to ‑0.15, for beech, spruce and young spruce respectively (Figure 8a-c). Similarly small differences were observed for bulk soil water SOI: mean SOI across all depths shifted from ‑0.08 to ‑0.03. But we saw a larger shift in mobile soil water SOI, from 0.21 to 0.12, across all depths between the drier and wetter summer sampling dates. The largest differences were observed in 10 cm (p < 0.05) and 40 cm (not significant) depths (SOI shift of ‑0.09), however, observed shifts between subsets were only significant for 10 and 80 cm (indicated by asterisks in Figure 8). Generally, we found that in campaigns with less antecedent precipitation, summer mobile waters contain more summer precipitation, but that trend was minimally apparent in xylem and bulk soil water signatures (Figure 8).