Figure 5: Isotope signature of precipitation in the 12 months prior to the bulk soil (10, 20, 40 and 80cm) and xylem sampling (beech, spruce, young spruce) on 27 July 2021 for 18O(a) and 2H (b) . The blue and yellow boxes indicate winter and summer precipitation, respectively. The boxplots (here and in all upcoming dual isotope plots) indicate the interquartile range for each species; the line indicates the median. The points surrounding the boxplots indicate the single measurements; the grey horizontal bar in the background marks the interquartile range of xylem isotope signatures across all three species.
Results from the mixing model suggest that beech trees typically used deeper water sources than spruce (and young spruce), which is in line with the expected differences in their rooting depths. While beech roots have heart roots, with most roots occupying the top 40 cm, spruce species have distributed roots in the shallow soil up to around 25 cm (Schmid and Kazda, 2002). Previous studies found that beech took up waters from deeper layers (Meißner et al. , 2012), especially when water closer to the surface was limited (Brinkmann et al. , 2019). However, caution is warranted as there are other factors that may complicate interpretation of such data. For example, in mixed beech and spruce plots, root development and spatial distribution of roots were different than at sites were only beech grew, even in places where water resources were typically not limited (Cahill et al. , 2010; Schmidet al. , 2015). So at our site where beech and spruce coexist, we’d expected that their root systems occupy separate soil layers to avoid competition for water resources (Meißner et al. , 2012). Spruce took up water from shallower layers, which is in line with the spruce water uptake depth of 10 to 20 cm reported by Bishop and Dambrine (1995).
Seasonality of precipitation, soil and xylem waters
We separately examined the data from each of the winter and the summer halves of the years, November through April and May through October, respectively. We excluded dormant-season beech xylem signatures, because these are likely to reflect stem evaporative enrichment rather than the source waters. Figure 6 shows that the xylem signatures of beech (in summer) and spruce and young spruce (in both winter and summer) resembled winter precipitation. This is also confirmed by the non-significant differences between winter precipitation and xylem waters as calculated from Wilcoxon signed rank tests (Table 2). Xylem waters were isotopically much lighter than summer precipitation, implying that throughout the year, tree xylem at our site mostly contained winter precipitation, despite Zurich typically receiving more precipitation during the summer months than during the winter months. During our two-year observation period, the total volume of precipitation falling during summer (May through October) was twice the volume of precipitation falling during winter (November through April).
Table 2: Wilcoxon signed rank tests comparing isotopic signals in winter and summer xylem versus winter and summer precipitation.