Figure 3: Fraction of new precipitation (from the 30 days preceding the xylem water sampling date - Fnew ) found in xylem water of beech (orange), spruce (dark green) and young spruce (light green) as calculated from δ2H.The results for Fnew from δ18O are shown in supplementary Figure S2.
We used a simple mixing model to calculate how much precipitation water vs. bulk soil water across all soil depths (10, 20, 40 and 80 cm), shallow bulk soil water (10 and 20 cm) and deep bulk soil water (40 and 80 cm) can be found in the xylem waters of beech, spruce and young spruce (Figure 4). We excluded results for beech during the dormant season (November to April), as beech are not actively transpiring, and xylem signatures are more likely to represent enrichment of waters stored in the stem. We found that for many of the xylem sampling dates, beech, spruce and young spruce xylem contained a mixture of soil waters rather than recent precipitation (gray bars in Figures 4a, 4c, and 4e). More explicitly, xylem water δ2H lay between the δ2H of shallow and deep bulk soil water (i.e., 6, 10 and 12 times out of 19 samplings for beech, spruce and young spruce; see Table 1). Only a few xylem samples carried the signature of soil layers shallower than 20 cm (1, 2, and 0 times for beech, spruce and young spruce), beech xylem samples carried the signature of soil layers deeper than 40 cm on 5 out of 19 sampling dates. However, during fall and spring (i.e., April, May, September, October, and November; see Table 1), xylem signatures were closer to recent precipitation than soil water, suggesting that these samples were dominated by recent precipitation (2, 4 and 4 times for beech, spruce and young spruce). Xylem waters were enriched in heavy isotopes during December and February, yielding isotope signatures that were much heavier than the waters found in the soil (10 to 40 cm) or recent precipitation. This is expected because during times when trees are not using water, evaporative enrichment in the stem will lead to heavier isotopic signatures (marked in orange in Table 1).