Abstract
Forest trees greatly influence both the routing of water downward into
the subsurface and the re-routing of water upward through water uptake
and transpiration. To reveal how the subsurface soil water pools used by
trees change across seasons, we analyzed two years of stable isotope
ratios of precipitation, soil water from different depths (using both
bulk sampling and suction-cup lysimeters), and xylem in a mixed beech
and spruce forest. Precipitation as well as mobile and bulk soil waters
all showed a distinct seasonal signature; the seasonal amplitude
decreased with depth, and mobile soil waters varied less than bulk soil
waters. Xylem water signatures in both tree species were similar to the
bulk soil water signatures and rather different from the mobile soil
water signatures. The beech and spruce trees had different isotope
ratios suggesting use of different water sources, and these differences
were larger under dry antecedent conditions than wet antecedent
conditions. Despite these differences, both species predominantly
transpired waters with a winter-precipitation isotopic signature
throughout the summer, including during wet conditions when more recent
precipitation was available. Over most of the sampling dates, the
fraction of recent precipitation (i.e., from the preceding 30 days) in
xylem water was low, despite both species typically demonstrating use of
both shallow and deeper soil waters. These results provide evidence that
the soil water storages used by these trees are largely filled in winter
and bypassed by recent precipitation, implying long residence times.