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.
Introduction
Plants drive water cycling at local to global scales, with their uptake
from subsurface water storages accounting for the majority of
terrestrial evapotranspiration (Nelson et al. , 2020).
Understanding the dynamics of how these plant-available subsurface water
storages are recharged and extracted can improve our ability to predict
transpiration fluxes and drought vulnerability. Stable isotope ratios of
water are widely used to identify water sources to plants (White, 1989),
and the combined use of plant and soil water isotope data has revealed
useful (and sometimes counterintuitive) findings concerning
plant-soil-water interactions (Kirchner et al. , 2023). For
example, Dawson and Ehleringer, 1991 found that the xylem signature of
riparian trees was different from that of streamflow, suggesting that
some trees use soil water held in tension even when streamwater is
available. Brooks et al. (2010) showed that infiltrating
precipitation can pass through soils and reach streams with apparently
little mixing with the stored water that supplies trees. Allen et
al. (2019a & 2019b) showed that recent precipitation can reach streams
even when soil water deficits exist, and that trees can access water
from previous seasons even when more recent water should be available.
Such processes generally conflict with conceptual models in which new
inputs refill antecedent deficits, rather than bypassing those
water-depleted storages. Our study builds on such findings, seeking to
understand how such hydrologic behavior occurs, when and where it is
expected to occur, and what implications it has for precipitation inputs
supplying transpiration.
Seasonal signals in precipitation, with isotopically heavier
precipitation in summer and lighter precipitation in winter, allow us to
track the relative abundance of precipitation from each season in
groundwaters (Jasechko, 2019; Jasechko et al. , 2014), streamflow
(Allen et al. , 2019a) and plants (Martin et al. , 2018;
Allen et al. , 2019b; Goldsmith et al. , 2022; Sprengeret al. , 2022). Soils carry the isotopic signature of many
previous precipitation events in any given layer, and trees may take up
water from multiple soil layers in different proportions (Warrenet al. , 2007). Despite this mixing, greater reliance on one
seasons’ precipitation versus another can be observed. These
observations have been made both in regions with dry growing seasons
(Brooks et al. , 2010; Rempe and Dietrich, 2018) and in regions
with year-round precipitation (Allen et al. , 2019b; Goldsmithet al. , 2022;). However, an important unknown is how new
precipitation inputs are used (or not used) by forest trees. Shifts in
water uptake depths during water limitations have been observed for
crops and trees (Rothfuss and Javaux, 2017; Sun et al. , 2022).
But what happens when dry periods are interrupted by new precipitation
inputs? Is there a rapid shift in sources to favor the use of those new
inputs?
Here we use a 2-year dataset of precipitation, soil water and xylem
water isotopes to identify how trees’ water sources vary across seasons
and wetness conditions. Our analysis focuses on two widespread species
in Europe, Norway spruce (Picea abies ) and European beech
(Fagus sylvatica ), which account for 44% and 18% of the total
Swiss forest inventory. Using these data, we address the following
research questions:
- What soil water sources are being used by forest trees? This is
assessed by comparing isotopic signals in xylem water to those of
precipitation, mobile soil waters and bulk soil waters across the
whole observation period and for single sampling dates.
- How do seasonal and event precipitation mix with water stored in
soils, by depth and across varying antecedent conditions?
- Does water from specific depths, precipitation from individual events,
or precipitation from specific seasons dominate the mixture of water
used by trees and, if so, does that dominance vary throughout the
year?
Study Site and Methods
Sampling and data collection
Our experimental field site is a small 0.3 km2catchment along a mixed forested hillslope dominated by spruce and beech
trees at a mean elevation of 510 m a.s.l. in Zurich, Switzerland. The
site is part of the larger “Waldlabor” Zürich
(www.waldlabor.ch) initiative.
The mean annual temperature of
the site is 9.3 °C, and mean annual precipitation is 1134 mm. Since
March 2020 we have measured and sampled various waters along the
hillslope: precipitation after each event, bulk & mobile soil waters,
as well as beech and spruce (and young spruce) xylem waters.