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:
  1. 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.
  2. How do seasonal and event precipitation mix with water stored in soils, by depth and across varying antecedent conditions?
  3. 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.