Seasonal origins of soil water used by trees

Abstract

Abstract. Rain recharges soil water storages and either percolates downward into aquifers and streams or is returned to the atmosphere through evapotranspiration. Although it is commonly assumed that summer rainfall recharges plant-available water during the growing season, the seasonal origins of water used by plants have not been systematically explored. We characterize the seasonal origins of waters in soils and trees by comparing their midsummer isotopic signatures (δ2H) to seasonal isotopic cycles in precipitation, using a new seasonal origin index. Across 182 Swiss forest sites, xylem water isotopic signatures show that summer rain was not the predominant water source for midsummer transpiration in any of the three sampled tree species. Beech and oak mostly used winter precipitation, whereas spruce used water of more diverse seasonal origins. Even in the same plots, beech consistently used more winter precipitation than spruce, demonstrating consistent niche partitioning in the rhizosphere. All three species' xylem water isotopes indicate that trees used more winter precipitation in drier regions, potentially mitigating their vulnerability to summer droughts. The widespread occurrence of winter isotopic signatures in midsummer xylem implies that growing-season rainfall may have minimally recharged the soil water storages that supply tree growth, even across diverse humid climates (690–2068 mm annual precipitation). These results challenge common assumptions concerning how water flows through soils and is accessed by trees. Beyond these ecological and hydrological implications, our findings also imply that stable isotopes of δ18O and δ2H in plant tissues, which are often used in climate reconstructions, may not reflect water from growing-season climates.

Persistence and Plasticity in Conifer Water‐Use Strategies

“…We refer to this as water-use plasticity where through either adjustments in the root systems or changes in the residence time and pathway of seasonal precipitation vectors, the seasonal origins of the water used by the trees evolves. In an alternative scenario, trees conserve their reliance on winter precipitation, which we refer to as the persistence scenario, and seems to be consistent with the ubiquitous use of winter precipitation as described by Allen et al (2019). To study these dynamics, decadal length datasets Journal of Geophysical Research: Biogeosciences 10.1029/2018JG004845 of plant water use are needed because many of the relevant subsurface processes such as residence time of deep water pools (Ghannam et al, 2016) and the turnover time of fine roots (Matamala et al, 2003) have multi-annual timescales.…”

“…To explore the processes that gave rise to the distinct clusters, we ran the cellulose model over the period from 1980 to 2016 with three distinct water use patterns: (1) exclusive reliance on snowmelt/winter precipitation as observed by Allen et al (2019), (2) reliance on growing season precipitation , and (3) the water use pattern observed from field observations during 2016, which was characterized by a mid season transition between reliance on winter to summer precipitation ( Figure 5). When the model was run with a winter precipitation water source, the average seasonal cycle over the 37-year simulation closely followed the structure of Cluster 1 (Figures 7a and 7e).…”

“…Despite this evidence of complexity in depth and spatial pattern of soil water isotopes, other recent work has found more homogenous patterns in the isotopic ratio of plant water, suggesting some of the isotopic heterogeneity observable within soil water profiles might be buffered as the signal is transferred to plants . The extensive survey of xylem waters across Switzerland by Allen et al (2019), noted that across a large domain the trees appeared to rely almost exclusively on winter precipitation. This observation could reflect multiple interacting processes including: the ubiquitous reliance of trees on deeper water pools; deeper water from a previous season migrating upward into the root zone; winter precipitation being retained in micropores; or that summer precipitation evaporated or passed through the root zone in macropores.…”

The Demographics of Water: A Review of Water Ages in the Critical Zone

“…Additionally, water age distribution of transpiration depends on species specific root distributions, for example, being older on average in Fagus sylvatica than for Picea abies at a Swiss study site (Brinkmann et al, 2018) and older for Pinus sylvestris than for Erica species at a Scottish study site (Sprenger, Tetzlaff, Buttle, Laudon, & Soulsby, 2018). Such species specific differences in water ages of plant water uptakes were recently also observed by Allen et al (2019), who found xylem water of beech and oak trees during midsummer to be isotopically similar to winter precipitation, while the source water for spruce trees was not clearly related to precipitation of a specific season. Independent of the species, trees across Switzerland were generally taking up older water at drier sites, indicating the trees' response to drought ; Figure 5).…”

Depth distribution of soil water sourced by plants at the global scale: A new direct inference approach

“…Third, most papers did not collect samples of soil and xylem waters at the same time or did not specify the collection time: this limits the assessment of the possible differences between the isotopic composition of xylem water and of its potential water sources in the light of the lag time between root water absorption and transport to the leaves, which can take days to weeks or even months (e.g., Allen et al, 2019;Brinkmann et al, 2018). However, given the large number of samples taken from different species and different climate regions, these possible differences are likely smoothed out.…”