On the Spatio-Temporal Under-Representation of Isotopic Data in Ecohydrological Studies

Rationale: New methods to measure stable isotopes of soil and tree water directly in the field enable us to increase the temporal resolution of obtained data and advance our knowledge on the dynamics of soil and plant water fluxes. Only few field applications exist. However, these are needed to further improve novel methods and hence exploit their full potential. Methods:We tested the borehole equilibration method in the field and collected in situ and destructive samples of stable isotopes of soil, trunk and root xylem water over a 2.5-month experiment in a tropical dry forest under natural abundance conditions and following labelled irrigation. Water from destructive samples was extracted using cryogenic vacuum extraction. Isotope ratios were determined with IRIS instruments using cavity ring-down spectroscopy both in the field and in the laboratory.Results: In general, timelines of both methods agreed well for both soil and xylem samples. Irrigation labelled with heavy hydrogen isotopes clearly impacted the isotope composition of soil water and one of the two studied tree species. Intermethod deviations increased in consequence of labelling, which revealed their different capabilities to cover spatial and temporal heterogeneities. Conclusions:We applied the novel borehole equilibration method in a remote field location. Our experiment reinforced the potential of this in situ method for measuring xylem water isotopes in both tree trunks and roots and confirmed the reliability of gas permeable soil probes. However, in situ xylem measurements should be further developed to reduce the uncertainty within the range of natural abundance and hence enable their full potential. | INTRODUCTIONPlant transpiration fuels the hydrological cycle by returning 35% to 90% of water from land surfaces to the atmosphere. 1,2 Therefore, transpiration greatly influences our climate and impacts water availability in consequence of land use and climate change. 3,4 However, why is the quantification of this essential water flux so uncertain? This comes down to knowledge gaps in the mechanistic functioning of root water uptake (RWU) as well as plant rooting depth that persist despite its crucial role in predicting the future of one of our most important resources. 5 A major constraint is the practical difficulty in observing below-ground processes. Moreover, the magnitude and location of RWU are the results of multiple influencing factors, such as extent of the root system and its hydraulic properties,

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“…They can be categorised into (a) methodological uncertainty and (b) true natural variability 66 and are discussed in this order hereafter.…”

Section: Tree Xylem Water δ Valuesmentioning

confidence: 99%

Continuous in situ measurements of water stable isotopes in soils, tree trunk and root xylem: Field approval

Kühnhammer

Dahlmann

Iraheta³

et al. 2022

Rapid Comm Mass Spectrometry

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“…Other studies have provided some indications that the isotopic offset could arise from δ 18 O fractionation processes (Marshall et al, 2020; Vargas et al, 2017), challenging the argument of negligible 18 O fractionation during root water uptake (Rothfuss & Javaux, 2017). We do not have evidence that goes beyond the behaviours recently discussed (Beyer & Penna, 2021; von Freyberg et al, 2020), but we cannot exclude that unsampled saprolite and weathered bedrock waters could act as additional sources potentially explaining this offset.…”

Section: Discussionmentioning

confidence: 76%

Transpiration patterns and water use strategies of beech and oak trees along a hillslope

et al. 2021

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The role of landscape topography in mediating subsurface water availability and ultimately tree transpiration is still poorly understood. To assess how hillslope position affects tree water use, we coupled sap velocity with xylem isotope measurements in a temperate beech-oak forest along a hillslope transect in Luxembourg. We generally observed greater sap velocities at the upslope locations in trees from average-sized trees, suggesting the presence of more suited growing conditions. We found a lower difference in sap velocity among hillslope positions for larger trees, likely due to the exploitation of deeper and more persistent water sources and the larger canopy light interception. Beech trees exploited a shallower and seasonally less persistent water source than oak trees, due to the shallower root system than oak trees. The different water exploitation strategy could also explain the stronger stomatal sensitivity of beech to vapour pressure deficit compared to oak trees. Xylem isotopic composition was seasonally variable at all locations, mainly reflecting the contribution of variable soil water sources and suggesting that groundwater did not contribute, or only marginally contributed, to tree transpiration. Overall, our results suggest that trees along the hillslope mainly rely on water stored in the unsaturated zone and that seasonally shallow groundwater table may not necessarily subsidize water uptake for species that do not tolerate anoxic conditions. Contrary to previous studies, at our site, we did not find higher sap velocity downslope as the subsurface hillslope structure promotes vertical water flux over lateral redistribution in the vadose zone.

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“…Significant open questions remain concerning field sampling of trees for isotopic analysis to estimate root water uptake. Water isotopic heterogeneity in the environment (i.e., soils, xylem, groundwater) has been identified as a primary complicating factor in the study of plant water uptake, storage, and transpiration (Barbeta et al, 2020;Beyer & Penna, 2021;Freyberg et al, 2020;Goldsmith et al, 2019;Oerter & Bowen, 2019). Across all months, the observed standard deviation of hemlock δ 18 O (Fig.…”

Section: Xylem Water Isotopic Spatio-temporal Heterogeneitymentioning

confidence: 99%

Water Use Strategy of Riparian Conifers Varies with Tree Size and Depends on Coordination of Water Uptake Depth and Internal Tree Water Storage

Knighton

2022

Preprint

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Abstract. Trees employ mechanisms to maintain safe xylem water transport including variations in trunk water storage and the depth of root water uptake. We tested the hypotheses that 1) trunk water storage is correlated with root water uptake in Eastern hemlock, 2) and that water use strategy varies with tree size. High spatiotemporal sampling of soil and hemlock xylem (30 trees) water isotopic ratios (2H, 18O) and tree tissue Relative Water Content (RWC) was conducted across seven months. Hemlock accessing more evaporatively enriched water from shallow soils stored less water within their trunks during dry periods, and more during wet periods. Soil and xylem water isotopic compositions revealed older and lower elevation hemlock primarily sourced water uptake from the upper 10 cm of soils, whereas younger and higher elevation trees sourced some water uptake from deeper soil layers. Larger diameter hemlock showed significant temporal changes in trunk RWC. In contrast, smaller diameter trees exhibited more temporally stable RWC. Observed species-level heterogeneity in xylem water isotope composition suggests the need for reporting of tree ages and a standardization of field sampling protocols to support our understanding of tree water use strategies. Our results inform the development of plant hydraulic strategies in ecohydrological- and terrestrial biosphere-models to understand forest responses to external stressors.

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On the Spatio-Temporal Under-Representation of Isotopic Data in Ecohydrological Studies

Cited by 32 publications

References 90 publications

Continuous in situ measurements of water stable isotopes in soils, tree trunk and root xylem: Field approval

Continuous in situ measurements of water stable isotopes in soils, tree trunk and root xylem: Field approval

Transpiration patterns and water use strategies of beech and oak trees along a hillslope

Water Use Strategy of Riparian Conifers Varies with Tree Size and Depends on Coordination of Water Uptake Depth and Internal Tree Water Storage

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