Isotopic fractionation from deep roots to tall shoots: A forensic analysis of xylem water isotope composition in mature tropical savanna trees

Grau, Adriana Vega; McDonnell, Jeffrey J.; Schmidt, Susanne; Annandale, Mark; Herbohn, John

doi:10.1016/j.scitotenv.2021.148675

Cited by 23 publications

(30 citation statements)

References 51 publications

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“…Given the mounting evidence, this isotopic depletion in xylem water may be ubiquitous (de la Casa et al, 2021), and Ellsworth and Williams (2007) have suggested it is related to isotopic fractionation during root uptake. Vega‐Grau et al (2021) found increasing deviation of xylem water isotopes from deep roots to stems in mature tropical savanna trees, suggesting fractionation along the transpiration stream. Other work by Poca et al (2019) shows the offset between xylem and source water can be enhanced by the presence of arbuscular mycorrhizal fungi, which are likely to be present in Magela Creek due to the inherent low nutrient status of the leached sands.…”

Section: Discussionmentioning

confidence: 99%

“…Trees in the area may have dimorphic root systems (Dawson & Pate, 1996; Ehleringer & Dawson, 1992) where wet season soil moisture drives nutrient mineralisation and access via shallow roots in the top soil supplemented by a more reliable water source accessed during the dry season via deeper roots. Tropical savanna trees in Australia tend to have a similar root architecture and strategy (Kelley et al, 2007; Vega‐Grau et al, 2021; Werner & Murphy, 2001). It is possible, however, that under more severe drought conditions trees may be entirely reliant on groundwater (e.g., in the event of lower rainfall in the previous wet seasons) (Lamontagne et al, 2005; O'Grady et al, 2006).…”

Section: Discussionmentioning

confidence: 99%

“…Depletion in δD of xylem water relative to source water has been increasingly reported across a range of bioregions and climates (Barbeta et al, 2019; Barbeta, Burlett, et al, 2020; Barbeta, Gimeno, et al, 2020; Carrière et al, 2020; De Deurwaerder et al, 2018; Evaristo et al, 2017; Poca et al, 2019; Zhao et al, 2016; Zhao & Wang, 2021). There are also less frequent reports of δD enrichment in xylem water relative to source water (Muñoz‐Villers et al, 2020; Vega‐Grau et al, 2021). All xylem water samples taken along the Magela Creek riparian corridor had negative δD offsets relative to both source water and local rainfall.…”

Section: Discussionmentioning

confidence: 99%

“…xylem water relative to source water(Muñoz-Villers et al, 2020;Vega-Grau et al, 2021). All xylem water samples taken along the Magela Creek riparian corridor had negative δD offsets relative to both source water and local rainfall.…”

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confidence: 96%

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Deuterium depletion in xylem water and soil isotopic effects complicate the assessment of riparian tree water sources in the seasonal tropics

et al. 2021

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Riparian trees located in seasonally dry environments may be reliant on groundwater supplies, but the prevalence and magnitude of groundwater uptake is often unclear. Using soil water matric potential and water stable isotopes, we examined the relative contributions of soil water and groundwater to the dry season water uptake of five riparian tree species along an intermittent river of tropical northern Australia. Because xylem water was depleted in deuterium relative to source water (average offset −14.0‰), we numerically removed this offset and assessed the effect of the correction on mixing model results. We also estimated the isotopic composition of unbound soil water (i.e., the portion of soil water not tightly bound to soil particles) from bulk soil water data by using an empirical formulation from the literature and tested whether considering unbound soil water as a source would affect our results. Despite the hot and dry surface environment, we found that soil moisture was available for trees at relatively shallow (~0.7–1.5 m) depths. When unbound soil water and corrected xylem water data were considered, most tree species used a combination of this soil moisture source and groundwater from the capillary fringe. However, not correcting for isotopic effects resulted in large underestimations of the groundwater contributions to tree water uptake. Our findings suggest that ignoring soil isotopic effects and deuterium depletion in xylem water may reduce the validity of source water partitioning assessments. Further research is needed on the likely causes for deuterium depletion in xylem water.

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Section: Discussionmentioning

confidence: 99%

Section: Discussionmentioning

confidence: 99%

Section: Discussionmentioning

confidence: 99%

mentioning

confidence: 96%

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Deuterium depletion in xylem water and soil isotopic effects complicate the assessment of riparian tree water sources in the seasonal tropics

et al. 2021

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“…For P. auritum , both δ T ‐ and δ CVE ‐values (Figures 3 and 4) were generally higher than for C. fairchildiana . Evaporative enrichment can occur along the stem (Martín‐Gómez et al, 2017; Vega‐Grau et al, 2021), affecting in particular less suberized plant species such as P. auritum . Higher values of P. auritum for δ T ‐ and δ CVE ‐values can also indicate plant water uptake from more shallow soil layers.…”

Section: Discussionmentioning

confidence: 99%

Tracing plant source water dynamics during drought by continuous transpiration measurements: An in‐situ stable isotope approach

Kübert

Dubbert²,

Bamberger

et al. 2022

Plant Cell & Environment

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The isotopic composition of xylem water (δX) is of considerable interest for plant source water studies. In‐situ monitored isotopic composition of transpired water (δT) could provide a nondestructive proxy for δX‐values. Using flow‐through leaf chambers, we monitored 2‐hourly δT‐dynamics in two tropical plant species, one canopy‐forming tree and one understory herbaceous species. In an enclosed rainforest (Biosphere 2), we observed δT‐dynamics in response to an experimental severe drought, followed by a 2H deep‐water pulse applied belowground before starting regular rain. We also sampled branches to obtain δX‐values from cryogenic vacuum extraction (CVE). Daily flux‐weighted δ18OT‐values were a good proxy for δ18OX‐values under well‐watered and drought conditions that matched the rainforest's water source. Transpiration‐derived δ18OX‐values were mostly lower than CVE‐derived values. Transpiration‐derived δ2HX‐values were relatively high compared to source water and consistently higher than CVE‐derived values during drought. Tracing the 2H deep‐water pulse in real‐time showed distinct water uptake and transport responses: a fast and strong contribution of deep water to canopy tree transpiration contrasting with a slow and limited contribution to understory species transpiration. Thus, the in‐situ transpiration method is a promising tool to capture rapid dynamics in plant water uptake and use by both woody and nonwoody species.

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Differences between stem and branch xylem water isotope composition in four tropical tree species

et al. 2023

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Stable isotope studies (δ2H and δ18O) of water within plants are providing new information on water sources, competitive interactions and water use patterns under natural conditions. This is based on the assumption that there is no fractionation at the time of water uptake or during its transport within trees. However, previous studies have found fractionation does occur in water taken up through the roots in halophytic and xerophytic plants. It is unclear how widespread such fractionation is in other species. In this study, we tested this fractionation by comparing stem and branch xylem water isotopes over the period of one day (from 8 am to 5 pm) in four wet tropical rainforest tree species (Dendrocnide photinophylla, Aphananthe philippinensis, Daphnandra repandula and Mallotus polyadenos). We found branch water isotope ratios (δ2H and δ18O) were enriched compared with stem xylem water isotopes. D. photinophylla had a significantly different branch δ18O ratio than A. philippinensis, D. repandula and M. polyadenos. In contrast, there were no significant differences in stem xylem δ18O among the four observed tree species. We found clear differences in the stable isotope ratios of δ18O for stem and branch xylem water for D. photinophylla and D. repandula of upto −0.85% and 0.50%, respectively. Remarkable δ2H differences were also found between stem and branch xylem water isotope ratios for A. philippinensis, D. repandula and M. polyadenos, being upto −12.14%, −16.17% and −9.65%, respectively. A dual isotope (δ2H–δ18O) plot showed branch water values were more enriched than stem xylem water values for D. photinophylla and D. repandula, indicating a clear difference between stem and branch xylem water. This study suggests that δ2H and δ18O fractionation could be a species‐specific phenomenon in tropical trees, which has important implications for plant water source identification and evapotranspiration partitioning.

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Isotopic fractionation from deep roots to tall shoots: A forensic analysis of xylem water isotope composition in mature tropical savanna trees

Cited by 23 publications

References 51 publications

Deuterium depletion in xylem water and soil isotopic effects complicate the assessment of riparian tree water sources in the seasonal tropics

Deuterium depletion in xylem water and soil isotopic effects complicate the assessment of riparian tree water sources in the seasonal tropics

Tracing plant source water dynamics during drought by continuous transpiration measurements: An in‐situ stable isotope approach

Differences between stem and branch xylem water isotope composition in four tropical tree species

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