Causes and consequences of pronounced variation in the isotope composition of plant xylem water
2020
Abstract. Stable isotopologues of water are widely used to derive relative root water
uptake (RWU) profiles and average RWU depth in lignified plants. Uniform
isotope composition of plant xylem water ( δxyl ) along the stem
length of woody plants is a central assumption of the isotope tracing
approach which has never been properly evaluated. Here we evaluate whether strong variation in δxyl within woody
plants exists using empirical field observations from French Guiana,
northwestern China, and Germany. In addition, supported by a mechanistic
plant hydraulic model, we test hypotheses on how variation in δxyl can develop through the effects of diurnal variation in RWU, sap
flux density, diffusion, and various other soil and plant parameters on the
δxyl of woody plants. The hydrogen and oxygen isotope composition of plant xylem water shows
strong temporal (i.e., sub-daily) and spatial (i.e., along the stem)
variation ranging up to 25.2 ‰ and
6.8 ‰ for δ2H and δ18O ,
respectively, greatly exceeding the measurement error range in all evaluated
datasets. Model explorations predict that significant δxyl variation
could arise from diurnal RWU fluctuations and vertical soil water
heterogeneity. Moreover, significant differences in δxyl emerge
between individuals that differ only in sap flux densities or are monitored
at different times or heights. This work shows a complex pattern of δxyl transport in the
soil–root–xylem system which can be related to the dynamics of RWU by
plants. These dynamics complicate the assessment of RWU when using stable
water isotopologues but also open new opportunities to study drought
responses to environmental drivers. We propose including the monitoring of sap
flow and soil matric potential for more robust estimates of average RWU
depth and expansion of attainable insights in plant drought strategies and
responses.
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