Abstract
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.
Highlights
The use of the stable isotope composition of water has strengthened ecohydrology studies by providing insights into phenomena that are otherwise challenging to observe, such as relative root water uptake depth (RWU depth) (Rothfuss and Javaux, 2017), belowground water competition, and hydraulic lift (Hervé-Fernández et al, 2016; Meunier et al, 2017)
A variety of methods are used to infer average RWU depth from the isotope composition of plant xylem water, but all rely on a direct relationship between the isotopic compositions of plant xylem and soil water (Ehleringer and Dawson, 1992)
The null model assumes the constant isotopic composition of root water uptake with only limited variance in isotopic composition introduced by extraction errors (β2HX < 3 ‰; δ18OX < 0.3 ‰)
Summary
The use of the stable isotope composition of water has strengthened ecohydrology studies by providing insights into phenomena that are otherwise challenging to observe, such as relative root water uptake depth (RWU depth) (Rothfuss and Javaux, 2017), belowground water competition, and hydraulic lift (Hervé-Fernández et al, 2016; Meunier et al, 2017). The technique is far less destructive and labor intensive. This makes it more flexible for studying multiple individuals across spatial and temporal scales (i.e., individual to ecosystem, daily to seasonal) (Dawson et al, 2002). The study of stable isotope composition of xylem water measures the real effects of RWU at different depths, whereas excavation yields only root distribution and architecture. All methods assume that xylem water provides a well-mixed isotope composition of water from different soil layers; sampled xylem water instantaneously reflects the distribution and water uptake of the roots independent of the timing or height of sampling
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
