Borehole Equilibration: Testing a New Method to Monitor the Isotopic Composition of Tree Xylem Water in situ

Forest water use has been difficult to quantify. One promising approach is to measure the isotopic composition of plant water, e.g., the transpired water vapor or xylem water. Because different water sources, e.g., groundwater versus shallow soil water, often show different isotopic signatures, isotopes can be used to investigate the depths from which plants take up their water and how this changes over time. Traditionally such measurements have relied on the extraction of wood samples, which provide limited time resolution at great expense, and risk possible artifacts. Utilizing a borehole drilled through a tree's stem, we propose a new method based on the notion that water vapor in a slow-moving airstream approaches isotopic equilibration with the much greater mass of liquid water in the xylem. We present two empirical data sets showing that the method can work in practice. We then present a theoretical model estimating equilibration times and exploring the limits at which the approach will fail. The method provides a simple, cheap, and accurate means of continuously estimating the isotopic composition of the source water for transpiration

Thermal imaging of increment cores: a new method to estimate sapwood depth in trees

The cells in tree sapwood form a network of interconnected conduits which enables the transport of water and nutrients from the tree roots to the canopy. Sapwood depth must be assessed when tree water use is estimated from sap flow velocities. However, current approaches to assess sapwood depth are either not applicable universally, or require expensive instruments, the application of chemicals or laborious field efforts. Here, we present a new method, which estimates sapwood depth by thermal imaging of increment cores. Using a low-cost thermal camera for mobile devices, we show that the sapwood-heartwood boundary is detectable by a sharp increase in temperature. Estimated sapwood depths agree with dye estimates (R-2 = 0.84). We tested our approach on a broad range of temperate and tropical tree species: Quercus robur, Pinus sylvestris, Swietenia macrophylla, Guazuma ulmifolia, Hymenaea courbaril, Sideroxylon capiri and Astronium graveolens. In nearly all species, the methods agreed within 0.6 cm. Thermal imaging of increment cores provides a straightforward, low-cost, easy-to-use, and species-independent tool to identify sapwood depth. It has further potential to reveal radial differences in sapwood conductivity, to improve water balance estimations on larger scales and to quickly develop allometric relationships

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

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. Inter-method 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

Causes and consequences of pronounced variation in the isotope composition of plant xylem water

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 (delta(xyl)) 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 delta(xyl) 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 delta(xyl) can develop through the effects of diurnal variation in RWU, sap flux density, diffusion, and various other soil and plant parameters on the delta(xyl) 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 parts per thousand and 6.8 parts per thousand for delta H-2 and delta O-18, respectively, greatly exceeding the measurement error range in all evaluated datasets. Model explorations predict that significant delta(xyl) variation could arise from diurnal RWU fluctuations and vertical soil water heterogeneity. Moreover, significant differences in delta(xyl) 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 delta(xyl) 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

Investigating the root plasticity response of Centaurea jacea to soil water availability changes from stable isotopic analysis

Root water uptake (RWU) is a key ecohydrological process for which a physically-based understanding has been developed in the past decades. Isotopic analysis of soil and sap xylem water allows for a quantification of RWU profiles relative to plant transpiration at the whole-plant scale and provides independent means for constraining these models. However, due to methodological constraints, mainly having to do with destructive sampling and subsequent extraction of water under vacuum, significant knowledge gaps remain about the plastic response of a whole plant root system to a rapidly changing environment. We designed a laboratory experimental setup consisting of a plant chamber coupled to a 60-cm long acrylic soil column equipped with gas-permeable tubing at eight different depths for the non-destructive monitoring of the stable isotopic compositions in plant transpiration of an herbaceous species (Centaurea jacea) and of water across the soil profile, respectively. In addition, soil water content was monitored at five different depths in the soil column. Isotopically labeled water was brought to the plant alternatively from the top and the bottom of the soil column to simulate a rain event and capillary rise of groundwater. Finally, root length density (RLD) was measured destructively at the end of the experimental period. Fast shifts in the isotopic composition of both soil and transpiration water could be observed with the setup and translated into dynamic and pronounced shifts of RWU profile by means of a statistical Bayesian multi-source mixing model. The incorporation of plant physiological and soil physical information into statistical modelling improved the model output. A simple exercise of water balance closure underlined the non-unique relationship between RWU profile on the one hand, and water content and RLD profiles on the other, illustrating the continuous adaptation of the plant RWU as a function of its root hydraulic architecture and soil water availability during the experiment