Xylem Water Isotope Research Articles

Revisiting streamside trees that do not use stream water: can the two water worlds hypothesis and snowpack isotopic effects explain a missing water source?

AbstractWe revisit a classic ecohydrological study that showed streamside riparian trees in a semiarid mountain catchment did not use perennial stream water. The original study suggested that mature individuals of Acer negundo, Acer grandidentatum, and other species were dependent on water from “deeper strata,” possibly groundwater. We used a dual stable isotope approach (δ18O and δ2H) to further examine the water sources of these trees. We tested the hypothesis that groundwater was the main tree water source, but found that neither groundwater nor stream water matched the isotope composition of xylem water during two growing seasons. Soil water (0–1 m depth) was closest to and periodically overlapped with xylem water isotope composition, but overall, xylem water was isotopically enriched compared to all measured water sources. The “two water worlds” hypothesis postulates that soil water comprises isotopically distinct mobile and less mobile pools that do not mix, potentially explaining this disparity. We further hypothesized that isotopic effects during snowpack metamorphosis impart a distinct isotope signature to the less mobile soil water that supplies summer transpiration. Depth trends in water isotopes following snowmelt were consistent with the two water worlds hypothesis, but snow metamorphic isotope effects could not explain the highly enriched xylem water. Thus, the dual isotope approach did not unambiguously determine the water source(s) of these riparian trees. Further exploration of physical, geochemical, and biological mechanisms of water isotope fractionation and partitioning is necessary to resolve these data, highlighting critical challenges in the isotopic determination of plant water sources.

Evidence of hydraulic lift in a young beech and oak mixed forest using 18O soil water labelling

Hydraulic lift (HL) by tree roots in a young, broad-leaved, mixed temperate European forest was investigated during the 2008 growing season by injecting 18O-enriched soil water at a depth of 75–90 cm under drought conditions experimentally imposed in a rain-exclusion system. Based on sap flow, leaf water potential, 2-D root distribution measurements, soil isotope profiles, and xylem water isotope composition, water acquisition and use by two tree species, beech (Fagus sylvatica) and oak (Quercus petraea) was compared. We showed that, unlike oak, beech experienced a marked decrease in sap flow and predawn leaf water potential with increasing soil drought. This behaviour was logical considering the shallower root system in beech than in oak. Six days after 18O-labelling, we observed isotopic enrichment in the shallower soil layers. Since the intermediate soil layers did not display any enrichment, our results clearly pointed to hydraulic lift by tree roots. The superficial enrichment that was observed in the vicinity of oak trunks and the increase in the isotopic signature of xylem sap in the oak trees but not in the beech trees confirmed the predominant role of oak in the hydraulic lift at our site. Even though facilitation for water acquisition among species was not observed here, our results suggest a potential positive contribution of species like oak toward maintaining species diversity in mixed forest ecosystems submitted to severe drought events.

Determining the Oxygen Isotope Composition of Evapotranspiration Using Eddy Covariance

The oxygen isotope composition of evapotranspiration (δ F ) represents an important tracer in the study of biosphere–atmosphere interactions, hydrology, paleoclimate, and carbon cycling. Here, we demonstrate direct measurement of δ F based on the eddy-covariance and tunable diode laser spectroscopy (EC-TDL) techniques. Results are presented from laboratory experiments and field measurements in agricultural ecosystems. The field measurements were obtained during the growing seasons of 2008 and 2009. Water vapour mixing ratios (χ w ) and fluxes (F) were compared using EC-TDL and traditional eddy-covariance and infrared gas analyser techniques over a soybean canopy in 2008. The results indicate that χ w and F agreed to within 1 and 6%, respectively. Measurements of δ F above a corn canopy in 2009 revealed a diurnal pattern with an expected progressive 18O enrichment through the day ranging from about −20‰ before sunrise to about −5‰ in late afternoon. The isotopic composition of evapotranspiration was similar to the xylem water isotope composition (δ x = −7.2‰) for short periods of time during 1400–1800 LST, indicating near steady-state conditions. Finally, the isotopic forcing values (I F ) revealed a diurnal pattern with mean maximum values of 0.09ms−1‰ at midday. The I F values could be described as an exponential relation of relative humidity confirming previous model calculations and measurements over a soybean canopy in 2006. These patterns and comparisons indicate that long-term continuous isotopic water vapour flux measurements based on the eddy-covariance technique are feasible and can provide new insights related to the oxygen isotope fractionation processes at the canopy scale.

Seasonal water use patterns of Juniperus ashei on the Edwards Plateau, Texas, based on stable isotopes in water

Summary Naturally occurring stable isotopes of oxygen and hydrogen were used to determine the seasonal water sources of Juniperus ashei and its relationship with soil water content on the Edwards Plateau of central Texas, Honey Creek State Natural Area. The patchy nature of juniper stands rooting in the thin soils over karstified carbonate bedrock imparts large spatial heterogeneity in soil water content and isotopic composition largely by differential shading, but also by canopy interception. There is a seasonal decrease in the oxygen isotope composition of juniper xylem water by ∼2‰ during the transition from hot, dry summer conditions to the cool, moist winter. Prior to this shift, the xylem water isotope composition is similar to local springs, presumed to represent local groundwater; after this shift, the xylem water composition is similar to soil water below 10 cm. These results suggest a change in juniper water use from a dominantly deep water source during the summer to a dominantly soil source during the winter over the study period, but the timing of this transition will likely vary from year to year.