Isotope Composition Of Xylem Water Research Articles

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

AbstractStable 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 repandulaandMallotus polyadenos). We found branch water isotope ratios (δ2H and δ18O) were enriched compared with stem xylem water isotopes.D. photinophyllahad a significantly different branch δ18O ratio thanA. philippinensis,D. repandulaandM. 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 forD. photinophyllaandD. repandulaof upto −0.85% and 0.50%, respectively. Remarkable δ2H differences were also found between stem and branch xylem water isotope ratios forA. philippinensis,D. repandulaandM. 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 forD. photinophyllaandD. 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.

黄土丘陵区优势造林树种水分来源对季节性干旱的响应

PDF HTML阅读 XML下载 导出引用 引用提醒 黄土丘陵区优势造林树种水分来源对季节性干旱的响应 DOI: 10.5846/stxb202106221657 作者: 作者单位: 作者简介: 通讯作者: 中图分类号: 基金项目: 山西省高等学校科技创新（2020L0141，2020L0146）；山西农业大学科技创新基金（博士科研启动）（2020BQ44）；山西省优秀博士来晋工作奖励（SXYBKY2020007） Responses of water source to seasonal drought of dominant afforestation tree species in the loess hilly region of China Author: Affiliation: Fund Project: 摘要 | 图/表 | 访问统计 | 参考文献 | 相似文献 | 引证文献 | 资源附件 | 文章评论 摘要:黄土丘陵区处于季节性干旱生态脆弱地带，探明水分环境变化对区域优势造林树种水分来源的影响，对干旱区植物水分利用及其共生关系具有重要意义。以该地区广泛种植的沙棘（Hippophae rhamnoides）+油松（Pinus tabuliformis）、沙棘（H.rhamnoides）+刺槐（Robinia pseudoacacia）混交林（HrPt，HrRp）及沙棘（Hr）、油松（Pt）和刺槐（Rp）纯林为对象，测定其茎秆水、土壤水和降水稳定同位素组成，采用IsoSource模型量化水分来源，相似性比例指数（PS）定量分析共生植物间水分利用关系。结果表明：（1）旱季，同一树种纯林和混交林间吸水层位存在差异，纯林中主要利用0-100 cm土层土壤水，其中Hr和Rp对40-100 cm的利用率为46.8%和43.8%，Pt对0-40 cm为83.9%；混交林中则更倾向于利用40-200 cm，其中HrPt（沙棘）对100-200 cm为58.8%，HrPt（油松）对40-100 cm为47.4%；HrRp（沙棘）对100-200 cm为58.2%，HrRp（刺槐）对100-200 cm为65.3%。（2）湿润季，纯林和混交林中各树种主要利用0-100 cm土层土壤水，利用率均在70%以上。（3）旱季PS指数显著大于湿润季，且HrRp的PS指数最大，表明沙棘和刺槐之间存在水源竞争现象。综上，相比HrRp，HrPt中共生植物间主要通过水分利用空间上的差异应对季节性干旱，使其组成的生态系统更好地适应区域脆弱的生态环境。研究结果可为区域退化生态恢复与重建及干旱适应性措施体系构建等提供依据。 Abstract:The loess hilly region is located in an ecologically fragile area with seasonal drought. Clarifying the impact of changes in the water environment on the water source of regional dominant afforestation tree species is of great significance to the water use of plants and their symbiosis in arid areas. Therefore, two kinds of mixed forests (Hippophae rhamnoides + Pinus tabuliformis, HtPt, H. rhamnoides+Robinia pseudoacacia, HtRp), the pure forests of H. rhamnoides (Ht), P. tabuliformis (Pt), and R. pseudoacacia (Rp) were selected. The isotope composition of xylem water, soil water and precipitation were determined. The water use and the relationship among symbiotic plants were quantitatively analyzed by using the IsoSource model and the proportional similarity (PS) index. The results indicated that:(1) during the dry season, there were differences in water absorption layer between pure forest and mixed forest of the same tree species. The tree species in the pure forest mainly used soil water in the 0-100 cm. The utilization ratios of Hr and Rp to 40-100 cm were 46.8% and 43.8%, and Pt from 0-40 cm was 83.9%. While, in the mixed forests were more inclined to use 40-200 cm. Among them, the utilization rate of HrPt (H. rhamnoides) from 100-200 cm layer was 58.8%, HrPt (P. tabuliformis) from 40-100 cm was 47.4%. HrRp (H. rhamnoides) and HrRp (R. pseudoacacia) from 100-200 cm were 58.2% and 65.3%, respectively. (2) During the wet season, tree species in pure and mixed forests were mainly use soil water in the 0-100 cm, and the utilization rates was above 70%. (3) The PS index in the dry season was significantly greater than that in the wet season, and in the HrRp was the highest, indicating that there might be water competition between H. rhamnoides and R. pseudoacacia. In summary, compared with HrRp, symbiotic plants in HrPt mainly respond to seasonal droughts through spatial differences in water use, which made it much easier for this ecosystem to be adaptive to the fragile ecological environment in the region. The research results can provide a basis for the restoration and reconstruction of regional degraded ecology and the construction of drought-adaptive measures system. 参考文献 相似文献 引证文献

Robust Estimation of Absorbing Root Surface Distributions From Xylem Water Isotope Compositions With an Inverse Plant Hydraulic Model

The vertical distribution of absorbing roots is one of the most influential plant traits determining plant strategy to access below ground resources. Yet little is known of natural variability in root distribution since collecting field data is challenging and labor-intensive. Studying stable water isotope compositions in plants could offer a cost-effective and practical solution to estimate the absorbing root surfaces distribution. However, such an approach requires developing realistic inverse modeling techniques that enable robust estimation of rooting distributions and associated uncertainty from xylem water isotopic composition observations. This study introduces an inverse modeling method that supports the assessment of the root allocation parameter (β) that defines the exponential vertical decay of a plants’ absorbing root surfaces distribution with soil depth. The method requires measurements obtained from xylem and soil water isotope composition, soil water potentials, and sap flow velocities when plants’ xylem water is sampled at a certain height above the rooting point. In a simulation study, we show that the approach can provide unbiased estimates of β and its associated uncertainty due to measuring errors and unmeasured environmental factors that can impact the xylem water isotopic data. We also recommend improving the accuracy and power of β estimation, highlighting the need for considering accurate soil water potential and sap flow monitoring. Finally, we apply the inverse modeling method to xylem water isotope data of lianas and trees collected in French Guiana. Our work shows that the inverse modeling procedure provides a robust analytical and statistical framework to estimate β. The method accounts for potential bias due to extraction errors and unmeasured environmental factors, which improves the viability of using stable water isotope compositions to estimate the distribution of absorbing root surfaces complementary to the assessment of relative root water uptake profiles.

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

Studies of plant water sources generally assume that xylem water integrates the isotopic composition (δ2H and δ18O) of water sources and does not fractionate during uptake or transport along the transpiration pathway. However, woody xerophytes, halophytes, and trees in mesic environments can show isotopic fractionation from source waters. Isotopic fractionation and variation in isotope composition can affect the interpretation of tree water sources, but most studies to date have been greenhouse experiments. Here we present a field-based forensic analysis of xylem water isotope composition for 12 Eucalyptus tetrodonta and Corymbia nesophila trees. We used a 25-tonne excavator to access materials from the trees' maximum rooting depth of 3 m to their highest canopies at 38 m. Substantial within-tree variation occurred in δ2H (−91.1‰ to −35.7‰ E. tetrodonta; −88.8‰ to −24.5‰ C. nesophila) and δ18O (−12.3‰ to −5.0‰ E. tetrodonta; −10.9‰ to −0.3‰ C. nesophila), with different root-to-branch isotope patterns in each species. Soil water δ2H and δ18O dual isotope slopes (7.26 E. tetrodonta, 6.66 C. nesophila) were closest to the Local Meteoric Water Line (8.4). The dual isotope slopes of the trees decreased progressively from roots (6.45 E. tetrodonta, 6.07 C. nesophila), to stems (4.61 E. tetrodonta, 5.97 C. nesophila) and branches (4.68 E. tetrodonta, 5.67 C. nesophila), indicative of fractionation along the xylem stream. Roots of both species were more enriched in 2H and 18O than soil water at all sampled depths. Bayesian mixing model analysis showed that estimated proportions of water sourced from different depths reflected the contrasting root systems of these species. Our study adds evidence of isotopic fractionation from water uptake and along the transpiration stream in mature trees in monsoonal environments, affecting the interpretation of water sources. We discuss the findings with view of interpreting aboveground xylem water isotopic composition, incorporating knowledge of root systems.

Tropical forest water source patterns revealed by stable isotopes: A preliminary analysis of 46 neighboring species

Stable isotope tracing of plant water use can illuminate plant water sources. But to date, the number of species tested at any given site has been minimal. Here, we sample 46 tropical hardwood tree species in a 0.32 ha plot with uniform soils. Soil water was characterized at 6 depths at 0.2 m intervals down to 1 m and showed simple and predictable depth patterns of δ2H and δ18O, and simple and spatially uniform isotope composition at each depth. Nevertheless, tree xylem water δ2H and δ18O showed remarkable variation covering the full range of soil composition, suggesting strong sorting and niche segregation across the small plot. Wood density, tree size and mean basal area increment together explained approximately 55% of the variance of xylem water isotope composition through principal component analysis. A Bayesian mixing model was applied to the data and showed that sampled trees were either sourcing their water from very shallow or deep soil layers, with very little contribution from the middle portion of the soil profile. The majority of the observed species relied on 0.0–0.2 m depth soil water. This layer contributed approximately 75% of the xylem water which was significantly higher than the contributions from all other depths. The contribution from shallow soil was highest for trees with high wood density, slow-growing trees and small-sized trees. Our work suggests that stable isotope tracers may aid a better understanding of tropical forest water uptake depths and their relation to tree functional traits and potential hydrological niche segregation among co-occurring tropical species.

A novel approach for estimating groundwater use by plants in rock-dominated habitats

Plant water use is an important component in the function of Earth’s critical zone and this can be examined by decomposing isotope composition of xylem water into contributions from precipitation stored in shallow soil layers and deeper groundwater. The usual procedure for estimating the proportional use of groundwater by plants is to sample the isotope composition of soil and groundwater and determine the most probable mixing coefficients from all potential sources. Here we propose and test a novel method for achieving the same goal without sampling soil water. The method is based on analyzing variability in the stem water isotope ratios of several members of a community and the known isotope ratio of groundwater to ‘triangulate’ the unknown isotope ratio of stored rainwater. Using a simple water balance model, parameterized to produce the best fit between actual and estimated stem water isotope ratios, we simulated seasonal variation in the volume and isotope ratio of rainwater storage, along with species-specific groundwater use ratios. The method was applied to eight woody plant species growing on two rocky outcrops in the South China karst. Estimated average proportional groundwater use over two seasons varied between 14% and 62% and was site-dependent. For the majority of species, groundwater use increased as estimated stored rainwater volume declined. The two species with highest groundwater use were taller, deciduous or semi-deciduous trees with lower wood densities. While the new method was inspired by the inability to sample water stored in the rocky outcrops, it may have broader use in any environment where the spatial variability of soil water isotope composition is a barrier to estimating average groundwater use ratios. The broader adoption of this or equivalent methods would greatly improve the study of the Earth’s critical zone.

Dynamics of competition over water in a mixed oak-pine Mediterranean forest: Spatio-temporal and physiological components

Understanding inter- and intra-specific plant interactions and competition over water is challenging because of the lack of effective approaches for accessing and monitoring root distribution and activity. In this context, stable isotopes are excellent eco-hydrological tracers that allow characterizing the dynamics of water uptake patterns in trees and shrubs. Here, we studied biotic interactions for water uptake between two typical Mediterranean tree species, Aleppo pine (Pinus halepensis) and holm oak (Quercus ilex), coexisting in a mixed forest. We measured stable isotope composition (δ18O and δ2H) of xylem water in all trees found in the studied stand during one growing season, covering an exceptionally long summer drought and subsequent recovery. We applied point-process statistics together with stand density information to evaluate tree-to-tree interactions for water use. In pines, we observed a clear uncoupling between soil and xylem water isotope composition after two months of persistent drought. Conversely, the isotope composition of xylem water in oaks tracked observed changes in the soil during the first two months of drought, but began to depart from soil values after three months. These results suggest that during drought the oaks were able to keep active for longer using alternative soil water sources, not available for the pines. Point-process statistics revealed more positive isotope compositions at distances below 4–6m, but only between con-specific individuals (i.e. pine-pine, oak-oak). These intra-specific responses were first seen in the pines (after two months of drought) and subsequently in oaks (after three months), coinciding with the onset of soil-xylem uncoupling for each species. On the other hand, the isotope composition of individual oaks decreased with increasing neighbor pine density, but increased in response to oak density. Conversely, the pines showed more positive values with increasing oak density. Our results suggest that the use of shallow water in oaks is limited by the presence of pines, which force them to shift to deep-soil water use, whereas pines have more restricted access to deep water in the presence of oaks, leading to more positive isotope values. According to the dynamics of interaction patterns, we conclude that inter-specific differences in pine-oak mixed forests hold two components: a static, spatial component determined by root distribution, and a dynamic, physiological component related to water uptake capacity within the soil profile.

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.

Point processes statistics of stable isotopes: analysing water uptake patterns in a mixed stand of Aleppo pine and Holm oak

Aim of study: Understanding inter- and intra-specific competition for water is crucial in drought-prone environments. However, little is known about the spatial interdependencies for water uptake among individuals in mixed stands. The aim of this work was to compare water uptake patterns during a drought episode in two common Mediterranean tree species, Quercus ilex L. and Pinus halepensis Mill., using the isotope composition of xylem water (δ18O, δ2H) as hydrological marker.Area of study: The study was performed in a mixed stand, sampling a total of 33 oaks and 78 pines (plot area= 888 m2). We tested the hypothesis that both species uptake water differentially along the soil profile, thus showing different levels of tree-to-tree interdependency, depending on whether neighbouring trees belong to one species or the other.Material and Methods: We used pair-correlation functions to study intra-specific point-tree configurations and the bivariate pair correlation function to analyse the inter-specific spatial configuration. Moreover, the isotopic composition of xylem water was analysed as a mark point pattern.Main results: Values for Q. ilex (δ18O= -5.3 ±0.2‰, δ2H=-54.3±0.7‰) were significantly lower than for P. halepensis (δ18O= -1.2±0.2‰, δ2H = -25.1±0.8‰), pointing to a greater contribution of deeper soil layers for water uptake by Q. ilex. Research highlights: Point-process analyses revealed spatial intra-specific dependencies among neighbouring pines, showing neither oak-oak nor oak-pine interactions. This supports niche segregation for water uptake between the two species.Key words: Cross-pair correlation function; Deuterium; Mark correlation function; Oxygen-18; Point patterns; Xylem.Abbreviations: d18O, oxygen isotope composition; d2H, hydrogen isotope composition; BA, basal area.

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.

Effect of groundwater freshening on riparian vegetation water balance

AbstractIn the floodplains of the lower River Murray (Australia), riparian open forests and woodlands dominated by River Red Gum (Eucalyptus camaldulensis Dehn.), Black Box (E. largiflorens F. Muell.) and River Cooba (Acacia stenophylla A. Cunn. Ex Benth.) have been severely impacted by salinization and drought. The ‘Bookpurnong Experiment’ was developed to test the hypothesis that watertable lowering combined with groundwater freshening would reduce tree water stress and improve floodplain vegetation health. Tree water use and water stress following groundwater freshening were examined using measurements of sapflow, pre‐dawn leaf water potentials and stable isotope composition of xylem water. Water use and water stress varied markedly across the floodplain. The open mixed forest lining the bank of the River Murray, with access to fresh groundwater, used five times more groundwater over the measurement period and maintained greater canopy vigour than the open mixed woodland at inland sites. The River Cooba trees approximately 80 m from the river responded to watertable lowering of 0·65 m and to groundwater freshening by increasing transpiration. Black Box trees at the same site experienced reduced plant water stress, with no increase in transpiration. However, at a third site, approximately 170 m from the river, where the saline watertable (total dissolved solids = 36 300 mg l−1) was lowered by 1 m, no change in plant water stress or pattern of plant water use was observed. These results indicate that groundwater lowering combined with groundwater freshening can provide an accessible water source for water‐stressed floodplain vegetation. Copyright © 2009 John Wiley & Sons, Ltd.

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.