Determining landscape use of Holocene mammals using strontium isotopes

The use of the landscape by animals is predicted to be a function of their body size. However, empirical data relating these two variables from an array of body sizes within a single mammalian community are scarce. We tested this prediction by assessing landscape use of mammals by analyzing strontium (Sr) isotope signatures found in mammalian hard tissues representing a 3,000-year record. We examined: (1) the Sr-determined landscape area of small (∼100 g), medium (∼1,500 g) and large (∼100,000 g) mammals, and; (2) whether the area used by these mammals varied during periods of environmental change. Strontium isotope values were obtained from 46 specimens from the Holocene paleontological deposits of Lamar Cave and Waterfall Locality in Wyoming, USA, as well as from 13 modern ungulate specimens from the same area. Our data indicate that medium- and large-sized species use larger percentages of the landscape than do species of small body size. The isotope values for specimens from each of the paleontological sites are similar across all stratigraphic levels, suggesting no change in home range over the last 3,000 years, even though climate is known to have fluctuated at these sites over this time period. Further, our study verifies that the fossil localities represent the local community. Where bedrock geology is appropriate, the use of strontium isotope analyses provides a valuable tool for discerning landscape use by vertebrate communities, an important though generally difficult aspect of an ancient species niche to identify. Electronic Supplementary Material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Connecting breeding and wintering grounds of Neotropical migrant songbirds using stable hydrogen isotopes: a call for an isotopic atlas of migratory connectivity

There is an overdue and urgent need to establish patterns of migratory connectivity linking breeding grounds, stopover sites, and wintering grounds of migratory birds. Such information allows more effective application of conservation efforts by applying focused actions along movement trajectories at the population level. Stable isotope methods, especially those using stable hydrogen isotope abundance in feathers (δ²H_f) combined with Bayesian assignment techniques incorporating prior information such as relative abundance of breeding birds, now provide a fast and reliable means of establishing migratory connectivity, especially for Neotropical migrants that breed in North America and molt prior to fall migration. Here we demonstrate how opportunistic sampling of feathers of 30 species of wintering birds in Cuba, Venezuela, Guatemala, Puerto Rico, and Mexico, regions that have typically been poorly sampled for estimating migratory connectivity, can be assigned to breeding areas in North America through both advanced spatial assignment to probability surfaces and through simpler map lookup approaches. Incorporating relative abundance information from the North American Breeding Bird Survey in our Bayesian assignment models generally resulted in a reduction in potential assignment areas on breeding grounds. However, additional tools to constrain longitude such as DNA markers or other isotopes would be desirable for establishing breeding or molt origins of species with broad longitudinal distributions. The isotope approach could act as a rapid means of establishing basic patterns of migratory connectivity across numerous species and populations. We propose a large‐scale coordinated sampling effort on the wintering grounds to establish an isotopic atlas of migratory connectivity for North American Neotropical migrants and suggest that isotopic variance be considered as a valuable metric to quantify migratory connectivity. This initiative could then act as a strategic template to guide further efforts involving stable isotopes, light‐sensitive geolocators, and other technologies.     

Improving provenance studies in migratory birds when using feather hydrogen stable isotopes

Hydrogen stable isotopes (δ²H) in feathers are used to determine the origin and migration strategy of birds. To identify the geographic location of the site of feather synthesis, calibration curves for the relation between feather δ²H and amount‐weighted growing‐season δ²H in precipitation are used to generate feather δ²H isoscapes. Factors like species, age and year might generate isotopic variation in calibration curves, but the extent to which accounting for variation may improve calibration curves and hence provenance determination of birds, is unknown. We compared three European calibration curves: 1) an existing multi‐species curve, uncorrected for age and year variation, and two species‐specific calibration curves, based on mallard Anas platyrhynchos feathers, of 2) varying age and year, and 3) juvenile natal origin, corrected for year variation. Calibration curves using ordinary least square linear regression (OLS) as opposed to standard major axis regression showed least bias in estimation. As expected, we found that a single species (mallard) OLS calibration curve corrected for age and year yielded the highest coefficient of determination, but was still surprisingly similar to the other two calibration curves. Nevertheless, when using feathers of known‐origin to assess provenance accuracy, the calibration curve that accounted for species, age and year variation yielded the best prediction in as many as 59% of the cases. Our study is the first to demonstrate implications of isotopic variation on assessing the origin of individual birds, but also highlights the relatively small gain in precision that is achieved by generating species, age and year specific calibration curves rather than resorting to more general alternatives.     

Determining biological tissue turnover using stable isotopes: the reaction progress variable

The reaction progress variable is applied to stable isotope turnover of biological tissues. This approach has the advantage of readily determining whether more than one isotope turnover pool is present; in addition, the normalization process inherent to the model means that multiple experiments can be considered together although the initial and final isotope compositions are different. Consideration of multiple isotope turnover pools allows calculation of diet histories of animals using a time sequence of isotope measurements along with isotope turnover pools. The delayed release of blood cells from bone marrow during a diet turnover experiment can be quantified using this approach. Turnover pools can also be corrected for increasing mass during an experiment, such as when the animals are actively growing. Previous growth models have been for exponential growth; the approach here can be used for several different growth models. Electronic Supplementary Material The online version of this article () contains supplementary material, which is available to authorized users.     

Using stable isotopes to investigate the dietary trophic level of fledgling songbirds

ABSTRACT Use of early successional habitat by mature forest birds during the postfledging period is well documented, but reasons for this habitat shift remain elusive. Although forest‐breeding songbirds are primarily insectivorous during the nestling and early fledgling periods due to high protein requirements, older fledglings may adopt a heavily frugivorous diet. Our objectives were to use stable isotopes to examine the dietary trophic level of juveniles of three species of mature forest songbirds to determine if juvenile songbirds heavily consume fruit resources during the postfledging period and to evaluate a possible link between diet and energetic condition. We collected the outer right rectrix and several body feathers from 34 Wood Thrushes (Hylocichla mustelina), 34 Ovenbirds (Seiurus aurocapilla), and 35 Scarlet Tanagers (Piranga olivacea) captured in regenerating clearcuts in southeastern Ohio in 2005 and 2006. We also collected fruit and arthropod samples from each clearcut. Isotopic values of body feathers were significantly higher (more enriched) than those of rectrices in all cases except values of δ¹³C for Ovenbirds where we found no difference between body and rectrix feathers. These results suggest that juvenile songbirds did not undergo a strong shift to frugivory during the postfledging period, and arthropods were the primary source of protein during the period when rectrix and body feathers were growing. In addition, the energetic condition of birds was not related to the isotopic signature of feathers. Although our results are inconsistent with the hypothesis that juveniles move into regenerating clearcuts enabling them to shift to a primarily frugivorous diet during the postfledging period, they may consume fruit for nonprotein requirements, such as lipids and carbohydrates.     

Source partitioning using stable isotopes: coping with too many sources

Stable isotopes are increasingly being used as tracers in environmental studies. One application is to use isotopic ratios to quantitatively determine the proportional contribution of several sources to a mixture, such as the proportion of various pollution sources in a waste stream. In general, the proportional contributions of n+1 different sources can be uniquely determined by the use of n different isotope system tracers (e.g., δ¹³C, δ¹⁵N, δ¹⁸O) with linear mixing models based on mass balance equations. Often, however, the number of potential sources exceeds n+1, which prevents finding a unique solution of source proportions. What can be done in these situations? While no definitive solution exists, we propose a method that is informative in determining bounds for the contributions of each source. In this method, all possible combinations of each source contribution (0–100%) are examined in small increments (e.g., 1%). Combinations that sum to the observed mixture isotopic signatures within a small tolerance (e.g., ±0.1‰) are considered to be feasible solutions, from which the frequency and range of potential source contributions can be determined. To avoid misrepresenting the results, users of this procedure should report the distribution of feasible solutions rather than focusing on a single value such as the mean. We applied this method to a variety of environmental studies in which stable isotope tracers were used to quantify the relative magnitude of multiple sources, including (1) plant water use, (2) geochemistry, (3) air pollution, and (4) dietary analysis. This method gives the range of isotopically determined source contributions; additional non-isotopic constraints specific to each study may be used to further restrict this range. The breadth of the isotopically determined ranges depends on the geometry of the mixing space and the similarity of source and mixture isotopic signatures. A sensitivity analysis indicated that the estimated ranges vary only modestly with different choices of source increment and mass balance tolerance parameter values. A computer program (IsoSource) to perform these calculations for user-specified data is available at http://www.epa.gov/wed/pages/models.htm.     

Stable isotopes in breath,blood, feces and feathers can indicate intra-individual changes in the diet of migratory songbirds

We used stable isotopes of C in breath, blood, feces and feathers to identify intra-individual changes in diet and the timescale of diet changes in free-living songbirds at a stopover site. Because accurate interpretation of differences between the ¹³C of breath, plasma, and red blood cells (RBCs) relative to diet requires knowing the turnover rate of C within them, we determined the rate of change of C in breath, plasma and RBCs for yellow-rumped warblers (Dendroica coronata). Half-lives of C in breath, plasma, and RBCs were 4.4±2.1 h, 24.8±12.3 h and 10.9±3.2 days, respectively, for yellow-rumped warblers. ¹³C of breath, plasma, RBCs and feces from wild-caught golden-crowned kinglets (Regulus satrapa), ruby-crowned kinglets (R. calendula) and gray catbirds (Dumetella carolinensis) indicated that they had maintained an isotopically consistent diet for an extended period of time. However, ¹³C of breath and plasma indicated that white-throated sparrows (Zonotrichia albicollis) had recently expanded their diet to include a C₄ dietary component. Likewise, ¹³C of breath, plasma, RBCs and feces indicated that some wild-caught yellow-rumped warblers had consumed foods with a more enriched protein signature prior to their arrival on Block Island, and since arrival, they had consumed mostly northern bayberry (Myrica pensylvanica). Therefore, comparisons of the ¹³C of breath, plasma, RBCs, feces and feathers from individual songbirds can indicate changes in diet and provide an estimate of the timescale of the diet change.     

Mapping critical areas for migratory songbirds using a fusion of remote sensing and distributional modeling techniques

Of the 338 species identified as Nearctic-Neotropical migrants occurring in North America, 98.5% have been recorded in Texas. The seasonal migration of these birds is a well-studied natural phenomenon – individuals weighing < 15 g will cross in the Gulf of Mexico approximately 965 km non-stop, completing a total distance of 1900–3200 km over the course of 26–80 h. The physiologically demanding nature of this feat makes the Texas coastline crucial to the success of these species. We used a fusion of multi-spectral remote sensing data and distributional modeling techniques to generate and evaluate predictive maps identifying critical areas for migratory passerines on the Texas coast. Imagery acquired from Landsat 8 OLI, maps provided by United States Geological Survey and the Texas Department of Transportation, and migratory bird occurrence records from the eBird citizen-contributed database were used to build predictive distribution models using three algorithms. Using the AUC to compare model performance, the Random Forest produced the most accurate distribution model, followed by MaxEnt, and Support Vector Machine (0.98, 0.81, and 0.79, respectively). We interpreted, from Boosted Regression Tree analysis, that elevation is the single most influential factor in determining migrant occupancy, with vegetative biomass the least influential predictor. Our approach here allows conservation biologists a more sophisticated approach to identifying critical areas for migratory passerines across large spatial extents in a short amount of time.     

Using stable isotopes to investigate migratory connectivity of the globally threatened aquatic warbler Acrocephalus paludicola

Understanding the links between breeding and wintering areas of migratory species has important ecological and conservation implications. Recently, stable isotope technology has been used to further our understanding. Stable isotope ratios vary geographically with a range of biogeochemical factors and isotope profiles in organisms reflect those in their food and environment. For inert tissues like feathers, isotope profiles reflect the environment in which they were formed. Following large-scale habitat destruction, the globally threatened aquatic warbler Acrocephalus paludicola has a fragmented breeding population across central Europe, largely in Belarus, Poland and Ukraine. The species sub-Saharan African wintering grounds have not yet been discovered, and this significantly hampers conservation efforts. Aquatic warblers grow their flight feathers on their wintering grounds, and we analysed stable isotope ratios (¹⁵N, ¹³C, D) in rectrices of adults from six main breeding sites (subpopulations) across Europe to determine whether different breeding subpopulations formed a single mixed population on the wintering grounds. ¹⁵N varies considerably with dietary trophic level and environmental factors, and D with the D in rainfall; neither varied between aquatic warbler subpopulations. Uniform feather ¹⁵N signatures suggest no major variation in dietary trophic level during feather formation. High variance and inter-annual differences in mean D values hinder interpretation of these data. Significant differences in mean ¹³C ratios existed between subpopulations. We discuss possible interpretations of this result, and consider differences in moulting latitude of different subpopulations to be the most parsimonious. ¹³C in plants and animals decreases with latitude, along a steep gradient in sub-Saharan Africa. Birds from the most north-westerly breeding subpopulation (Karsibor, Poland) had significantly lower variance in ¹³C and ¹⁵N than birds from all other sites, suggesting either that birds from Karsibor are less geographically dispersed during moult, or moult in an area with less isotopic heterogeneity. Mean ¹³C signatures from winter-grown feathers of different subpopulations were positively correlated with the latitude and longitude of breeding sites, suggesting a strong relationship between European breeding and African winter moulting latitudes. The use of stable isotopes provides novel insights into migratory connectivity and migration patterns in this little-known threatened species.     

Elucidating the nutritional dynamics of fungi using stable isotopes

Mycorrhizal and saprotrophic (SAP) fungi are essential to terrestrial element cycling due to their uptake of mineral nutrients and decomposition of detritus. Linking these ecological roles to specific fungi is necessary to improve our understanding of global nutrient cycling, fungal ecophysiology, and forest ecology. Using discriminant analyses of nitrogen (δ¹⁵N) and carbon (δ¹³C) isotope values from 813 fungi across 23 sites, we verified collector-based categorizations as either ectomycorrhizal (ECM) or SAP in > 91% of the fungi, and provided probabilistic assignments for an additional 27 fungi of unknown ecological role. As sites ranged from boreal tundra to tropical rainforest, we were able to show that fungal δ¹³C (26 sites) and δ¹⁵N (32 sites) values could be predicted by climate or latitude as previously shown in plant and soil analyses. Fungal δ¹³C values are likely reflecting differences in C-source between ECM and SAP fungi, whereas ¹⁵N enrichment of ECM fungi relative to SAP fungi suggests that ECM fungi are consistently delivering ¹⁵N depleted N to host trees across a range of ecosystem types.     

Estimating the timing of diet shifts using stable isotopes

Stable isotope analysis has become an important tool in studies of trophic food webs and animal feeding patterns. When animals undergo rapid dietary shifts due to migration, metamorphosis, or other reasons, the isotopic composition of their tissues begins changing to reflect that of their diet. This can occur both as a result of growth and metabolic turnover of existing tissue. Tissues vary in their rate of isotopic change, with high turnover tissues such as liver changing rapidly, while relatively low turnover tissues such as bone change more slowly. A model is outlined that uses the varying isotopic changes in multiple tissues as a chemical clock to estimate the time elapsed since a diet shift, and the magnitude of the isotopic shift in the tissues at the new equilibrium. This model was tested using published results from controlled feeding experiments on a bird and a mammal. For the model to be effective, the tissues utilized must be sufficiently different in their turnover rates. The model did a reasonable job of estimating elapsed time and equilibrial isotopic changes, except when the time since the diet shift was less than a small fraction of the half-life of the slowest turnover tissue or greater than 5–10 half-lives of the slowest turnover tissue. Sensitivity analyses independently corroborated that model estimates became unstable at extremely short and long sample times due to the effect of random measurement error. Subject to some limitations, the model may be useful for studying the movement and behavior of animals changing isotopic environments, such as anadromous fish, migratory birds, animals undergoing metamorphosis, or animals changing diets because of shifts in food abundance or competitive interactions.     

Estimating the latitudinal origins of migratory birds using hydrogen and sulfur stable isotopes in feathers: influence of marine prey base

Hydrogen stable isotope analysis of feathers is an important tool for estimating the natal or breeding latitudes of nearctic-neotropical migratory birds. This method is based on the latitudinal variation of hydrogen stable isotope ratios in precipitation in North America (deltaD(p)) and the inheritance of this variation in newly formed feathers (deltaD(f)). We hypothesized that the typically strong relationship between deltaD(p) and deltaD(f) would be decoupled in birds that forage in marine food webs because marine waters have relatively high deltaD values compared to deltaD values for local precipitation. Birds that forage on marine prey bases should also have feathers with high delta(34)S values, since delta(34)S values for marine sulfate are generally higher than delta(34)S values in terrestrial systems. To examine this potential marine effect on feather stable isotope ratios, we measured deltaD and delta(34)S in the feathers of nine different species of raptors from both inland and coastal locations across North America. Feathers from coastal bird-eating raptors had consistently higher deltaD and delta(34)S values than feathers from inland birds. Birds that had high delta(34)S values also deviated strongly from the typical relationship between deltaD(p) and deltaD(f). We recommend measuring both sulfur and hydrogen stable isotope ratios in feathers when some members of a migrant population could potentially forage in marine habitats. We suggest using a practical cutoff of delta(34)S >10 per thousand to remove marine-foraging birds from a migrant sample when using stable isotopes of hydrogen to estimate the latitudinal origins of migrants because high deltaD(f) values for marine-foraging birds could potentially distort estimates of origins.     

Measuring the trophic ecology of ants using stable isotopes

Ants are prominent components of most terrestrial arthropod food webs, yet due to their highly variable diet, the role ants play in arthropod communities can be difficult to resolve. Stable isotope analysis is a promising method for determining the dietary history of an organism, and has the potential to advance our understanding of the food web ecology of social insects. However, some unique characteristics of eusocial organisms can complicate the application of this technique to the study of their trophic ecology. Using stable isotopes of N and C, we investigated levels of intraspecific variation both within and among colonies. We also examined the effect of a common preservation technique on δ¹⁵N and δ¹³C values. We discuss the implications of our results on experimental design and sampling methods for studies using stable isotopes to investigate the trophic ecology of social insects. Received 4 February 2005; revised 23 June 2005; accepted 4 July 2005.     

Diffuse migratory connectivity in two species of shrubland birds: evidence from stable isotopes

Connecting seasonal ranges of migratory birds is important for understanding the annual template of stressors that influence their populations. Brewer’s sparrows (Spizella breweri) and sagebrush sparrows (Artemisiospiza nevadensis) share similar sagebrush (Artemisia spp.) habitats for breeding but have different population trends that might be related to winter location. To link breeding and winter ranges, we created isoscapes of deuterium [stable isotope ratio (δ) of deuterium; δ ²H] and nitrogen (δ ¹⁵N) for each species modeled from isotope ratios measured in feathers of 264 Brewer’s and 82 sagebrush sparrows and environmental characteristics at capture locations across their breeding range. We then used feather $\delta^{2} {\text{H}}_{\text{f}}$ and $\delta^{15} {\text{N}}_{\text{f}}$ measured in 1,029 Brewer’s and 527 sagebrush sparrows captured on winter locations in southwestern United States to assign probable breeding ranges. Intraspecies population mixing from across the breeding range was strong for both Brewer’s and sagebrush sparrows on winter ranges. Brewer’s sparrows but not sagebrush sparrows were linked to more northerly breeding locations in the eastern part of their winter range. Winter location was not related to breeding population trends estimated from US Geological Survey Breeding Bird Survey routes for either Brewer’s or sagebrush sparrows. Primary drivers of population dynamics are likely independent for each species; Brewer’s and sagebrush sparrows captured at the same winter location did not share predicted breeding locations or population trends. The diffuse migratory connectivity displayed by Brewer’s and sagebrush sparrows measured at the coarse spatial resolution in our analysis also suggests that local environments rather than broad regional characteristics are primary drivers of annual population trends.     

Uncertainty in source partitioning using stable isotopes

Stable isotope analyses are often used to quantify the contribution of multiple sources to a mixture, such as proportions of food sources in an animal's diet, or C₃ and C₄ plant inputs to soil organic carbon. Linear mixing models can be used to partition two sources with a single isotopic signature (e.g., '¹³C) or three sources with a second isotopic signature (e.g., '¹⁵N). Although variability of source and mixture signatures is often reported, confidence interval calculations for source proportions typically use only the mixture variability. We provide examples showing that omission of source variability can lead to underestimation of the variability of source proportion estimates. For both two- and three-source mixing models, we present formulas for calculating variances, standard errors (SE), and confidence intervals for source proportion estimates that account for the observed variability in the isotopic signatures for the sources as well as the mixture. We then performed sensitivity analyses to assess the relative importance of: (1) the isotopic signature difference between the sources, (2) isotopic signature standard deviations (SD) in the source and mixture populations, (3) sample size, (4) analytical SD, and (5) the evenness of the source proportions, for determining the variability (SE) of source proportion estimates. The proportion SEs varied inversely with the signature difference between sources, so doubling the source difference from 2‰ to 4‰ reduced the SEs by half. Source and mixture signature SDs had a substantial linear effect on source proportion SEs. However, the population variability of the sources and the mixture are fixed and the sampling error component can be changed only by increasing sample size. Source proportion SEs varied inversely with the square root of sample size, so an increase from 1 to 4 samples per population cut the SE in half. Analytical SD had little effect over the range examined since it was generally substantially smaller than the population SDs. Proportion SEs were minimized when sources were evenly divided, but increased only slightly as the proportions varied. The variance formulas provided will enable quantification of the precision of source proportion estimates. Graphs are provided to allow rapid assessment of possible combinations of source differences and source and mixture population SDs that will allow source proportion estimates with desired precision. In addition, an Excel spreadsheet to perform the calculations for the source proportions and their variances, SEs, and 95% confidence intervals for the two-source and three-source mixing models can be accessed at http://www.epa.gov/wed/pages/models.htm.     

Simple test of intestinal calcium absorption measured by stable strontium.

A clinical test of intestinal calcium absorption has been developed using non-radioactive stable strontium as a calcium tracer. In nine elderly subjects there was a close correlation between the fractional absorption of strontium and radioactive calcium (45Ca) during a five hour period after the simultaneous oral administration of the two tracers. Comparable precision was achieved with each tracer in six subjects in whom the test was repeated after two weeks. The effect of food on strontium absorption was examined in a further 33 normal subjects (age 21-60 years), and the administration of the strontium with a standard breakfast was shown to reduce the variance at individual time points. A simplified test in which serum strontium concentration was measured four hours after the oral dose given with a standard breakfast was adopted as the routine procedure. The normal range (mean (2 SD], established over 97 tests in 53 patients, was 7.0-18.0% of the dose in the extracellular fluid. A further 30 patients with possible disorders of calcium absorption (10 with primary hyperparathyroidism and 20 with coeliac disease) were studied by this standard test. In both groups of patients the mean four hour strontium values were significantly different from normal. This standard strontium absorption test allows assessment of calcium absorption with sufficient sensitivity and precision to have a wide application in clinical practice.     

Energy sources for aquatic animals in the Orinoco River floodplain: evidence from stable isotopes

Summary Stable carbon and nitrogen isotope ratios in autotrophs, aquatic invertebrates and fishes from the Orinoco River floodplain of Venezuela reveal that microalgae, including both phytoplankton and epiphytic (attached) forms, are predominant energy sources for many aquatic animals, even though aquatic vascular plants are much more abundant. Floating mats of the grass Paspalum repens and the water hyacinth Eichhornia spp. harbor particularly high densities of aquatic animals, but isotopic evidence indicates that few species are dependent on organic carbon originating from these plants. The stable isotopic evidence for the trophic importance of algae contradicts traditional interpretations of food webs in freshwater wetlands, which are generally thought to be based largely on detritus originating from vascular plants.     

利用氮、氧稳定同位素识别水体硝酸盐污染源研究进展

水体硝酸盐污染已经成为一个相当普遍且重要的环境问题.为了保证人类的身体健康、水环境的良性演化,有效识别水体中硝酸盐污染的来源就显得尤为重要.水体中不同来源的硝酸盐具有不同的氮、氧稳定同位素组成,因此,可以利用氮、氧稳定同位素对水体中的硝酸盐污染进行源识别.本文介绍了氮、氧稳定同位素在氮循环主要过程中的分馏系数和主要硝酸盐来源的氮、氧稳定同位素组成,对比了5种硝酸盐氮、氧同位素分析预处理方法的优缺点,综述了国内外学者在该方向的研究进展并划分为3个阶段:单独使用氮稳定同位素;同时使用氮、氧稳定同位素;结合数学模型的应用.最后,对该领域今后的研究方向进行了展望.     

稳定氢氧同位素定量植物水分来源的不确定性解析

稳定氢氧同位素技术被广泛运用于生态系统、特别是干旱区生态系统中植物水分来源的研究,其理论假设为"水分被植物根系吸收并向木质部运输过程中不发生氢氧同位素分馏"。生态系统中不同水源的氢氧同位素组成普遍存在显著差异,为从水源混合体中区分出各水源的贡献率提供了前提条件。但在实际应用过程中,诸多因素导致稳定氢氧同位素技术定量植物水分来源的结果具有不确定性。综合已有研究并加以分析,举证说明植物吸收水分相对于水源同位素变化的滞后性、水源同位素的季节性变化、蒸发作用和水源之间的混合作用对水源同位素的影响等导致植物水分来源定量结果不确定性的几个因素,以期为今后稳定氢氧同位素技术在植物水分来源领域的应用提供参考。