Long-term, high-frequency atmospheric CO _2 measurements at multiple sites in Salt Lake City (SLC), Utah, reveal that annual and monthly CO _2 variability aligns with a priori estimates of emissions from anthropogenic and biological sources. In this study, we investigate whether short-term fluctuations in anthropogenic emissions, as captured in the Vulcan3 dataset for the United States, can be detected in atmospheric CO _2 observations. Specifically, we focus on Thanksgiving holidays, when traffic and energy usage patterns differ from the rest of November. Onroad CO _2 emissions exhibit a double peak during weekday morning and evening rush hours but remain relatively low on weekends and Thanksgiving. Interestingly, CO _2 mole fractions during Thanksgiving were higher than the rest of November at all SLC monitoring sites, particularly from 2008 to 2013. This increase is partially attributed to elevated energy-related emissions — especially residential sources — and meteorological factors such as weak wind speeds, cold temperature, and a low planetary boundary layer height (PBLH). While CO _2 emissions and mole fraction patterns align over time, notable spatial differences exist. For instance, the near-highway site in Murray shows the highest CO _2 mole fractions despite low local emissions, suggesting pollution transport via highways and wind advection. Random Forest model-based SHapley Additive exPlanations (SHAP) analysis reveals that onroad emissions dominate CO _2 contributions on weekdays and weekends, while energy-related emissions play a larger role during Thanksgiving, alongside meteorological drivers such as wind speed and PBLH. Across six urban cities, CO _2 emissions display a consistent pattern: residential and commercial emissions peak during Thanksgiving with substantial year-to-year variability, while onroad emissions peak during weekdays, with minimal variability. These findings highlight that urban CO _2 variability is driven by the combined influence of emissions and meteorology, underscoring the need for integrated mitigation strategies. Additionally, multi-site measurements are essential for accurate source attribution and effective policy interventions.

Trees harbor large stores of nonstructural carbohydrates, some of which are quite old (> 10 yr), yet we know little of how these older stores may be used for woody growth. Crucially, the use of old carbohydrates during cellulose biosynthesis could confound climate reconstructions that rely on tree ring stable isotope ratios. We analyzed tree-ring cellulose Δ14C and δ13C in earlywood of two pine species from montane forests in western North America using tree rings produced during the radiocarbon bomb pulse (1966-1980). Comparison of the Δ14C from ponderosa pine in Utah with estimates of atmospheric 14CO2 showed that the cellulose Δ14C values can be explained using only carbon fixed in the same growing season as ring construction. In the more arid Arizona pine, the cellulose Δ14C values indicate that up to 50% of the carbon used in tree-ring construction can be from photosynthate assimilated the year before ring construction. Correlations between cellulose δ13C time series and aridity indices validated the results obtained from Δ14C values. The results reveal that in some semiarid coniferous forests, tree-ring isotope composition could partially reflect the climate from at least one previous growing season, but that carbon sources older than 2 yr are likely seldom used.

This study assessed variations in leaf intrinsic water use efficiency (iWUE) and δ15N values among Encelia, a genus of drought-deciduous shrubs distributed across arid regions of southwestern North America between 1972 and 1980 when climates were cooler than today. We hypothesized that geographical variations in climate would significantly influence iWUE, a response to water-related climate constraints, and δ15N values, a proxy for the balance between N2 fixation and denitrification. Leaf samples were collected from six species of Encelia across 78 sites representing the genus range. The δ15N and δ13C values of these samples were measured and analyzed to identify drivers of spatial variability. Significant variations among iWUE and δ15N values were observed as a function of climate, along a spring-summer precipitation gradient. Precipitation and vapor pressure deficit (VPD) were significant drivers of variations in iWUE values, with iWUE increasing with VPD and/or decreasing precipitation, as would be predicted based on water-related constraints on leaf gas exchange. Climate values were significant drivers of variations in δ15N values, with lower δ15N values occurring in cooler temperature, spring-growing plants (northern latitudes) than in warmer summer-growing plants (southern latitudes). Encelia leaf iWUE and δ15N observations suggest few, if any, species-specific differences; but more likely that there is high plasticity in these values driven by variations in climate.

RationaleHydrogen isotope (δ2H) analysis of keratinaceous bulk tissues has been used in forensic science to reconstruct an individual's travel history or determine their region‐of‐origin. Here, we use a compound‐specific approach to examine patterns of individual amino acid δ2H values in relation to those of local tap water, bulk scalp hair tissues, and region‐of‐origin.MethodsWe measured δ2H values of amino acids in anonymously collected scalp hair (n = 67) and tap water from 28 locations in the United States. Samples were hydrolyzed into their constituent amino acids, derivatized alongside in‐house reference materials, and analyzed in triplicate using a GC‐C‐IRMS system.ResultsNon‐essential amino acid (AANESS) δ2H values and their corresponding tap water samples varied systematically across continental regions. Hydrogen isotope values of alanine, glutamic acid, and glycine were significantly correlated with tap water and an estimated 42%–51% of the hydrogen atoms in these AANESS were derived from tap water. We used linear discriminate analysis (LDA) to explore regional patterns in scalp hair bulk tissue and amino acid δ2H values. For the model that included AANESS data, 87% of the variance was explained by the first linear discriminant axis (LD1), and was driven by bulk hair tissue, alanine, and proline. This model had an overall 72% successful reclassification with samples from the south and northwest regions reclassifying correctly 92% and 78% of the time, respectively. For the model that included AAESS data, LD1 explained 81% of the variation and was driven bulk hair, threonine, valine, phenylalanine, and isoleucine. The overall reclassification rate for the model that included AAESS was 70%.ConclusionsOur findings suggest that δ2H analyses of AANESS and AAESS could help improve geolocation models for human and wildlife forensics by simultaneously providing information about both dietary and tap water inputs of hydrogen to tissue synthesis.

Over fifty years have passed since the publication of Harold Mooney’s formative paper, “The Carbon Balance of Plants” on pages 315–346 of Volume 3 (1972) of Annual Review of Ecology and Systematics. Arguably, the conceptual framework presented in that paper, and the work by Mooney and his students leading up to the paper, provided the foundational principles from which core disciplines emerged in plant economic theory, functional trait theory and, more generally, plant physiological ecology. Here, we revisit the primary impacts of those early discoveries to understand how researchers constructed major concepts in our understanding of plant adaptations, and where those concepts are likely to take us in the near future. The discipline of functional trait ecology, which is rooted in the principles of evolutionary and economic optimization, has captured the imagination of the plant physiological ecology research community, though its emphasis has shifted toward predicting species distributions and ecological roles across resource gradients. In the face of ‘big-data’ research pursuits that are revealing trait expression patterns at the cellular level and mass and energy exchange patterns at the planetary scale, an opportunity exists to reconnect the principles of plant carbon balance and evolutionary optimization with trait origins at the genetic and cellular scales and trait impacts at the global scale.

We describe establishment of Encelia farinosa, a drought-deciduous shrub common to the Mojave and Sonoran Deserts, based on annual observations of two populations between 1980 and 2020. Only 11 establishment events of 50 + yearlings (0.02–0.03 individuals m−2) occurred during this monitoring period; in 68% of the years fewer than 10 yearlings were established. Yearling survival to adulthood (age 4) ranged from 88 to 5% and was significantly related to cumulative precipitation. Juvenile survival rates were lowest during the current megadrought period. We calculated intrinsic water-use efficiency (iWUE) and observed the widest variations in iWUE values among the youngest plants. Among juveniles, surviving yearlings with the lowest iWUE values exhibited upward ontogenetic shifts in iWUE values, whereas those yearlings with the highest initial iWUE values exhibited little if any change. Juvenile size, higher iWUE values, and greater likelihood of surviving were all positively related with each other over the past several decades. Furthermore, iWUE and photosynthetic capacity were positively related to each other, providing a mechanistic explanation for why increased iWUE values among juveniles could lead to greater survival rates and to larger plants under water-deficit conditions. We posit that there is bi-directional selection for genotypic variations in iWUE values among E. farinosa and that this variation is selected for because of interannual environmental heterogeneity in precipitation and VPD associated with both high- and low-frequency climate cycles. Extreme drought cycles may favor plants with higher iWUE values, whereas more mesic periods may allow for greater persistence of lower iWUE genotypes.

Urban regions emit a large fraction of anthropogenic emissions of greenhouse gases (GHG) such as carbon dioxide (CO2) and methane (CH4) that contribute to modern-day climate change. As such, a growing number of urban policymakers and stakeholders are adopting emission reduction targets and implementing policies to reach those targets. Over the past two decades research teams have established urban GHG monitoring networks to determine how much, where, and why a particular city emits GHGs, and to track changes in emissions over time. Coordination among these efforts has been limited, restricting the scope of analyses and insights. Here we present a harmonized data set synthesizing urban GHG observations from cities with monitoring networks across North America that will facilitate cross-city analyses and address scientific questions that are difficult to address in isolation.

Concern about adolescent diets, obesity, and the associated health risks have been growing in the United States. This inspired former First Lady Michelle Obama to spearhead the Healthy Hunger-Free Kids Act (HHFKA), which made changes to the national school lunch program by increasing servings of whole grains, fruits, and vegetables. Our study examined the variability of student carbohydrate sources throughout the day and before and after the implementation of HHFKA using a stable isotope dietary biomarker. This method uses carbon stable isotope values of exhaled CO2 breath (δ13Cbreath) and provides a quantitative, non-invasive measure. δ13Cbreath samples were collected throughout the day from students (n = 31) that attended a public high school in Salt Lake City, UT. δ13Cbreath measurements reflected the short-term carbohydrate inputs from the previous meal. Carbohydrate sources were not consistent throughout the day; most students had their lowest inputs of corn/sugar-based carbohydrates after lunch. We compared our results with an earlier study that had been conducted pre-HHFKA. After-lunch δ13Cbreath values decreased significantly between the two time points, suggesting an increase in whole grain, fruit, and vegetable carbohydrates in the lunch program. Our results demonstrated that δ13Cbreath measurements provide a valuable tool to examine carbohydrate sources in an individual's diet throughout the day. We believe that this tool could be beneficial to studies examining the relationship between sugar sweetened beverages, added sugars, and refined carbohydrates and health outcomes like diabetes and obesity in both adolescent and adult populations.

Isotope ratios of tree-ring cellulose are a prominent tool to reconstruct paleoclimate and plant responses to environmental variation. Current models for cellulose isotope ratios assume a transfer of the environmental signals recorded in bulk leaf water to carbohydrates and ultimately into stem cellulose. However, the isotopic signal of carbohydrates exported from leaf to branch may deviate from mean leaf values if spatial heterogeneity in isotope ratios exists in the leaf. We tested whether the isotopic heterogeneity previously observed along the length of a ponderosa pine (Pinus ponderosa) leaf water was preserved in photosynthetic products. We observed an increase in both sugar and bulk tissue δ18O values along the needle, but the increase in carbohydrate δ18O values was dampened relative to the trend observed in leaf water. In contrast, δ13C values of both sugar and bulk organic matter were invariant along the needle. Phloem-exported sugar measured in the branch below the needles did not match whole-needle values of δ18O or δ13C. Instead, there was a near-constant offset observed between the branch and needle sugar δ13C values, while branch δ18O values were most similar to δ18O values observed for sugar at the base of the needle. The observed offset between the branch and needle sugar δ18O values likely arises from partial isotope oxygen exchange between sugars and water during phloem loading and transport. An improved understanding of the conditions producing differential δ13C and δ18O isotope effects between branch phloem and needle sugars could improve tree-ring-based climate reconstructions.

While plant delta N-15 values have been applied to understand nitrogen (N) dynamics, uncertainties regarding intraspecific and temporal variability currently limit their application. We used a 28 yr record of delta N-15 values from two Mojave Desert populations of Encelia farinosa to clarify sources of population-level variability. We leveraged > 3500 foliar delta N-15 observations collected alongside structural, physiological, and climatic data to identify plant and environmental contributors to delta N-15 values. Additional sampling of soils, roots, stems, and leaves enabled assessment of the distribution of soil N content and delta N-15, intra-plant fractionations, and relationships between soil and plant delta N-15 values. We observed extensive within-population variability in foliar delta N-15 values and found plant age and foliar %N to be the strongest predictors of individual delta N-15 values. There were consistent differences between root, stem, and leaf delta N-15 values (spanning c. 3 parts per thousand), but plant and bulk soil delta N-15 values were unrelated. Plant-level variables played a strong role in influencing foliar delta N-15 values, and interannual relationships between climate and delta N-15 values were counter to previously recognized spatial patterns. This long-term record provides insights regarding the interpretation of delta N-15 values that were not available from previous large-scale syntheses, broadly enabling more effective application of foliar delta N-15 values.

Two populations of the common shrub Encelia farinosa in the northern and southern portions of the Mojave Desert have been surveyed each spring for nearly 40 years, providing an opportunity to assess highly variable shrub mortality in an arid ecosystem. Most of the newly established shrubs experienced mortality during the juvenile stage, with median survival time of about three years in both populations yet, a small number of shrubs lived for at least a dozen years or even decades. Applying machine learning techniques, we predicted shrub mortality at different life-history stages using random forest and logistic regression. First, we examined seedling survival to become yearlings (one-year old plants), finding that less than 3% of seedlings in both populations survived to become established yearling shrubs. Second, we predicted whether or not yearlings would die prior to reaching the mature adult stage (four years old). The models achieved an Area Under the Receiver Operating Characteristic (AUC) in the 0.80 range for the Oatman population (southern Mojave Desert) and 0.90 range for the Death Valley population (northern Mojave Desert). We found yearling characteristics of smaller shrub size, low leaf coverage, and location in specific microsites associated with experiencing mortality before reaching the mature stage. Third, using only the average juvenile plant characteristics over the first four years of life, we predicted whether or not new adult shrubs were likely to experience mortality within the next eight years. The performance in this application achieved AUC in the 0.72 range for both populations. We found adult Encelia farinosa shrubs that had juvenile characteristics of smaller size, flowered less frequently, and had smaller interplant distances for the Oatman population were associated with increased mortality within the next eight years. Overall, the size of the shrub was the most important feature for the mortality modeling applications. No significant difference in AUC was found for random forest and logistic regression.

Plants make leaf-level trade-offs between photosynthetic carbon assimilation and water loss, and the optimal balance between the two is dependent, in part, on water availability. "Conservative" water-use strategies, in which minimizing water loss is prioritized over assimilating carbon, tend to be favored in arid environments, while "aggressive" water-use strategies, in which carbon assimilation is prioritized over water conservation, are often favored in mesic environments. When derived from foliar carbon isotope ratios, intrinsic water-use efficiency (iWUE) serves as a seasonally integrated indicator of the balance of carbon assimilation to water loss at the leaf level. Here, we used a multi-decadal record of annual iWUE, growth, and flowering from a single population of Encelia farinosa in the Mojave Desert to evaluate the effect of iWUE on plant performance across interannual fluctuations in water availability. We identified substantial variability in iWUE among individuals and found that iWUE interacted with water availability to significantly influence growth and flowering. However, the relationships between iWUE, water availability, and plant performance did not universally suggest that "conservative" water-use strategies were advantageous in dry years or that "aggressive" strategies were advantageous in wet years. iWUE was positively related to the odds of growth regardless of water availability and to the odds of flowering in dry years, but negatively related to growth rates in dry years. In addition, we found that leaf nitrogen content affected interannual plant performance and that an individual's iWUE plasticity in response to fluctuations in aridity was negatively related to early life drought survival and growth.

Accurate human provenancing using stable isotopes depends directly on solid understandings of the geographic and individual factors affecting isotope variability and incorporation into human tissues. Transfer of isotopic, and therefore spatial, information between environmental water and biological tissues is mediated by the isotopic composition of body water. Thus, there is a need to study body water isotope ratios at a population level and over a large isotopic and geographic range. We evaluated oxygen (δ18Obw) and hydrogen (δ2Hbw) isotope values of body water from 72 volunteers in 10 different cities across the US, and over a 5-10-day period. We analyzed covariates (e.g., water intake, physical activity, biometrics, gender) that might explain individual stable isotope ratio variations and tested a predictive model that incorporates the δ-values of drinking water, food, and O2 as well as individual variables to predict the δ-values of body water. The individual variability in body water isotope values overtime (mean 0.3‰ for δ18Obw and 2.3‰ for δ2Hbw) was lower than the intra-city variability (mean 0.9‰ for δ18Obw and 6.9‰ for δ2Hbw). Body water isotope values differed among cities (ANOVA: δ18ObwF = 97.2, p < 0.001; δ2HbwF = 176.2, p < 0.001). However, significant overlap among some cities with different drinking water was discovered. We detected significant covariation of measured drinking water and human body water isotope values (both isotope systems R2 ≥ 0.89, p < 0.001) and small but significant effects of the average daily exercise and amount of fluid intake. The differences between measured and model-predicted body water values (mean 0.12 ± 1.2‰ for Δδ18O and -1.2 ± 8.2‰ for Δδ2H) were statistically indistinguishable from zero (Δδ18O t = -0.751, p = 0.45; Δδ2H t = 1.133, p = 0.26). Here we show that community level variation exists in the δ18Obw and δ2Hbw values and the primary drivers are the regional differences in drinking water isotopes. Consistency of the body water isotope composition over the study period suggests that tissues would incorporate a stable isotope signal over time. The amount of drinking water and physical activity influence body water values, while the variation in the isotopic values of food may contribute to regional level variability, but that still remains to be assessed further. The human body water model provides accurate estimates for measured values, capturing and reproducing the main features of the body water isotope variation across space.

Herd mobility is a key parameter of many agro-pastoral strategies. Sequential variations of oxygen, carbon and strontium isotope ratios in ruminant teeth are commonly used for characterizing herd mobility along altitudinal gradients and across geological areas. To what extent is the variation and covariation of isotopic ratios along a tooth actually reflecting vertical and geological mobility? The interpretation of such data fom archaeological material widely depends on our ability to answer to this question. Few modern baselines are available to assess covariation of oxygen and carbon isotope ratios along the tooth of modern sheep experiencing vertical mobilities. There are no such baselines for the covariation of strontium, oxygen and carbon isotope ratios. Our study aimed at analyzing the variation and covariation of isotope ratios of oxygen, carbon, and strontium in the teeth and guard hair of domestic ruminants experiencing different pastoral mobilities along altitudinal gradients and across geological areas. We sampled lower molars from one cow and 17 sheep, as well as the guard hair from five goats, all grazing in different mountainous areas of Azerbaijan (Nakhchivan, Ganja-Qazakh, and Lankaran regions) at least part of the year. Our results suggest that the covariation of the isotope ratios of oxygen and carbon allow to spot easily the specimens moving seasonally between the plains and the highlands and also between the subalpine zone and the highland meadows. To a lesser extent it is possible to sort two-staged annual mobility from four-staged annual mobility. The variations in strontium isotope ratios during the period of enamel formation also adequately reflect seasonal mobility but it deeply depends on the number of samples analyzed along the tooth crown. These results allow us to interpret isotope ratio variations in archaeological domestic ruminant teeth in order to analyze the role of herd mobilities in past agro-pastoral communities.

Globally, intrinsic water-use efficiency (iWUE) has risen dramatically over the past century in concert with increases in atmospheric CO2 concentration. This increase could be further accelerated by long-term drought events, such as the ongoing multidecadal "megadrought" in the American Southwest. However, direct measurements of iWUE in this region are rare and largely constrained to trees, which may bias estimates of iWUE trends toward more mesic, high elevation areas and neglect the responses of other key plant functional types such as shrubs that are dominant across much of the region. Here, we found evidence that iWUE is increasing in the Southwest at one of the fastest rates documented due to the recent drying trend. These increases were particularly large across three common shrub species, which had a greater iWUE sensitivity to aridity than Pinus ponderosa, a common tree species in the western United States. The sensitivity of both shrub and tree iWUE to variability in atmospheric aridity exceeded their sensitivity to increasing atmospheric [CO2]. The shift to more water-efficient vegetation would be, all else being equal, a net positive for plant health. However, ongoing trends toward lower plant density, diminished growth, and increasing vegetation mortality across the Southwest indicate that this increase in iWUE is unlikely to offset the negative impacts of aridification.

The Craig–Gordon type (C–G) leaf water isotope enrichment models assume a homogeneous distribution of enriched water across the leaf surface, despite observations that Δ 18 O can become increasingly enriched from leaf base to tip. Datasets of this ‘progressive isotope enrichment’ are limited, precluding a comprehensive understanding of (a) the magnitude and variability of progressive isotope enrichment, and (b) how progressive enrichment impacts the accuracy of C–G leaf water model predictions. Here, we present observations of progressive enrichment in two conifer species that capture seasonal and diurnal variability in environmental conditions. We further examine which leaf water isotope models best capture the influence of progressive enrichment on bulk needle water Δ 18 O. Observed progressive enrichment was large and equal in magnitude across both species. The magnitude of this effect fluctuated seasonally in concert with vapour pressure deficit, but was static in the face of diurnal cycles in meteorological conditions. Despite large progressive enrichment, three variants of the C–G model reasonably successfully predicted bulk needle Δ 18 O. Our results thus suggest that the presence of progressive enrichment does not impact the predictive success of C–G models, and instead yields new insight regarding the physiological and anatomical mechanisms that cause progressive isotope enrichment.

High δ 13C in human tissues in Brazil indicate high consumption of C4-based sources due to the consumption of highly processed food and animal protein. The significant positive correlation between the human developed index (HDI) developed by the United Nations Development Program, and fingernail δ 13C at the county level proved to be useful as a new proxy in tracking human nutrition. Regions with higher HDI are those with higher consumption of highly processed food.

While tree rings have enabled interannual examination of the influence of climate on trees, this is not possible for most shrubs. Here, we leverage a multidecadal record of annual foliar carbon isotope ratio collections coupled with 39 y of survey data from two populations of the drought-deciduous desert shrub Encelia farinosa to provide insight into water-use dynamics and climate. This carbon isotope record provides a unique opportunity to examine the response of desert shrubs to increasing temperature and water stress in a region where climate is changing rapidly. Population mean carbon isotope ratios fluctuated predictably in response to interannual variations in temperature, vapor pressure deficit, and precipitation, and responses were similar among individuals. We leveraged the well-established relationships between leaf carbon isotope ratios and the ratio of intracellular to ambient CO2 concentrations to calculate intrinsic water-use efficiency (iWUE) of the plants and to quantify plant responses to long-term environmental change. The population mean iWUE value increased by 53 to 58% over the study period, much more than the 20 to 30% increase that has been measured in forests [J. Peñuelas, J. G. Canadell, R. Ogaya, Glob. Ecol. Biogeogr. 20, 597-608 (2011)]. Changes were associated with both increased CO2 concentration and increased water stress. Individuals whose lifetimes spanned the entire study period exhibited increases in iWUE that were very similar to the population mean, suggesting that there was significant plasticity within individuals rather than selection at the population scale.

Determining coffee region-of-origin is most appropriately addressed through analyses of the product available to the consumer. We analyzed the concentrations of 44 trace elements in 53 samples of roasted Arabica coffee beans (Coffea arabica) from 21 different countries. Variations in absolute elemental concentrations of coffee beans arise through varying degrees of roasting (from green through dark roasts). Since trace elements are not volatilized at roasting temperatures, we conducted analyses of element ratios to evaluate concentration-related differences among beans of different origins. We used kernel density estimates to compare the distributions of 1892 element ratios for each of these countries with the combined distribution of coffee samples from the other countries. Using this quantitative approach, we demonstrated that many of the world's coffee-producing regions can be distinguished from other regions of the world on the basis of element ratios.

In this chapter, the authors lay out basic concepts related to hair and isotope incorporation, as well as known spatial patterns that provide the basis for human provenancing. They consider it is of uttermost importance that researchers considering these analyses understand how hair records the isotopic signals related to diet, drinking water and geographical environment. The overall high content of C, N, S, O and H; the high stability that translates into high rates of preservation; and the continuous growth pattern of hair make it a great candidate for stable isotope analysis. The authors discuss patterns at the individual level related to metabolic states that can change the expected isotope ratios and also serve as a tool aiding in individual identification. The use of the “dietary isotopes” for region-of-origin assignment of humans is based on the principle that these isotopes reflect geographically distinct dietary patterns.