James R. Ehleringer

Postblast analyses of military and terrorist events will benefit from the capacity to learn more about the explosive materials used in an event. Here stable isotope ratio analyses (δ¹³C, δ¹⁵N) can provide additional information to complement identification of the explosive components. Controlled detonations using different types of military-grade explosives were conducted in 55-gal barrels. Additionally, soil analyses were conducted following Mark-84 field detonations. Swab materials and soils were purified to analyze explosive compounds using established HPLC and IRMS techniques. Explosive materials were recovered in the residues of TNT (aromatic-based explosive) and RDX (nonaromatic explosive) detonations in barrel experiments. Explosive residues were not recovered from PETN (nonaromatic explosive). Postblast δ¹³C and δ¹⁵N values of TNT residues were similar to δ values in the source explosive, suggesting minimal enrichment in δ residues. While δ¹⁵N values of RDX in postblast residues were also similar to preblast source values, postblast RDX δ¹³C values were enriched by almost 2‰ relative to the preblast explosive. Similar patterns were observed in HMX, RDX, and NT recovered from soils following Mark-84 detonations. δ¹³C and δ¹⁵N values can be effectively measured on explosive compounds recovered in residues following detonations. Residue δ¹³C and δ¹⁵N values can be linked to δ values of undetonated explosive compounds. Additional field studies should be conducted to verify these results.

Triple oxygen isotope values of xylem water were measured along the length of smooth horsetail stems ( Equisetum laevigatum ). Extreme isotope enrichment is observed moving from base to stem tip. δ ¹⁸ O values range from −8.3‰ at the base to 82.6‰ at the tip. Δ′ ¹⁷ O values range from 0 to −1,797 per meg. The δ ¹⁸ O and Δ′ ¹⁷ O values are the most extreme measured for any terrestrial material and expand the known range of Δ′ ¹⁷ O values by fivefold for mass-dependent fractionation on Earth. The extreme isotope enrichments are explained using a hybrid evaporation/chain-of-lakes model, allowing us to refine the leaf respiration coefficient to θ _k = 0.511 ± 0.001. This new value is required to explain the low Δ′ ¹⁷ O values previously measured in desert plants and animals and is critical when using fossil samples for paleoclimate reconstruction. Coexisting phytoliths and stem water were also measured. The 1000 ln ¹⁸ α _silica-water value at the plant base (35.89‰) appears to be in isotopic equilibrium, with far smaller fractionations of 10.3‰ near the tip. The smaller fractionations at higher levels are explained by continual silica deposition as the plant elongates and the δ ¹⁸ O values of each segment become higher. The overall integrated phytolith value is a combination of early and late silica growth. The Δ′ ¹⁷ O _silica − Δ′ ¹⁷ O _water values are not in equilibrium, explained by a kinetic isotope effect, with a λ value of 0.5205 vs. 0.5244 for equilibrium. Phytolith isotope values may lead to erroneous interpretations for paleoclimate reconstruction.

Summary: 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.

Long-term, high-frequency atmospheric CO₂ measurements at multiple sites in Salt Lake City (SLC), Utah, reveal that annual and monthly CO₂ 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 CO₂ observations. Specifically, we focus on Thanksgiving holidays, when traffic and energy usage patterns differ from the rest of November. Onroad CO₂ emissions exhibit a double peak during weekday morning and evening rush hours but remain relatively low on weekends and Thanksgiving. Interestingly, CO₂ 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 CO₂ emissions and mole fraction patterns align over time, notable spatial differences exist. For instance, the near-highway site in Murray shows the highest CO₂ 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 CO₂ 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, CO₂ 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 CO₂ 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.

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.

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.

Significance: Photosynthesis involves a tradeoff between the uptake of carbon and the loss of water. Intrinsic water-use efficiency is an indicator of this tradeoff that is pivotal for understanding plant responses to climate change. Global increases in atmospheric CO ₂ concentration have increased intrinsic water-use efficiency, but this relationship is also modulated by water availability. Here, we have identified that a severe, multidecadal drought in the American Southwest has caused some of the largest increases in plant water-use efficiency ever observed. The increase was particularly large in shrubs, which dominate much of the landscape in the region. Given that water scarcity in the American Southwest is expected to worsen, these relationships have important implications for plant health and carbon and water cycling.

Summary: While plant δ¹⁵N 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 δ¹⁵N values from two Mojave Desert populations of Encelia farinosa to clarify sources of population‐level variability. We leveraged > 3500 foliar δ¹⁵N observations collected alongside structural, physiological, and climatic data to identify plant and environmental contributors to δ¹⁵N values. Additional sampling of soils, roots, stems, and leaves enabled assessment of the distribution of soil N content and δ¹⁵N, intra‐plant fractionations, and relationships between soil and plant δ¹⁵N values. We observed extensive within‐population variability in foliar δ¹⁵N values and found plant age and foliar %N to be the strongest predictors of individual δ¹⁵N values. There were consistent differences between root, stem, and leaf δ¹⁵N values (spanning c. 3‰), but plant and bulk soil δ¹⁵N values were unrelated. Plant‐level variables played a strong role in influencing foliar δ¹⁵N values, and interannual relationships between climate and δ¹⁵N values were counter to previously recognized spatial patterns. This long‐term record provides insights regarding the interpretation of δ¹⁵N values that were not available from previous large‐scale syntheses, broadly enabling more effective application of foliar δ¹⁵N values.