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We utilize traffic density and Convolvulus arvensis leaf chemistry to understand spatial patterns linking atmospheric pollution and household income in the Salt Lake City metropolitan area, USA. We hypothesize that traffic density will explain variation in atmospheric NOx and O3 concentrations. In addition, we expect foliar %N and nitrogen isotope ratios (δ15N) to be elevated and carbon isotope ratios (δ13C) to be depleted on high traffic density roads. These hypotheses were supported: we found the highest NOx and lowest O3 concentrations on high‐traffic density roads. Also, NOx concentrations were higher in low‐income neighborhoods. Low‐income neighborhoods contained a greater density of high‐traffic roads than high‐income neighborhoods. Foliar %N was highest in low‐income neighborhoods and was correlated with NOx and O3, while δ15N was more depleted with increasing O3. These findings indicate that the distribution of high‐traffic density roads plays a significant role in inequities in pollution exposure between high‐ and low‐income neighborhoods, which is reflected in leaf chemistry. Key Points: High‐traffic density roads contained higher NOx and lower O3 concentrations than low‐traffic density roadsLow‐income neighborhoods had higher NOx concentrations and a greater density of high‐traffic roads than high‐income neighborhoodsThe relationship between income and air pollution may be driven by the distribution and spatial footprint of high‐trafficked roads [ABSTRACT FROM AUTHOR]

While human activities have altered the urban nitrogen (N) and carbon (C) cycles, the relationships between social and biophysical processes in cities are not well understood. Here we evaluated relationships between sociodemographic variables (median household income and housing age) and N and C contents and stable isotope ratios of vegetation in the Los Angeles Metropolitan Area (LA), California, and the Salt Lake Valley, Utah. We hypothesized that (1) N content and stable isotope ratios would be negatively related to income via increased N deposition in lower‐income areas; (2) N and C content and N stable isotope ratios would have a positive relationship with age due to soil organic matter accumulation and increased exposure to N losses over time, respectively; and (3) δ13C would increase with income as a result of increased fossil fuel‐derived CO2 concentrations in lower‐income areas. We found that δ15N values decreased with median household income in both cities and N content decreased with income in LA. In addition, atmospheric NO2 concentrations decreased with income in LA. Soil N and C content increased with housing age. However, δ15N had opposing relationships with age in the two cities. Finally, foliar δ13C values were more isotopically depleted with both increasing household income and increasing NO2 concentrations in LA. These results show that urban foliar and soil isotopic composition are associated with sociodemographic variables and that affluence, as well as housing age, influences urban plant and soil function. Key Points: Leaf and soil C and N content and isotopic composition were correlated with income and housing age in Los Angeles and Salt Lake CityLeaf N was higher in low‐income areas of Los Angeles and isotopically enriched in low income areas in both Los Angeles and Salt Lake CityResults may be explained by spatial patterns in atmospheric pollution, which disproportionately affects low‐income areas in Los Angeles [ABSTRACT FROM AUTHOR]

The seasonal pattern of the carbon isotope content (δ13C) of atmospheric CO2 depends on local and nonlocal land-atmosphere exchange and atmospheric transport. Previous studies suggested that the δ13C of the net land-atmosphere CO2 flux ( δsource) varies seasonally as stomatal conductance of plants responds to vapor pressure deficit of air (VPD). We studied the variation of δsource at seven sites across the United States representing forests, grasslands, and an urban center. Using a two-part mixing model, we calculated the seasonal δsource for each site after removing background influence and, when possible, removing δ13C variation of nonlocal sources. Compared to previous analyses, we found a reduced seasonal (March-September) variation in δsource at the forest sites (0.5‰ variation). We did not find a consistent seasonal relationship between VPD and δsource across forest (or other) sites, providing evidence that stomatal response to VPD was not the cause of the global, coherent seasonal pattern in δsource. In contrast to the forest sites, grassland and urban sites had a larger seasonal variation in δsource (5‰) dominated by seasonal transitions in C3/C4 grass productivity and in fossil fuel emissions, respectively. Our findings were sensitive to the location used to account for atmospheric background variation within the mixing model method that determined δsource . Special consideration should be given to background location depending on whether the intent is to understand site level dynamics or regional scale impacts of land-atmosphere exchange. The seasonal amplitude in δ13C of land-atmosphere CO2 exchange ( δsource ) varied across land cover types and was not driven by seasonal changes in vapor pressure deficit. The largest seasonal amplitudes of δsource were at grassland and urban sites, driven by changes in C3/C4 grass productivity and fossil fuel emissions, respectively. Mixing model approaches may incorrectly calculate δsource when background atmospheric observations are remote and/or prone to anthropogenic influence. [ABSTRACT FROM AUTHOR]

Evapotranspiration ( ET), an essential process in biosphere-atmosphere interactions, is highly uncertain in cities that maintain cultivated and irrigated landscapes. We estimated ET of irrigated landscapes in Los Angeles by combining empirical models of turfgrass ET and tree transpiration derived from in situ measurements with previously developed remotely sensed estimates of vegetation cover and ground-based vegetation surveys. We modeled irrigated landscapes as a two-component system comprised of trees and turfgrass to assess annual and spatial patterns of ET. Annual ET from vegetated landscapes ( ETveg) was 1110 ± 53 mm/yr and ET from the whole city (vegetated and nonvegetated areas, ETland) was three times smaller, reflecting the fractional vegetation cover. With the exception of May and June, monthly ETland was significantly higher than predicted by the North American Land Data Assimilation System. ETveg was close to potential ET, indicating abundant irrigation inputs. Monthly averaged ETveg varied from 1.5 ± 0.1 mm/d (December) to 4.3 ± 0.2 mm/d (June). Turfgrass was responsible for ∼70% of ETveg. For trees, angiosperm species (71% of all trees) contributed over 90% to total tree transpiration, while coniferous and palm species made very small contributions. ETland was linearly correlated with median household income across the city, confirming the importance of social factors in determining spatial distribution of urban vegetation. These estimates have important implications for constraining the municipal water budget of Los Angeles and improving regional-scale hydrologic models, as well as for developing water-saving practices. The methodology used in this study is also transferable to other semiarid regions for quantification of urban landscape ET. [ABSTRACT FROM AUTHOR]

Owens Valley, California, USA, is an important source of water for the city of Los Angeles; however, recent studies have documented losses of grass cover coinciding with decreased watertable depths in many locations. These changes in community composition are assumed to be associated with shallower rooting depths and greater vulnerability to declining watertable depths in grasses compared to neighbouring shrubs. However, the hydraulic properties and water stress resistance of most species in Owens Valley have not been measured. In addition, the relationships between groundwater depth and other aspects of ecosystem function such as nutrient cycling are not well understood. In this study, we measured grass and shrub cover, vulnerability to cavitation, and plant and soil isotopic and chemical composition at 9 sites along a depth to watertable gradient of 0·3-5·7 m in Owens Valley. Contrary to expectations, the grass species was more resistant to water stress-induced cavitation than either shrub species. However, grass cover declined in sites with deeper watertables while shrub cover remained constant. Water isotopes indicated shallower rooting depths in grasses than in shrubs, although the phreatophytic shrub Ericameria nauseosa had enriched leaf water isotopes at deep groundwater sites, indicating water stress. Sites with lower grass cover contained less soil nitrogen (N) that was also more isotopically enriched, which is indicative of greater ecosystem N losses. These results show that groundwater depth is correlated with a number of ecosystem traits in these Great Basin Desert ecosystems and should be considered when evaluating future changes in groundwater depth. Copyright © 2010 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Approximately 75–80% of the population of North America currently lives in urban areas as defined by national census bureaus, and urbanization is continuing to increase. Future trajectories of fossil fuel emissions are associated with a high degree of uncertainty; however, if the activities of urban residents and the rate of urban land conversion can be captured in urban systems models, plausible emissions scenarios from major cities may be generated. Integrated land use and transportation models that simulate energy use and traffic-related emissions are already in place in many North American cities. To these can be added a growing dataset of carbon gains and losses in vegetation and soils following urbanization, and a number of methods of validating urban carbon balance modeling, including top down atmospheric monitoring and urban ‘metabolic’ studies of whole ecosystem mass and energy flow. Here, we review the state of our understanding of urban areas as whole ecosystems with regard to carbon balance, including both drivers of fossil fuel emissions and carbon cycling in urban plants and soils. Interdisciplinary, whole-ecosystem studies of the socioeconomic and biophysical factors that influence urban carbon cycles in a range of cities may greatly contribute to improving scenarios of future carbon balance at both continental and global scales. [ABSTRACT FROM AUTHOR]

Estimates of terrestrial carbon isotope discrimination are useful to quantify the terrestrial carbon sink. Carbon isotope discrimination by terrestrial ecosystems may vary on seasonal and interannual time frames, because it is affected by processes (e.g. photosynthesis, stomatal conductance, and respiration) that respond to variable environmental conditions (e.g. air humidity, temperature, light). In this study, we report simulations of the temporal variability of canopy-scale C3 photosynthetic carbon isotope discrimination obtained with an ecophysiologically based model (ISOLSM) designed for inclusion in global models. ISOLSM was driven by half-hourly meteorology, and parameterized with eddy covariance measurements of carbon and energy fluxes and foliar carbon isotope ratios from a pine forest in Metolius (OR). Comparing simulated carbon and energy fluxes with observations provided a range of parameter values that optimized the simulated fluxes. We found that the sensitivity of photosynthetic carbon isotope discrimination to the slope of the stomatal conductance equation ( m, Ball–Berry constant) provided an additional constraint to the model, reducing the wide parameter space obtained from the fluxes alone. We selected values of m that resulted in similar simulated long-term discrimination as foliar isotope ratios measured at the site. The model was tested with 13C measurements of ecosystem ( δR) and foliar ( δf) respiration. The daily variability of simulated 13C values of assimilated carbon ( δA) was similar to that of observed δf, and higher than that of observed and simulated δR. We also found similar relationships between environmental factors (i.e. vapor pressure deficit) and simulated δR as measured in ecosystem surveys of δR. Therefore, ISOLSM reasonably simulated the short-term variability of δA controlled by atmospheric conditions at the canopy scale, which can be useful to estimate the variability of terrestrial isotope discrimination. Our study also shows that including the capacity to simulate carbon isotope discrimination, together with simple ecosystem isotope measurements, can provide a useful constraint to land surface and carbon balance models. [ABSTRACT FROM AUTHOR]

The article focuses on ecohydrology in riparian forests. Riparian ecosystems provide essential habitat for a variety of species in the desert southwest of the United States. In these areas, "Fremont cottonwood" is a common riparian species that becomes established following flooding and depositional events in alluvial plains. Human-caused alterations in hydrology, grazing, and invasive species have had a large impact on riparian areas once dominated by Fremont cottonwood and its co-occurring species. Stands of Fremont cottonwood have been reduced to a small fraction of their former extent, having been replaced by secondary successional species, upland species, and exotic invasives. A number of mechanisms have been ascribed to the decline of Fremont cottonwood, including drought stress, salinity stress, and inter-specific competition. Management and restoration of riparian ecosystems requires an understanding of the importance of potential mechanisms of ecosystem degradation and shifts in community structure.

SummaryPlant responses to elevated atmospheric CO2 have been characterized generally by stomatal closure and enhanced growth rates. These responses are being increasingly incorporated into global climate models that quantify interactions between the biosphere and atmosphere, altering climate predictions from simpler physically based models. However, current information on CO2 responses has been gathered primarily from studies of crop and temperate forest species. In order to apply responses of vegetation to global predictions, CO2 responses in other commonly occurring biomes must be studied. A Free Air CO2 Enrichment (FACE) study is currently underway to examine plant responses to high CO2 in a natural, undisturbed Mojave Desert ecosystem in Nevada, USA. Here we present findings from this study, and its companion glasshouse experiment, demonstrating that field-grown Ephedra nevadensis and glasshouse-grown Larrea tridentata responded to high CO2 with reductions in the ratio of transpirational surface area to sapwood area (LSR) of 33% and 60%, respectively. Thus, leaf-specific hydraulic conductivity increased and stomatal conductance remained constant or was increased under elevated CO2. Field-grown Larrea did not show a reduced LSR under high CO2, and stomatal conductance was reduced in the high CO2 treatment, although the effect was apparent only under conditions of unusually high soil moisture. Both findings suggest that the common paradigm of 20–50% reductions in stomatal conductance under high CO2 may not be applicable to arid ecosystems under most conditions. [ABSTRACT FROM AUTHOR]

SummaryPlant responses to elevated atmospheric CO2 have been characterized generally by stomatal closure and enhanced growth rates. These responses are being increasingly incorporated into global climate models that quantify interactions between the biosphere and atmosphere, altering climate predictions from simpler physically based models. However, current information on CO2 responses has been gathered primarily from studies of crop and temperate forest species. In order to apply responses of vegetation to global predictions, CO2 responses in other commonly occurring biomes must be studied. A Free Air CO2 Enrichment (FACE) study is currently underway to examine plant responses to high CO2 in a natural, undisturbed Mojave Desert ecosystem in Nevada, USA. Here we present findings from this study, and its companion glasshouse experiment, demonstrating that field-grown Ephedra nevadensis and glasshouse-grown Larrea tridentata responded to high CO2 with reductions in the ratio of transpirational surface area to sapwood area (LSR) of 33% and 60%, respectively. Thus, leaf-specific hydraulic conductivity increased and stomatal conductance remained constant or was increased under elevated CO2. Field-grown Larrea did not show a reduced LSR under high CO2, and stomatal conductance was reduced in the high CO2 treatment, although the effect was apparent only under conditions of unusually high soil moisture. Both findings suggest that the common paradigm of 20–50% reductions in stomatal conductance under high CO2 may not be applicable to arid ecosystems under most conditions. [ABSTRACT FROM AUTHOR]

ABSTRACTResponses of stomatal conductance (gs) to increasing vapour pressure deficit (D) generally follow an exponential decrease described equally well by several empirical functions. However, the magnitude of the decrease – the stomatal sensitivity – varies considerably both within and between species. Here we analysed data from a variety of sources employing both porometric and sap flux estimates of gs to evaluate the hypothesis that stomatal sensitivity is proportional to the magnitude of gs at low D (≤ 1 kPa). To test this relationship we used the function gs = gsref – m · lnD where m is the stomatal sensitivity and gsref = gs at D = 1 kPa. Regardless of species or methodology, m was highly correlated with gsref (average r2 = 0·75) with a slope of approximately 0·6. We demonstrate that this empirical slope is consistent with the theoretical slope derived from a simple hydraulic model that assumes stomatal regulation of leaf water potential. The theoretical slope is robust to deviations from underlying assumptions and variation in model parameters. The relationships within and among species are close to theoretical predictions, regardless of whether the analysis is based on porometric measurements of gs in relation to leaf-surface D (Ds), or on sap flux-based stomatal conductance of whole trees (GSi), or stand-level stomatal conductance (GS) in relation to D. Thus, individuals, species, and stands with high stomatal conductance at low D show a greater sensitivity to D, as required by the role of stomata in regulating leaf water potential. [ABSTRACT FROM AUTHOR]