[Show abstract][Hide abstract] ABSTRACT: Urbanization is increasing rapidly in semi-arid environments and is predicted to alter atmospheric deposition of nutrients and pollutants to cities as well as to ecosystems downwind. We examined patterns of wet and coarse dry deposition chemistry over a five-year period at 7 sites across the Central Arizona-Phoenix (CAP) study area, one of two urban sites within the National Science Foundation's Long-Term Ecological Research (LTER) program. Wet and dry deposition of organic carbon (oC) were significantly elevated in the urban core; in contrast, mean annual wet and dry fluxes of nitrogen (N) were low (<6 kg ha(-1) yr(-1)) compared to previous estimates and did not differ significantly among sites. Wet deposition of sulfate (SO(4)2-) was high across CAP (mean 1.39 kg ha(-1) yr(-1) as S) and represented the dominant anion in rainfall. Dry deposition rates did not show strong seasonal trends with the exception of oC, which was 3-fold higher in winter than in summer; ammonium (NH4+) deposition was high but more variable. Dry deposition of NO3- and oC was strongly correlated with particulate base cations and dust-derived soluble reactive phosphorus (SRP), suggesting that urban-derived dust is scrubbing the atmosphere of acidic gases and entrained particles and increasing local deposition. Differences between measured and predicted rates of dry N deposition to the urban core may be explained by incomplete collection of gas phase N on surrogate deposition surfaces in this hot and arid environment. The extent of urban enhancement of cations and oC inputs to desert ecosystems appears to be restricted to the urbanized metropolitan area rather than extending far downwind, although a low number of sites make it difficult to resolve this spatial pattern. Nevertheless, wet and dry inputs may be important for biogeochemical cycles in nutrient and carbon-poor desert ecosystems within and near arid cities.
Science of The Total Environment 08/2008; 402(1):95-105. · 3.16 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Ecologists increasingly use plot-scale data to inform research and policy related to regional and global environmental change. For soil chemistry research, scaling from the plot to the region is especially difficult due to high spatial variability at all scales. We used a hierarchical Bayesian model of plot-scale soil nutrient pools to predict storage of soil organic carbon (oC), inorganic carbon (iC), total nitrogen (N), and available phosphorus (avP) in a 7962-km2 area including the Phoenix, Arizona, USA, metropolitan area and its desert and agricultural surroundings. The Bayesian approach was compared to a traditional approach that multiplied mean values for urban mesic residential, urban xeric residential, nonresidential urban, agricultural, and desert areas by the aerial coverage of each land-use type. Both approaches suggest that oC, N, and avP are correlated with each other and are higher (in g/m2) in mesic residential and agricultural areas than in deserts or xeric residential areas. In addition to traditional biophysical variables, cultural variables related to impervious surface cover, tree cover, and turfgrass cover were significant in regression models predicting the regional distribution of soil properties. We estimate that 1140 Gg of oC have accumulated in human-dominated soils of this region, but a significant portion of this new C has a very short mean residence time in mesic yards and agricultural soils. For N, we estimate that 130 Gg have accumulated in soils, which explains a significant portion of "missing N" observed in the regional N budget. Predictions for iC differed between the approaches because the Bayesian approach predicted iC as a function of elevation while the traditional approach employed only land use. We suggest that Bayesian scaling enables models that are flexible enough to accommodate the diverse factors controlling soil chemistry in desert, urban, and agricultural ecosystems and, thus, may represent an important tool for ecological scaling that spans land-use types. Urban planners and city managers attempting to reduce C emissions and N pollution should consider ways that landscape choices and impervious surface cover affect city-wide soil C, N, and P storage.
[Show abstract][Hide abstract] ABSTRACT: Urban ecosystems are profoundly modified by human activities and thereby provide a unique “natural laboratory” to study potential
ecosystem responses to anthropogenic environmental changes. Because urban environments are now affected by urban heat islands,
carbon dioxide domes, and high-level nitrogen deposition, to some extent they portend the future of the global ecosystem.
Urbanization in the metropolitan region of Phoenix, Arizona (USA) has resulted in pronounced changes in air temperature (T
air), atmospheric CO2 concentration, and nitrogen deposition (Ndep). In this study, we used a process-based ecosystem model to explore how the Larrea tridentata dominated Sonoran Desert ecosystem may respond to these urbanization-induced environmental changes. We found that water availability
controls the magnitude and pattern of responses of the desert ecosystem to elevated CO2, air temperature, N deposition and their combinations. Urbanization effects were much stronger in wet years than normal and
dry years. At the ecosystem level, aboveground net primary productivity (ANPP) and soil organic matter (SOM) both increased
with increasing CO2 and Ndep individually and in combinations with changes in T
air. Soil N (Nsoil) responded positively to increased N deposition and air temperature, but negatively to elevated CO2. Correspondingly, ANPP and SOM of the Larrea ecosystem decreased along the urban–suburban–wildland gradient, whereas Nsoil peaked in the suburban area. At the plant functional type (FT) level, ANPP generally responded positively to elevated CO2 and Ndep, but negatively to increased T
air. C3 winter annuals showed a greater ANPP response to higher CO2 levels (>420 ppm) than shrubs, which could lead over the long term to changes in species composition, because competition
among functional groups is strong for resources such as soil water and nutrients. Overall, the combined effects of the three
environmental factors depended on rainfall variability and nonlinear interactions within and between plant functional types
and environmental factors. We intend to use these simulation results as working hypotheses to guide our field experiments
and observations. Experimental testing of these hypotheses through this process should improve our understanding of urban
ecosystems under increasing environmental stresses.
[Show abstract][Hide abstract] ABSTRACT: Modeling the multivariate spatial distribution of soil carbon and nutrients has been a challenge for ecosystem ecologists. There is a need for explanatory models, which give insight into socio-economic and biophysical controls on soil spatial variability. We propose a hierarchical Bayesian modeling specification, an approach that takes into account the spatial covariates as well as the inter-dependent nature of soil nutrients and carbon pools. We develop the model to explain variability in soil nutrient and carbon pools in the Central Arizona Phoenix Metropolitan region where soil-composition has changed considerably over the years due to socio-economic factors. A fully Bayesian statistical analysis of how these changes have affected soil nutrients provides insight as to how socio-economics influence changes in ecology. Our model included five geomorphic, ecological, and socio-economic independent variables that were used to predict soil total N, organic C, inorganic C, and extractable . Using six levels of hierarchy, we fit a suitable spatial hierarhical model. Using a Bayesian co-kriging strategy, we generate appropriate values used for predictions at new locations where covariate information is unavailable. We compare prediction results from standard models and show that our model is richer and so is the interpretation. To the best of our knowledge, this is the first work that applies hierarchical Bayesian modeling techniques and kriging strategies to study multivarate soil nutrient and carbon concentrations. We conclude a discussion of our findings and the broader ecological applicability of our modeling style.
Communication in Statistics- Simulation and Computation 02/2008; 37:434-453. · 0.29 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Though there is a growing appreciation of the importance of research on urban ecosystems, the question of what constitutes
an urban ecosystem remains. Although a human-dominated ecosystem is sometimes considered to be an accurate description of
an urban ecosystem, describing an ecosystem as human-dominated does not adequately take into account the history of development,
sphere of influence, and potential impacts required in order to understand the true nature of an urban ecosystem. While recognizing
that no single definition of “urban” is possible or even necessary, we explore the importance of attaching an interdisciplinary,
quantitative, and considered description of an urban ecosystem such that projects and findings are easier to compare, repeat,
and build upon. Natural science research about urban ecosystems, particularly in the field of ecology, often includes only
a tacit assumption about what urban means. Following the lead of a more developed social science literature on urban issues,
we make suggestions towards a consistent, quantitative description of urban that would take into account the dynamic and heterogeneous
physical and social characteristics of an urban ecosystem. We provide case studies that illustrate how social and natural
scientists might collaborate in research where a more clearly understood definition of “urban” would be desirable.
[Show abstract][Hide abstract] ABSTRACT: Cities are rapidly growing throughout the world and are altering biologic processes in many regions, with global consequences. Urbanization in the Phoenix, USA metropolitan region has dramatically altered regional ecosystem patterns, but little is known about how these changes have influenced soil organic matter, total nitrogen, and the distribution of nitrogen stable isotopes. Because urban development is a phenomenon occurring at multiple scales, ecological consequences of urbanization will likely differ between individual patches and the entire metropolitan region. To investigate such changes we conducted spatially explicit surveys including three dominant land-use types in this region: native desert, agriculture, and mesic residential. These data were combined for analysis with previously collected samples from a synoptic regional survey. A landscape scaling approach was implemented to compare the dependence of soil variability on the sampled extent and the uncertainty associated with scaling from points to patches, land-use types, and the Phoenix metropolitan region. The multiple-scale analysis of soil properties showed that variation in total soil nitrogen, soil organic matter, and δ5N content of soils differed between patch and regional scales. The majority of variation in the urbanized patch types was exhibited between patches while for the native desert the majority of variation was observed within individual patches. These differences show the impact of urbanization on the scaling relations of ecosystem components. Overall, urbanization in this region appears to have increased soil organic matter by 44%, total nitrogen by 48%, and has elevated δ15N by 21%.
[Show abstract][Hide abstract] ABSTRACT: Rapid urbanization has become an area of crucial concern in conservation owing to the radical changes in habitat structure and loss of species engendered by urban and suburban development. Here, we draw on recent mechanistic ecological studies to argue that, in addition to altered habitat structure, three major processes contribute to the patterns of reduced species diversity and elevated abundance of many species in urban environments. These activities, in turn, lead to changes in animal behavior, morphology and genetics, as well as in selection pressures on animals and plants. Thus, the key to understanding urban patterns is to balance studying processes at the individual level with an integrated examination of environmental forces at the ecosystem scale.
[Show abstract][Hide abstract] ABSTRACT: Accurate and up-to-date data describing land use and land cover change support studies of urban growth such as quantifying the amount of rural to urban change and identifying change trajectories. This paper compares three methods for identifying urban land use/land cover, based on aerial photography, satellite imagery, and ground observations. While it might be natural to assume that classification based on ground observations would be the most accurate, this may not always be the case. Here we aim to quantify to what degree these three different classification approaches agree or contradict each other and to understand why. Land use/land cover data derived from these three sources were compared for the Phoenix metropolitan area, an arid urban region undergoing rapid urbanization. Our results show that satellite data are well suited to classify land use/land cover where land use categories are associated with homogeneous land cover at the subpixel level, but that for land use categories with subpixel land cover complexity, aerial photographs or ground observations are needed to aid in the classification.
Computers Environment and Urban Systems 05/2006; · 1.79 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We examined how growth of the Phoenix urban landscape has changed spatial patterns in native Sonoran desert plant diversity. Combining data from the U.S. Census with a probability-based field inventory, we used spatial and multivariate statistics to show how plant diversity across the region is influenced by human actions. Spatial variations in plant diversity among sites were best explained by current and former land use, income, housing age, and elevation. Despite similar average diversity in perennial plant genera between desert and urban sites, numerous imported exotics have significantly increased variation in plant generic composition among urban sites, with a “luxury effect” of higher plant diversity at sites in wealthier neighborhoods. We conclude that controls on natural spatially autocorrelated desert plant diversity are replaced by a variable suite of site-specific human factors and legacy effects, which require an integration of ecology and social science to be fully understood.
Society and Natural Resources 02/2006; 19(2):101-116. · 1.09 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Aim In this paper, we explore the relationship between pollen concentration in surface soil and extant perennial vegetation across a rapidly urbanizing arid ecosystem along with its surrounding agricultural and native Sonoran Desert land. We also investigate whether taxa behave as expected from known pollination characteristics (e.g. anemophilous vs. zoophilous) and whether the pollen–plant relationship differs between the undeveloped desert and agro-urban regions.Location We used a probability-based sampling scheme at 200, 900-m2 sites across the Central Arizona–Phoenix region, a 6400-km2 area of the south-western USA.Methods Pollen–plant abundances were mapped across the study area using interpolation techniques, summary bar charts and scatterplots, then two statistical approaches were applied to examine the data quantitatively. Firstly, we used regression analysis of paired pollen concentration and plant cover data; secondly, pc-ord was used on a cross-matrix (contingency table) containing the presence/absence scores of both pollen and plants for each taxon at each site.Results Mapping and quantitative analysis revealed that pollen–plant relationships do vary both between anemophilous and zoophilous groupings and by individual taxon. They also revealed that distribution patterns of both pollen and plant abundance vary consistently across the three main landscape types (desert, urban, agricultural) and that pollen taxa groupings are consistent with pollination method and source plant distribution regardless of landscape type.Main conclusions These findings provide a broad range of comparative data to facilitate palynological reconstruction of past vegetation and aid in assessing types of prehistoric impacts on this vegetation. They also document that significant variation can occur in pollen rain across a city, even for taxa with widely distributed pollen, leading to the conclusion that forecasting pollen distribution patterns for allergy-related pollen types using only one or a few pollen traps is likely to entail substantial error.
[Show abstract][Hide abstract] ABSTRACT: Urbanization represents the extreme case of human influence on an ecosystem. Biogeochemical cycling of nitrogen (N) in cities
is very different from that of non-urban landscapes due to the large input of reactive forms of N and the heterogeneous distribution
of various land uses that alters landscape connections. To quantify the likely effects of human activities on soil N and other
soil properties in urban ecosystems, we conducted a probability-based study to sample 203 plots randomly distributed over
the 6,400 km2 Central Arizona-Phoenix Long-Term Ecological Research (CAP LTER) area, which encompasses metropolitan Phoenix with its 3.5
million inhabitants. Soil inorganic N concentrations were significantly higher in urban residential, non-residential, agricultural,
transportation, and mixed sites than in the desert sites. Soil water content and organic matter were both significantly higher
under urban and agricultural land uses, whereas bulk density was lower compared to undeveloped desert. We calculated that
farming and urbanization on average had caused an accumulation of 7.23 g m−2 in soil inorganic N across the CAP study area. Average soil inorganic N of the sampled desert sites (3.23 g m−2) was much higher than the natural background level reported in the literature. Laboratory incubation studies showed that
many urban soils exhibited net immobilization of inorganic N, whereas desert and agricultural soils showed small, but positive,
net N mineralization. The large accumulation of inorganic N in soils (mostly as nitrate) was highly unusual in terrestrial
ecosystems, suggesting that in this arid urban ecosystem, N is likely no longer the primary limiting resource affecting plants,
but instead poses a threat to surface and groundwater contamination, and influences other N cycling processes such as denitrification.
[Show abstract][Hide abstract] ABSTRACT: We explored variations in inorganic soil nitrogen (N) concentrations across metropolitan Phoenix, Arizona, and the surrounding
desert using a probability-based synoptic survey. Data were examined using spatial statistics on the entire region, as well
as for the desert and urban sites separately. Concentrations of both NO3-N and NH4-N were markedly higher and more heterogeneous amongst urban compared to desert soils. Regional variation in soil NO3-N concentration was best explained by latitude, land use history, population density, along with percent cover of impervious
surfaces and lawn, whereas soil NH4-N concentrations were related to only latitude and population density. Within the urban area, patterns in both soil NO3-N and NH4-N were best predicted by elevation, population density and type of irrigation in the surrounding neighborhood. Spatial autocorrelation
of soil NO3-N concentrations explained 49% of variation among desert sites but was absent between urban sites. We suggest that inorganic
soil N concentrations are controlled by a number of ‘local’ or ‘neighborhood’ human-related drivers in the city, rather than
factors related to an urban-rural gradient.
[Show abstract][Hide abstract] ABSTRACT: We explored variations in inorganic soil nitrogen (N) concentrations across metropolitan Phoenix, Arizona, and the surrounding desert using a probability-based synoptic survey. Data were examined using spatial statistics on the entire region, as well as for the desert and urban sites separately. Concentrations of both NO 3 -N and NH 4 -N were markedly higher and more heterogeneous amongst urban compared to desert soils. Regional variation in soil NO 3 -N concentration was best explained by latitude, land use history, population density, along with percent cover of impervious surfaces and lawn, whereas soil NH 4 -N concentrations were related to only latitude and population density. Within the urban area, patterns in both soil NO 3 -N and NH 4 -N were best predicted by elevation, population density and type of irrigation in the surrounding neighborhood. Spatial autocorrelation of soil NO 3 -N concentrations explained 49% of variation among desert sites but was absent between urban sites. We suggest that inorganic soil N concentrations are controlled by a number of 'local' or 'neighborhood' human-related drivers in the city, rather than factors related to an urban-rural gradient.
[Show abstract][Hide abstract] ABSTRACT: Amounts of readily soluble nutrients on asphalt parking lot surfaceswere measured at four locations in metropolitan Phoenix, Arizona, U.S.A. Using a rainfall simulator, short intense rainfall events were generated to simulate `first flush' runoff. Samples were collected from 0.3 m2 sections of asphalt at 8 to 10 sites on each of four parkinglots, during the pre-monsoon season in June-July 1998 and analyzed for dissolved NO3
+-N, soluble reactive phosphate (SRP), and dissolved organic carbon (DOC). Runoff concentrations varied considerably for NO3
--N and NH4
+-N (between 0.1 and 115.8 mg L-1) and DOC (26.1 to 295.7 mg L-1), but less so for SRP (0.1 to 1.0 mg L-1), representing average surface loadings of 191.3, 532.2, and 1.8 mg m-2 respectively. Compared with similar data collected from undeveloped desert soil surfaces outside the city, loadings of NO3
--N and NH4
+-N on asphalt surfaces were greater by factors of 91 and 13, respectively. In contrast, SRP loads showed little difference between asphalt and desert surfaces. Nutrient fluxes in runoff from a storm that occurred shortly after the experiments were used to estimate input-output budgets for 3 of the lots under study. Measured outputs of DOC and SRP were similar to those predicted using rainfall and experimentally determined surface loadings, but for NH4
+-N and particularly for NO3
--N, estimated rainfall inputs and surface runoff were significantly higher than exports in runoff. This suggests that parking lots may be important sites for nutrient accumulation and temporary storage in arid urban catchments.
[Show abstract][Hide abstract] ABSTRACT: Arbuscular mycorrhizal fungal (AMF) species richness, composition, spore density and diversity indices were evaluated in the Phoenix metropolitan area, Arizona, USA at 20 sampling sites selected to represent the four predominant land-use types found in the greater urban area: urban-residential, urban non-residential, agriculture and desert. AMF spores were extracted and identified from soil samples and from trap cultures established using soil collected at each site. Data were analyzed according to land use, land-use history, soil chemistry and vegetation characteristics at each site. Current agricultural sites were associated with decreased spore densities and historically agricultural sites with decreased species richness. Overall species composition was similar to that previously reported for the Sonoran desert, but composition at each sampling site was influenced by the vegetation from which samples were collected. Sites with the highest degrees of similarity in AMF species composition were also similar to each other in native plants or land use. Conversely, sites with the lowest similarity in AMF composition were those from which the majority of samples were collected from non-mycorrhizal plants, predominately ectomycorrhizal plants or bare soil. Spores of Glomus microggregatum were most abundant in urban sites, while those of G. eburneum were most abundant in desert and agricultural sites. Further studies are needed to determine the functional implications of shifts in AMF communities in urban ecosystems, including effects on plant primary productivity.
[Show abstract][Hide abstract] ABSTRACT: Spatial variation in plant diversity has been attributed to heterogeneity in resource availability for many ecosystems. However, urbanization has resulted in entire landscapes that are now occupied by plant communities wholly created by humans, in which diversity may reflect social, economic, and cultural influences in addition to those recognized by traditional ecological theory. Here we use data from a probability-based survey to explore the variation in plant diversity across a large metropolitan area using spatial statistical analyses that incorporate biotic, abiotic, and human variables. Our prediction for the city was that land use, along with distance from urban center, would replace the dominantly geomorphic controls on spatial variation in plant diversity in the surrounding undeveloped Sonoran desert. However, in addition to elevation and current and former land use, family income and housing age best explained the observed variation in plant diversity across the city. We conclude that a functional relationship, which we term the "luxury effect," may link human resource abundance (wealth) and plant diversity in urban ecosystems. This connection may be influenced by education, institutional control, and culture, and merits further study.
Proceedings of the National Academy of Sciences 08/2003; 100(15):8788-92. · 9.81 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Annual fluxes of NOx-derived dry deposition in the Phoenix metropolitan area, along with its temporal and spatial characteristics, were predicted using a diagnostic model as well as the Community Multiscale Air Quality Model (Models-3/CMAQ). Input data for the diagnostic model included hourly pollutant concentrations measured at six air quality monitoring stations, meteorological variables and detailed land cover characteristics of the study area. NOx dry deposition fluxes were simulated in the urban core area for the years 1996 and 1998. A Models-3/CMAQ simulation for 22-23 July, 1996 was used to predict spatial patterns of NOx and nitric acid dry deposition over the entire study area. Average annual NOx-derived N deposition fluxes were found to be about 9 kg N ha-1 year-1 in the urban core area, 1.5 kg N ha-1 year-1 in the upwind desert and 10 kg N ha-1 year-1 downwind of the urban core. Nitric acid and NOx dry deposition contributed 25 percent and 75 percent respectively to the total N deposition flux. Nitrogen dry deposition to the entire area was estimated to be 13.4 Gg yr-1, 20 percent of total annual N inputs and therefore a significant term in the nitrogen mass balance of the urban ecosystem.