Central Arizona-Phoenix Long-Term Ecological Research Research Articles

Microplastics are ubiquitous and increasing in soil of a sprawling urban area, Phoenix (Arizona)

Microplastics are environmental contaminants that have been extensively studied in marine and aquatic environments; terrestrial ecosystems, where most microplastics originate and have the potential to accumulate, typically receive less attention. This study aims to investigate the spatial and temporal soil concentrations of microplastics in a large desert metropolitan area, the Central Arizona–Phoenix Long-Term Ecological Research (CAP-LTER) area. Soil samples from the Ecological Survey of Central Arizona (ESCA) surveys (2005 and 2015) were leveraged to study spatial distributions and the temporal change of microplastic abundances. The temporal soil microplastics data were supplemented by microplastics deposition fluxes in a central location within the area (Tempe, AZ) for a period of one year (Oct 5th, 2020 to Sept 22nd, 2021). Samples were processed and microplastics were counted under an optical microscope to obtain quantitative information of their distribution in soil. Results for the spatial variation of the microplastic abundances in soil samples in Phoenix and the surrounding areas of the Sonoran Desert from 2015 depict microplastics as ubiquitous and abundant in soils (122 to 1299 microplastics/kg) with no clear trends between different locations. Microplastics deposition fluxes show substantial deposition in the local area (71 to 389 microplastics/m2/day with an average deposition flux of 178 microplastics/m2/day) but the role of resuspension and redistribution by dust storms to deposition may contribute to the unclear spatial trends. Comparison between the 2005 and 2015 surveys show a systematic increase in the abundance of microplastics and a decrease in microplastics size. Micro-Raman spectroscopy identified a variety of plastics including PE, PS, PVC, PA, PES and PP. However, a majority of microplastics remained chemically unidentifiable. Polyethylene was present in 75 % of the sampling sites and was the most abundant polymer on average in all soil samples.

The co-production of sustainable future scenarios

Scenarios are a tool to develop plausible, coherent visions about the future and to foster anticipatory knowledge. We present the Sustainable Future Scenarios (SFS) framework and demonstrate its application through the Central Arizona-Phoenix Long-term Ecological Research (CAP LTER) urban site. The SFS approach emphasizes the co-development of positive and long-term alternative future visions. Through a collaboration of practitioner and academic stakeholders, this research integrates participatory scenario development, modeling, and qualitative scenario assessments. The SFS engagement process creates space to question the limits of what is normally considered possible, desirable, or inevitable in the face of future challenges. Comparative analyses among the future scenarios demonstrate trade-offs among regional and microscale temperature, water use, land-use change, and co-developed resilience and sustainability indices. SFS incorporate diverse perspectives in co-producing positive future visions, thereby expanding traditional future projections. The iterative, interactive process also creates opportunities to bridge science and policy by building anticipatory and systems-based decision-making and research capacity for long-term sustainability planning.

Waterbird community composition, abundance, and diversity along an urban gradient

Urban riparian corridors have the capacity to maintain high levels of bird abundance and biodiversity. How riparian corridors in cities are used by waterbirds has received relatively little focus in urban bird studies. The principal objective of our study was to determine how habitat and landscape elements affect waterbird biodiversity in an arid city. We surveyed 36 transects stratified across a gradient of urbanization and water availability along the Salt River, a riparian corridor that is monitored as part of the Central Arizona-Phoenix Long-Term Ecological Research study system located in Phoenix, Arizona, USA. Habitat and landscape variables were reduced via Principal Component Analysis to be used in a constrained ordination that identified waterbird community composition patterns, and then used to model the responses of guild abundance and diversity. Habitat and landscape components from the constrained ordination explained 39% of the variation in the waterbird community. Land use components were related to the suite of species at each site, but had a weaker relationship to guild abundance or diversity. Habitat-level components (water physiognomy, shoreline composition, and terrestrial vegetation cover) were more important in predicting both guild abundance and diversity. We found that water physiognomy was the strongest driver shaping waterbird community parameters. The implications of our study are relevant to urban planning in arid cities, offering the opportunity to design and improve wildlife habitat while providing other important public amenities.

Seasonality and land cover characteristics drive aphid dynamics in an arid city

Urbanization of arid environments results in biotic communities that differ from the surrounding desert. The growth of cities has lowered biodiversity and increased abundance of generalist species, known as urbanophiles. However, the mechanisms by which specific organisms can dominate urban ecosystems remain unclear. Using an 11-year data set from the Central Arizona-Phoenix Long-Term Ecological Research program, we evaluated how aphids, an arthropod urbanophile, were affected by habitat type and seasonality in Phoenix, Arizona, USA. Twenty-five sites were selected in habitat types varying in land use and land cover characteristics. Aphids varied along a gradient of water availability and vegetation, rather than level of urbanization. Seasonal aphid abundance was the highest in the spring and lowest in the summer, a pattern that did not differ between habitat types. We developed a mathematical model parallel to our empirical study to explain how temperature may affect the temporal patterns. The analysis of our model demonstrated that although seasonal patterns were similar across habitats, slight shifts in microclimate can result in dramatic variation of population dynamics. We conclude that both land cover and climate have huge impacts on aphids and that urbanophiles are able to take advantage of favorable environmental conditions caused by urbanization.

Variation in monsoon precipitation drives spatial and temporal patterns of Larrea tridentata growth in the Sonoran Desert

Summary 1. Broad-scale patterns of above-ground net primary production (ANPP) are closely coupled to climate features, particularly the distribution and magnitude of rainfall. In arid and semi-arid regions, however, the precipitation regime, together with local geomorphology and plant life history traits, combine to regulate soil water availability and patterns of growth, complicating simple correlations with climatic variables. 2. To better understand the drivers of plant growth in desert ecosystems, we characterized the rates and spatial heterogeneity of stem elongation by the dominant perennial shrub, creosote bush (Larrea tridentata) in the northern Sonoran Desert of Arizona (USA). Estimates of stem growth were made over a 5-year period (2006–2010) from 60 plots at 15 sites spanning c. 120 km across the Central Arizona–Phoenix (CAP) Long-term Ecological Research (LTER) area. 3. We observed both the highest and lowest rates of stem growth during summer, and these patterns were related to the amount of monsoon rainfall and local rates of water infiltration. The relationship between growth and precipitation in the summer was nonlinear, with rates increasing more than eightfold at plots receiving more than 100 mm of seasonal rainfall. Conversely, growth during the winter/spring was intermediate in magnitude, similar among years and poorly correlated with seasonal precipitation. 4. The spatial heterogeneity of stem growth also differed markedly between seasons and was greater both within and among sites during summer compared to winter/spring growing periods. At broad scales, spatial heterogeneity of shrub growth was correlated with seasonal changes in the spatial variability of rainfall across the study area. At small spatial scales, greater heterogeneity during the summer appears linked to local soil properties that influence infiltration and water availability following monsoon storms. 5. Overall, the strong, nonlinear growth response by L. tridentata to monsoon rainfall suggests that the recurrence interval of wet summer seasons is an important driver of ANPP for this long-lived shrub. More generally, our results illustrate how regional climate variability and local soil properties can interact to influence the rates and heterogeneity of desert plant growth at multiple scales.

Phosphorus in Phoenix: a budget and spatial representation of phosphorus in an urban ecosystem

As urban environments dominate the landscape, we need to examine how limiting nutrients such as phosphorus (P) cycle in these novel ecosystems. Sustainable management of P resources is necessary to ensure global food security and to minimize freshwater pollution. We used a spatially explicit budget to quantify the pools and fluxes of P in the Greater Phoenix Area in Arizona, USA, using the boundaries of the Central Arizona-Phoenix Long-Term Ecological Research site. Inputs were dominated by direct imports of food and fertilizer for local agriculture, while most outputs were small, including water, crops, and material destined for recycling. Internally, fluxes were dominated by transfers of food and feed from local agriculture and the recycling of human and animal excretion. Spatial correction of P dynamics across the city showed that human density and associated infrastructure, especially asphalt, dominated the distribution of P pools across the landscape. Phosphorus fluxes were dominated by agricultural production, with agricultural soils accumulating P. Human features (infrastructure, technology, and waste management decisions) and biophysical characteristics (soil properties, water fluxes, and storage) mediated P dynamics in Phoenix. P cycling was most notably affected by water management practices that conserve and recycle water, preventing the loss of waterborne P from the ecosystem. P is not intentionally managed, and as a result, changes in land use and demographics, particularly increased urbanization and declining agriculture, may lead to increased losses of P from this system. We suggest that city managers should minimize cross-boundary fluxes of P to the city. Reduced P fluxes may be accomplished through more efficient recycling of waste, therefore decreasing dependence on external nonrenewable P resources and minimizing aquatic pollution. Our spatial approach and consideration of both pools and fluxes across a heterogeneous urban ecosystem increases the utility of nutrient budgets for city managers. Our budget explicitly links processes that affect P cycling across space with the management of other resources (e.g., water). A holistic management strategy that deliberately couples the management of P and other resources should be a priority for cities in achieving urban sustainability.

The conservation value of residential yards: linking birds and people

Urbanization is recognized as one of the greatest threats to biodiversity throughout the world. However, the vegetation within an urbanized landscape is diverse and includes a variety of native and exotic plant species. This variation allows for testing whether certain landscape designs outperform others in the support of native biodiversity. Residential yards represent a large component of an urban landscape and, if managed collectively for birds and other wildlife, could offset some of the negative effects of urbanization. In addition, many urbanites have their primary interaction with the natural world in their front and back yards. Therefore, ensuring positive wildlife experiences for them is essential in promoting urban biodiversity. At the Central Arizona-Phoenix Long-Term Ecological Research site we tested the efficacy of native landscaping in residential yards in attracting native birds. We also explored the links between socioeconomic factors, landscape designs, and urban gradient measurements with the urban bird communities. A redundancy analysis suggested that native desert bird species increased in abundance in neighborhoods with desert landscaping designs, neighborhoods closer to large desert tracts, and higher-income neighborhoods. Variance partitioning showed that collectively these three sets of environmental variables explained almost 50% of the variation in the urban bird community. Results suggested racial and economic inequities in access to biodiversity, whereby predominantly Hispanic and lower-income neighborhoods had fewer native birds. We also found that residents' satisfaction with bird diversity was positively correlated with actual bird diversity. Our study provides new insights into the relative importance of socioeconomic variables and common urban ecological measurements in explaining urban bird communities. Urban planners can use this information to develop residential landscapes that support the well-being of both birds and people.

A multi‐scale analysis of fragmentation effects on remnant plant species richness in Phoenix, Arizona

AbstractAim Understanding complex ecological phenomena, such as the determinants of species richness, is best achieved by investigating their properties at different spatial scales. Factors significantly affecting the number of species occurring at one scale may not impact on richness at other scales. While this scale dependence has become increasingly recognized, there still remains a need to elucidate exactly how richness is structured across scales, and which mechanisms are influential for determining this important community property. This study explores how woody plant species richness varies in a fragmented system at multiple scales, and which factors are primarily responsible for these patterns.Location The study area is located in the Sonoran Desert within the bounds of metropolitan Phoenix, Arizona, which is the locus of the Central Arizona–Phoenix Long‐Term Ecological Research (CAP‐LTER) site.Methods Estimates of local and fragment plant species richness were generated from field data collected from 22 sites. Independent variables describing fragment sites were also calculated, including area, habitat heterogeneity, density of individuals, mean elevation, and extent of isolation. Structural equation modelling, multiple regression, and analysis of covariance were used to assess the contribution of independent variables to richness at the fragment and local scales.Results Fragment species richness was significantly influenced by area, though not isolation, habitat heterogeneity, mean elevation, or density of individuals. Local richness was not significantly related to fragment area, but was positively related to fragment richness, plant density, and elevation.Main conclusions The fragment species–area effect resulted from larger remnants supporting higher numbers of individuals at comparable densities, increasing richness through either passive sampling of progressively less common species and/or lower extinction rates among larger populations. Without using multi‐temporal data it is not possible to disentangle these mechanisms. We found that patterns evident at one scale are not necessarily apparent at other scales, as elevation and density of individuals significantly affected richness at the local scale but not at the fragment scale. These results lend support to the concept that mechanisms influencing the species richness of natural communities may be operable only within certain domains and that relevant scales should be specified.

Estimating vegetation cover in an urban environment based on Landsat ETM+ imagery: A case study in Phoenix, USA

Studies of urban ecological systems can be greatly enhanced by combining ecosystem modelling and remote sensing which often requires establishing statistical relationships between field and remote sensing data. At the Central Arizona–Phoenix Long‐Term Ecological Research (CAPLTER) site in the south‐western USA, we estimated vegetation abundance from Landsat ETM+ acquired at three dates by computing vegetation indices (NDVI and SAVI) and conducting linear spectral mixture analysis (SMA). Our analyses were stratified by three major land use/land covers—urban, agricultural, and desert. SMA, which provides direct measures of vegetation end member fraction for each pixel, was directly compared with field data and with the independent accuracy assessment dataset constructed from air photos. Vegetation index images with highest correlation with field data were used to construct regression models whose predictions were validated with the accuracy assessment dataset. We also investigated alternative regression methods, recognizing the inadequacy of traditional Ordinary Least Squares (OLS) in biophysical remote sensing. Symmetrical regressions—reduced major axis (RMA) and bisector ordinary least squares (OLSbisector)—were evaluated and compared with OLS. Our results indicated that SMA was a more accurate approach to vegetation quantification in urban and agricultural land uses, but had a poor accuracy when applied to desert vegetation. Potential sources of errors and some improvement recommendations are discussed.

Soil Characteristics and the Accumulation of Inorganic Nitrogen in an Arid Urban Ecosystem

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.

Urban, Residential, and Rural Climate Comparisons from Mobile Transects and Fixed Stations: Phoenix, Arizona

Fixed stations versus automobile transects were compared for a 27-day period between July and November 2001 to investigate urban, residential, and rural temperatures and dew points post-sundown for a sub-region in the metropolitan area of Phoenix, Arizona. Temperature and dew point differences were determined among urban (urb), residential (res), and rural (rur) from the fixed sites (fix) and the mobile transect route (tran). On average, transects revealed a mean ΔT(urb-rur)tran of 7.3°C (standard deviation, σ, of 2.02°C) and ΔT(res-rur)tran of 3°C (σ = 1.09°C); whereas the fixed site results were ΔT(urb-rur)fix of 4.8°C (σ = 2.78°C) and ΔT(res-rur)fix of 2.3°C (σ = 1.81°C). Comparisons of dew points for ΔTd(urb-rur)tran and ΔTd(res-rur)tran were −3.21°C (σ = 4.56°C) and −1.0°C (σ = 1.44°C), respectively; whereas the ΔTd(urb-rur)fix and ΔTd(res-rur)fix values averaged 1.27°C (σ = 2.85°C) and −0.10°C (σ = 2.75°C), respectively. The use of a Soil-Adjusted Vegetation Index (SAVI) assists in interpreting the overall results of the differences among sites and highlights the important role of surface vegetation and moisture in reducing temperatures in this desert urban setting. An ongoing mobile climate sampling system is planned within the Central Arizona-Phoenix Long-Term Ecological Research program (CAP LTER) to maintain seasonal and long-term climate sampling in order to determine local details linking urban ecology and land cover change to climate change in this metropolitan region.

Simulating the dynamics of primary productivity of a Sonoran ecosystem: Model parameterization and validation

Modeling has played a crucial role in understanding the structural and functional dynamics of forest and grassland ecosystems in the past decades, but relatively few ecosystem models have been developed for deserts. Adapting an existing desert ecosystem model to new regions with different community components and environmental settings may testify to the generality of model applicability, further verify model structure and formulations, and provide new insight into understanding desert ecosystem functioning. In this paper, we use a desert ecosystem model that was originally developed for the Chihuahuan Desert, Patch Arid Land Simulator-Functional Types (PALS-FT), to estimate the aboveground annual net primary productivity (ANPP) of a creosotebush ( Larrea tridentata)-dominated Sonoran Desert ecosystem in the Phoenix metropolitan area, home to the Central Arizona-Phoenix Long-Term Ecological Research Project (CAP LTER). We modified and parameterized the model using meteorological data, ecophysiological parameters for different plant functional types, and site characteristic data from the CAP LTER study area and an independent test site in the San Simon Valley of southeastern Arizona. Model predictions were validated and calibrated using field observations from the San Simon Valley test site. The results showed that PALS-FT was able to simulate ANPP of this typical Sonoran Desert ecosystem reasonably well, with a relative error of ±2.4% at the ecosystem level and generally less than ±25% at the functional-type level. We then used the model to simulate ANPP and its seasonal and inter-annual dynamics for a similar ecosystem in the CAP LTER study area. The model predicted average annual ANPP of 72.3 g m −2 y −1, ranging from 11.3 g m −2 y −1 to 229.6 g m −2 y −1 in a 15-year simulation. The simulated average ANPP of the Sonoran Desert ecosystem is close to field observations in other areas of the Sonoran Desert, and the range of variation also is close to that reported by other researchers for arid and semiarid ecosystems. The dynamics of ecosystem ANPP in response to fluctuations in annual precipitation simulated by the model agreed well with the known relationship between ANPP and precipitation in arid and semiarid systems. A closer examination of this relationship at the level of plant functional types further revealed that seasonal distribution of rainfall significantly affected ANPP. A comparison between the PALS-FT model prediction and two regression models for North American warm deserts showed that both regression models underestimated the Larrea ecosystem ANPP, while the process-based PALS-FT model provided the most accurate prediction among the three models. This study provides a validation for use of the PALS-FT model to investigate Sonoran desert ecosystem responses to environmental changes.

URBANIZATION AND SPIDER DIVERSITY: INFLUENCES OF HUMAN MODIFICATION OF HABITAT STRUCTURE AND PRODUCTIVITY

As a part of the Central Arizona–Phoenix Long-Term Ecological Research project, we determined how land-use alteration influenced spider and harvestman diversity. We sampled spiders in six habitat types (desert parks, urban desert remnants, industrial, agricultural, xeric- and mesic-residential yards) and tested how habitat type and productivity affected spider diversity and abundance. As expected, agricultural fields and mesic yards were more productive than the other, xeric habitats. These more productive habitats were characterized by higher abundances but lower spider diversity and were dominated by Lycosidae (wolf spiders), followed by Linyphiidae (sheet-web weavers). The increase in wolf spider abundance was positively correlated with habitat productivity and negatively correlated with the abundance of other predatory arthropods that might compete with, or prey upon, wolf spiders. Temporal changes in productivity affected spider abundance. After an El-Niño winter (May 1998), spider abundance was five times higher than after an extremely dry winter (May 2000). The differences in spider abundance between agricultural fields and the four xeric habitats were profound in 2000 but moderate in 1998, suggesting an interaction between the effects of natural and anthropogenic factors on spider populations. Compared with xeric habitats, the El-Niño effect was less profound in agricultural sites, suggesting that human land modification mollifies seasonal effects. We suggest that habitat structure and productivity alteration may change community structure, as the urban or agricultural habitats favor one or a few preadapted taxa over many others. Incorporation of large fragments of natural habitats into future landscape planning in urban environments may be important for conservation of rich spider communities.

Human Dimensions of Ecosystem Studies

Over the course of this century, academic life has progressed in such a manner that scholars who formerly approached inquires from a broad perspective now often constrain themselves within the boundaries of a single discipline. This trend has been a response to the need to effectively engage one’s subject matter by developing specialized methodologies, vocabularies, and expectations. Whereas this specialization has proved to be a successful approach for many inquiries, it has also acted to divide scholars in both their training and their investigations. Nowhere has this trend been more pronounced than in environmental sciences whose subject matter naturally divides itself into four domains: biotic, human, geologic, and ‘‘built’’ (Figure 1). Despite well-entrenched disciplinary divisions, most practicing scientists recognize that when treating a comprehensive subject, like human ecosystems, inquiries can not be effectively pursued by specialists working in one of these domains in isolation from the others. For it is by working at the junction of these domains that processes can be best understood and the greatest scientific breakthroughs will be made. Yet, building intellectual bridges between the life, earth, engineering, and social sciences remains extremely difficult. To each of us with our own disciplinary training ‘‘human–environmental interactions’’ has come to mean something quite distinct. For sociologists the ‘‘environment’’ is the social conditions created by humans, for anthropologists the interaction focuses on subsistence pursuits, and for the biologists human participation in an ecosystem is often viewed as a ‘‘disturbance.’’ Yet, increasingly people trained in each of these perspectives are recognizing the value—no, the necessity—of bridging these disciplinary chasms to create a new means of synthesis. This article reports on the efforts being made by the Central Arizona-Phoenix Long-Term Ecological Research (CAPLTER) project to build these bridges. In a special section of a recent issue of the journal Science several distinguished ecologists reached the conclusion that ‘‘most aspects of the structure and functioning of Earth’s ecosystems cannot be understood without accounting for the strong, often dominant influence of humanity’’ (Vitousek and others 1997, p 494). Furthermore, in a letter to the editor of Science only a few months ago, 20 prominent ecologists called for their colleagues to devote increased energy to understanding and better managing human influences over ecosystem function (Bazzaz and others 1998). Ecologists have long recognized that humans have had significant impacts on their study areas, but these have most often been treated as a form of external disturbance. Although adequate in some respects, it underestimates the pervasiveness and complexity of human influence. To truly understand human actions and influences upon ecosystems, it is essential to thoroughly integrate the approaches developed in the social and economic sciences with those of life and earth scientists. Recognition that ecosystems are affected by biological, geological (including climate), and human (both social and engineering) forces is a first step in the process of integration. Although most ecologists acknowledge that there are fundamental ‘‘drivers’’ behind biological and geological processes, they have little familiarity with the ‘‘drivers’’ behind human action. Without taking into account these drivers and the interactions they engender, our understanding of ecosystem dynamics both at the Received 18 February 1999; accepted 3 March 1999. E-mail: charles.redman@asu.edu Ecosystems (1999) 2: 296–298 ECOSYSTEMS r 1999 Springer-Verlag