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Redlining was a racially discriminatory housing policy established by the federal government’s Home Owners’ Loan Corporation (HOLC) during the 1930s. For decades, redlining limited access to homeownership and wealth creation among racial minorities, contributing to a host of adverse social outcomes, including high unemployment, poverty, and residential vacancy, that persist today. While the multigenerational socioeconomic impacts of redlining are increasingly understood, the impacts on urban environments and ecosystems remain unclear. To begin to address this gap, we investigated how the HOLC policy administered 80 years ago may relate to present-day tree canopy at the neighborhood level. Urban trees provide many ecosystem services, mitigate the urban heat island effect, and may improve quality of life in cities. In our prior research in Baltimore, MD, we discovered that redlining policy influenced the location and allocation of trees and parks. Our analysis of 37 metropolitan areas here shows that areas formerly graded D, which were mostly inhabited by racial and ethnic minorities, have on average ~23% tree canopy cover today. Areas formerly graded A, characterized by U.S.-born white populations living in newer housing stock, had nearly twice as much tree canopy (~43%). Results are consistent across small and large metropolitan regions. The ranking system used by Home Owners’ Loan Corporation to assess loan risk in the 1930s parallels the rank order of average percent tree canopy cover today.

 

Milldams and their legacies have significantly influenced fluvial processes and geomorphology. However, less is known about their effects on riparian zone hydrology, biogeochemistry, and water quality. Here, we discuss the potential effects of existing and breached milldams on riparian nitrogen (N) processing through multiple competing hypotheses and observations from complementary studies. Competing hypotheses characterize riparian zone processes that remove (sink) or release (source) N. Elevated groundwater levels and reducing soil conditions upstream of milldams suggest that riparian zones above dams could be hotspots for N removal via denitrification and plant N uptake. On the other hand, dam removals and subsequent drops in stream and riparian groundwater levels result in drained, oxic soils which could increase soil nitrification and decrease riparian plant uptake due to groundwater bypassing the root zone. Whether dam removals would result in a net increase or decrease of N in riparian groundwaters is unknown and needs to be investigated. While nitrification, denitrification, and plant N uptake have typically received the most attention in riparian studies, other N cycle processes such as dissimilatory nitrate reduction to ammonium (DNRA) need to be considered. We also propose a novel concept of riparian discontinuum, which highlights the hydrologic and biogeochemical discontinuities introduced in riparian zones by anthropogenic structures such as milldams. Understanding and quantifying how milldams and similar structures influence the net source or sink behavior of riparian zones is urgently needed for guiding watershed management practices and for informed decision making with regard to dam removals.

 

Spatial spillovers from renovations can bolster urban redevelopment efforts by multiplying scarce city funds, creating a reinforcing feedback cycle of redevelopment. Using spatially explicit data on renovations from Baltimore, MD, we model the extent of spatial spillovers in housing renovations and estimate the effect of previous renovations on the likelihood to renovate while controlling for underlying spatial correlation. We find strong evidence of spatial spillovers with a neighbouring renovation increasing renovation likelihood by at least 1.8% These positive feedback cycles indicate the potential for significant private market multipliers from city‐led urban revitalization policies that encourage renovation.

 

Urban ecosystems present an opportunity to study ecological communities in the context of unprecedented environmental change. In the face of urban land conversion, ecologists observe new patterns of species composition, dominance, behaviour and dispersal. We propose a hypothetical socioeconomic template that describes a gradient in human investment in community composition to aid in organizing the human role in shaping urban biodiversity. We asked: (1) what is the relative magnitude of taxonomic and functional turnover of urban woody plant communities across different land‐use types; and (2) do land uses exhibiting higher intensity of human management of biodiversity support higher turnover over those with less human influence?

Baltimore,MD,USA(39°17′ N, 76°38′ W).

We examined patterns in woody plant biodiversity across 209 plots of different urban land uses. Six land‐use types were arranged along a gradient in the intensity through which humans are hypothesized to manage species composition at the plot scale. We calculated local, or α‐diversity, and compositional turnover, or β‐diversity, of taxonomic and functional diversity across plots within each land‐use type. We compared the magnitude of these biodiversity measures between land uses to test our conceptual template for how the intensity of human management can predict urban woody plant biodiversity.

We observed high taxonomic turnover in residential and commercial plots compared with vacant or open space land‐use areas. This was associated with a weaker, but similar, pattern in functional diversity. This was associated with low total abundance in residential and commercial plots. Furthermore, the number of unique species was extremely high in the same land‐use types.

Our observations help explain why turnover can be high in heavily managed plots relative to vacant land. In patches without heavy human management, we found low levels of turnover. This highlights the importance of assessing diversity both locally and at the level of turnover between patches. Management and policy can benefit from the perspective embodied in the conceptual approach tested here.

 

Many research and monitoring networks in recent decades have provided publicly available data documenting environmental and ecological change, but little is known about the status of efforts to synthesize this information across networks. We convened a working group to assess ongoing and potential cross‐network synthesis research and outline opportunities and challenges for the future, focusing on the US‐based research network (the US Long‐Term Ecological Research network, LTER) and monitoring network (the National Ecological Observatory Network, NEON). LTER‐NEON cross‐network research synergies arise from the potentials for LTER measurements, experiments, models, and observational studies to provide context and mechanisms for interpreting NEON data, and for NEON measurements to provide standardization and broad scale coverage that complement LTER studies. Initial cross‐network syntheses at co‐located sites in the LTER and NEON networks are addressing six broad topics: how long‐term vegetation change influences C fluxes; how detailed remotely sensed data reveal vegetation structure and function; aquatic‐terrestrial connections of nutrient cycling; ecosystem response to soil biogeochemistry and microbial processes; population and species responses to environmental change; and disturbance, stability and resilience. This initial study offers exciting potentials for expanded cross‐network syntheses involving multiple long‐term ecosystem processes at regional or continental scales. These potential syntheses could provide a pathway for the broader scientific community, beyond LTER and NEON, to engage in cross‐network science. These examples also apply to many other research and monitoring networks in the US and globally, and can guide scientists and research administrators in promoting broad‐scale research that supports resource management and environmental policy.

 

In urban areas, anthropogenic drivers of ecosystem structure and function are thought to predominate over larger‐scale biophysical drivers. Residential yards are influenced by individual homeowner preferences and actions, and these factors are hypothesized to converge yard structure across broad scales. We examined soil total C and total δ¹³C, organic C and organic δ¹³C, total N, and δ¹⁵N in residential yards and corresponding reference ecosystems in six cities across the United States that span major climates and ecological biomes (Baltimore, Maryland; Boston, Massachusetts; Los Angeles, California; Miami, Florida; Minneapolis‐St. Paul, Minnesota; and Phoenix, Arizona). Across the cities, we found soil C and N concentrations and soil δ¹⁵N were less variable in residential yards compared to reference sites supporting the hypothesis that soil C, N, and δ¹⁵N converge across these cities. Increases in organic soil C, soil N, and soil δ¹⁵N across urban, suburban, and rural residential yards in several cities supported the hypothesis that soils responded similarly to altered resource inputs across cities, contributing to convergence of soil C and N in yards compared to natural systems. Soil C and N dynamics in residential yards showed evidence of increasing C and N inputs to urban soils or dampened decomposition rates over time that are influenced by climate and/or housing age across the cities. In the warmest cities (Los Angeles, Miami, Phoenix), greater organic soil C and higher soil δ¹³C in yards compared to reference sites reflected the greater proportion of C₄plants in these yards. In the two warm arid cities (Los Angeles, Phoenix), total soil δ¹³C increased and organic soil δ¹³C decreased with increasing home age indicating greater inorganic C in the yards around newer homes. In general, soil organic C and δ¹³C, soil N, and soil δ¹⁵N increased with increasing home age suggesting increased soil C and N cycling rates and associated¹²C and¹⁴N losses over time control yard soil C and N dynamics. This study provides evidence that conversion of native reference ecosystems to residential areas results in convergence of soil C and N at a continental scale. The mechanisms underlying these effects are complex and vary spatially and temporally.

 

The steadily rising global urban population has placed substantial strain on urban water quality, and this strain is projected to increase for the foreseeable future. Considerable attention has been given to the hydrological and physico‐chemical effects of urbanization on stream ecosystems. However, due to the relative infancy of the field of urban ecology, long‐term water quality analyses in urban streams are sparse. Using a 15‐yr stream chemistry monitoring record from Baltimore, Maryland, we quantified long‐term trends in nitrate, phosphate, total nitrogen, total phosphorus, chloride, and sulfate export at several sites along a rural–urban gradient. We found no significant change in solute export at most sites, although we did find specific patterns of interest for certain solutes. For example, nitrogen export declined at the most headwater urban site, while phosphorus export declined at the most downstream urban site. Coupling long‐term monitoring with data on gray and green infrastructure management throughout the landscape, we established relationships between solute export at the most downstream urban monitoring site and sanitary sewer overflows (SSOs), best management practice (BMP) implementation, and road salt application rates. Phosphorus export was correlated with BMP implementation in the watershed, whereas nitrogen export was related to SSOs. Despite highly urbanized watersheds, water quality does not appear to be declining at most of these sites, suggesting that current management may have limited further impairment. Results of our study suggest that both gray and green infrastructure are key for maintaining and improving water quality in this highly urbanized watershed.

 

Cultivation and spread of non‐native plant species may result in either phylogenetic homogenization (increasing similarity) or differentiation (decreasing similarity) of urban floras. However, it is unknown how non‐native species influence homogenization of cultivated versus spontaneously occurring species in cities, and which traits are associated with species that promote homogenization versus differentiation. In this study, we compared homogenization effects of cultivated and spontaneous non‐native species in yard floras across and within seven widely distributed U.S. cities. Additionally, we explored which traits explained their particular contribution to homogenization. We recorded plant presence/absence in 178 private yards distributed among seven metropolitan statistical areas in the United States. We compared phylogenetic homogenization effects of non‐native species within both the cultivated and spontaneous species pools using phylogenetic dissimilarities and the homogenization index. Then, we expressed contributions of non‐native species to the homogenization of each pool as a function of two different sets of plant functional traits using phylogenetic generalized least square (PGLS) models across and within cities. Across cities, spontaneous non‐native species homogenized, and cultivated non‐native species differentiated, yard floras. Within the spontaneous pool, short, small‐seeded non‐native plants and non‐native grasses significantly homogenized yard floras. Within the cultivated pool, species contribution to homogenization was best predicted by plant height, presence of showy flowers, and growth form, with non‐native grasses significantly homogenizing cultivated yard floras. Within cities, non‐native species—whether they were cultivated or spontaneous—consistently homogenized yard floras of the three northern cities and differentiated yard floras of three of the four southern cities, suggesting that homogenization processes are context‐ and scale‐dependent. Likewise, traits explaining homogenization differed substantially among cities. The inconsistent patterns among cities in the plant traits that promoted homogenization of both cultivated and spontaneous species suggest that local environmental and anthropogenic conditions of individual cities imposed strong constraints on trait selection. Linking plant functional traits that promote homogenization with residents’ preferences for vegetation may further enhance understanding of how yard plant communities assemble at regional and local scales.

 

Abstract There are urgent calls for developing a comprehensive and globally-relevant urban science that emphasizes convergence among disciplines and practice. Advancing theory and conceptual frameworks is critical to developing a new urban systems science. We synthesize five frameworks that address features identified in calls for global urban science. The frameworks address the overarching urban conditions of complexity, diffuseness, connectivity, and diversity of cities across the globe. The frameworks also help evaluate how a project or study may advance sustainability. The metacity concept, a spatially scalable representation of mosaic change in urban systems, demonstrates how the frameworks apply to increasingly extensive, spatially heterogeneous, and dynamic urban regions. The metacity concept helps avoid static and isolated plans and management approaches and provides a conceptual foundation for an interdisciplinary urban systems science. The frameworks suggest a practical checklist that may help interventions, strategies, and research better align with goals for transforming urban systems toward sustainability. 

Despite the social and ecological importance of residential spaces across the country, scant research examines urban yard management policies in the U.S. Governance scholarship points to the implementation challenges of navigating policy language. Our research provides an exploration of yard ordinance language, with implications for communities across the U.S. Specifically, we sought to determine whether—and in what instances—vegetation- and groundcover-related yard ordinances in the U.S. are ambiguous or clear. Our findings suggest that ordinances are often ambiguous when referring to the state or quality of the constituent parts that make up the residential yard (e.g., “neat” or “orderly”). Yet they are clear when providing guidance about what plant species are or are not allowed, or when articulating specific requirements regarding the size or dimensions of impervious surfaces and plants. We discuss the policy implications of these findings, especially in the context of how such policies may invite the subjective judgment by enforcers, leaving open the potential for discriminatory enforcement, especially with regard to marginalized communities where different cultural values and esthetics may be expressed in yards.