Leaf isotopes and remote sensing reveal legacies of redlining in Midwestern cities

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Elizabeth Hrycyna, Jennings Mergenthal, Saiido Noor and Mary Heskel, Biology, Macalester College, Saint Paul, MN, Xiaomei Xu, Earth System Science, University of California, Irvine, Irvine, CA

Background/Question/Methods
Legacy impacts of historic racist policies can be observed in the urban ecology of US cities, ranging from tree cover, to the biodiversity of species populations, to air quality and heat wave impacts on human health. Redlining, which broadly refers to the racist, classist, and anti-immigrant policies of borrowing and lending in urban areas during the 1930-1960s by the Home Owners Loan Commission (HOLC), while eliminated on paper, can still be traced to current-day environmental injustice and inequity. Using a hybrid-methods approach of radiocarbon isotope ratios and remote sensing, we asked the following questions: (1) can leaves of local trees and annual plants integrate a signal of fossil fuel emissions that reflect differences amongst neighborhoods that were historically ‘redlined’ and ‘greenlined’?; and (2) can differences in fossil fuels emissions amongst these neighborhoods also be detected remotely using satellite imagery? To address these questions, we focused on St. Paul, MN, USA as a pilot site, and collected leaves from plants representing different HOLC classifications (n=12), as well as a remote, non-urban site. Radiocarbon isotope data of these leaves was measured to indicate the proportion of fossil-fuel derived carbon assimilated within the 2020 growing season, providing a local signal of CO2 emissions. We then expanded our study to 10 US Midwestern cities with historic HOLC classifications with spatial different proportions of ‘red’ to ‘green’ neighborhoods to evaluate current emissions of NOx, CO, and Ozone gasses using TROPOMI (Tropospheric monitoring instrument) satellite data.
Results/Conclusions
Our data indicate plant leaves integrate a local fossil fuel emission signal, largely influenced by the co-varying drivers of proximity to large roadways and historic neighborhood categorization. Radiocarbon isotopes in leaves may be used to further examine how redlining policies have shaped emissions exposure to populations over long periods of time by using dendrochronological approaches. We also found variation in the emissions of NOx, CO, and Ozone across midwestern cities that is driven both by seasonality and local neighborhoods. Emissions values and patterns amongst and within cities point to ongoing inequity in air quality. Together, these data present a novel hybrid-methods approach to examining historic legacy impacts on modern emissions exposure to urban populations.