•A multi-scale LCA explores the pathways of utilizing urban tree waste in the US•Utilizing urban tree waste can yield nationwide environmental benefits in the US•Highlighting needs to move from landfilling and combusting to valuable utilization•Environmental benefits have geospatial variations at the state and city levels More than 45 million dry tonnes of urban tree waste are generated every year in the US. Landfilling these wastes generates greenhouse gas emissions that contribute to climate change and nutrient-related emissions causing eutrophication that results in water crises such as harmful algal boom and fish kills. Converting urban tree waste into valuable products can help mitigate climate change and eutrophication, but it is important to understanding the extent to which reusing tree waste reduces these impacts. We find that converting urban tree waste to compost, lumber, chips, and biochar substantially reduces national environmental emissions compared with landfilling waste. Such benefits vary by location within the US, and the most environmentally beneficial combination is using merchantable logs for lumber and residues for biochar. Our results highlight the feasibility and environmental benefits of recycling/reusing urban tree wastes. Substantial urban tree waste is generated and underutilized in the US. Circular utilization of urban tree wastes has been explored in the literature, but the life-cycle environmental implications of varied utilization pathways have not been fully understood. Here we quantify the life-cycle environmental benefits of utilizing urban tree wastes at process, state, and national levels in the US. Full utilization of urban tree wastes to produce compost, lumber, chips, and biochar substantially reduces nationwide global warming potential (127.4–251.8 Mt CO2 eq./year) and eutrophication potential (93.9–192.7 kt N eq./year) compared with landfilling. Such benefits vary with state-level locations due to varied urban tree waste availability and types. Process-level comparisons identify the most environmentally beneficial combination as using merchantable logs for lumber and residues for biochar. The results highlight the climate change and eutrophication mitigation potential of different circular utilization pathways, supporting the development of circular bioeconomy in the urban environment. Substantial urban tree waste is generated and underutilized in the US. Circular utilization of urban tree wastes has been explored in the literature, but the life-cycle environmental implications of varied utilization pathways have not been fully understood. Here we quantify the life-cycle environmental benefits of utilizing urban tree wastes at process, state, and national levels in the US. Full utilization of urban tree wastes to produce compost, lumber, chips, and biochar substantially reduces nationwide global warming potential (127.4–251.8 Mt CO2 eq./year) and eutrophication potential (93.9–192.7 kt N eq./year) compared with landfilling. Such benefits vary with state-level locations due to varied urban tree waste availability and types. Process-level comparisons identify the most environmentally beneficial combination as using merchantable logs for lumber and residues for biochar. The results highlight the climate change and eutrophication mitigation potential of different circular utilization pathways, supporting the development of circular bioeconomy in the urban environment. Urban forest, as a vital component of the urban system, provides many benefits to both human and natural systems, including temperature and microclimatic modification, pollution mitigation, noise reduction, biodiversity and habitat enhancement, and recreational opportunities.1Nowak D.J. Dwyer J.F. Understanding the benefits and costs of urban forest ecosystems.in: Kuser J.E. Urban and community forestry in the northeast. Springer, 2007: 25-46Crossref Google Scholar, 2Zhang Z. Meerow S. Newell J.P. Lindquist M. Enhancing landscape connectivity through multifunctional green infrastructure corridor modeling and design.Urban For. Urban Green. 2019; 38: 305-317Crossref Scopus (101) Google Scholar, 3Speak A. Escobedo F.J. Russo A. Zerbe S. Total urban tree carbon storage and waste management emissions estimated using a combination of LiDAR, field measurements and an end-of-life wood approach.J. Clean. Prod. 2020; 256: 120420Crossref Scopus (17) Google Scholar, 4Zhang Z. Stevenson K.T. 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Urban Green. 2021; 57: 126869Crossref Scopus (7) Google Scholar However, few studies have systematically quantified and compared the life-cycle emissions of products made from urban tree waste with the counterpart products made from virgin materials. Substantial variabilities throughout the life cycle of waste-derived and virgin-material-based products affect the environmental benefits of diverse urban tree waste utilization pathways, which have not been explored previously. Furthermore, previous studies rarely considered the counterfactual end-of-life cases (e.g., landfill or field application) affecting the carbon balances and environmental emissions. Here we address those research gaps by developing a multi-scale cradle-to-grave life cycle assessment (LCA) integrating process and Monte Carlo simulation (MCS). This study aims to tackle the research question of what the life-cycle environmental implications are of managing and utilizing the US urban tree waste in various pathways. To answer this question, this study chose varied urban tree waste utilization pathways to produce compost, mulch, electricity, lumber and chips, and biochar. They were selected based on their potential economic and environmental benefits discussed in the previous literature.7Nowak D.J. Greenfield E.J. Ash R.M. Annual biomass loss and potential value of urban tree waste in the United States. Urban for.Urban For. Urban Green. 2019; 46: 126469Crossref Scopus (20) Google Scholar,17Lan K. Kelley S.S. Nepal P. Yao Y. Dynamic life cycle carbon and energy analysis for cross-laminated timber in the Southeastern United States.Environ. Res. Lett. 2020; 15: 124036Crossref Scopus (19) Google Scholar, 18Lan K. Ou L. Park S. Kelley S.S. Nepal P. Kwon H. Cai H. Yao Y. Dynamic life-cycle carbon analysis for fast pyrolysis biofuel produced from pine residues: implications of carbon temporal effects.Biotechnol. Biofuels. 2021; 14: 191-217Crossref PubMed Scopus (6) Google Scholar, 19Sahoo K. Bilek E. 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The stakeholders and policymakers can further use the results presented in this study to tailor their strategies or policy toward sustainable management of urban tree waste. In this study, the system boundary includes raw material extraction, production, transportation, and end of life. This study developed and coupled process simulation models with LCA. The process simulation models provided LCI data (e.g., mass and energy balances, and environmental emissions) of composting, wood product manufacturing (i.e., lumber and chips), and biochar production by pyrolysis. These process simulation models also allow for investigating the impacts of parameter variations (e.g., variations of biomass composition, conversion process operations, and end of life) on LCI. 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Dynamic life cycle carbon and energy analysis for cross-laminated timber in the Southeastern United States.Environ. Res. Lett. 2020; 15: 124036Crossref Scopus (19) Google Scholar) of merchantable trees and non-merchantable trees are classified as residues. In this study, five scenarios were established to understand the impacts of different pathways in processing the urban tree waste (see section “scenario analysis”, Table 1, and Figure 5 for details). Scenarios were developed based on the sustainability assessment guideline for urban forests developed by the US Department of Agriculture (USDA) Forest Service.35Leff M. The Sustainable Urban Forest A Step-by-step Approach. Davey Institute/USDA Forest Service, 2016Google Scholar The guideline provides four levels of urban wood utilization. Landfilling is ranked as low utilization, while chips and mulch are considered as fair utilization. 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