Air Quality and Climate Change

Urban forests, providing multiple ecosystem services, offer a natural and sustainable solution for improving the environmental quality of urban areas through air purification, temperature regulation, and carbon sequestration. Trees, individually and collectively as part of urban forests, play a crucial role in enhancing air quality by filtering pollutants such as nitrogen dioxide (NO₂) and sulphur dioxide (SO₂). By absorbing these pollutants, trees metabolize and convert them into less harmful substances. They also trap particulate matter (PM) on their leaves, needles, and bark, effectively reducing their concentrations in the air. Research by Nowak et al. (2018) has shown that urban forests in Canada remove substantial amounts of air pollutants annually, leading to significant improvements in air quality and co-benefiting public health. An urban tree can absorb anywhere between 10 and 40 kg of CO₂ each year and can intercept up to 4.5 kg of pollutants such as NO₂, SO₂, dust, soot, and smoke (EcoTree, 2024; Vallet, 2005; Greener Seasons, 2022). 

Urban forests play a crucial role in helping communities deal with the impacts of climate change. In addition to their air-purifying functions, trees aid in mitigating the urban heat island (UHI) effect, which exacerbates air pollution levels. Trees cool down urban areas by providing shade and releasing moisture through transpiration, thereby reducing ground-level ozone formation (McDonald et al., 2016). These cooling benefits are significant as cities face warmer summers and more frequent and intense heat waves. Furthermore, the cooling effect of trees can result in lower energy usage in buildings, indirectly reducing greenhouse gas emissions. For instance, research conducted in Montreal demonstrated that planting more trees along streets could significantly reduce the UHI effect, leading to lower temperatures and improved thermal comfort (Wang & Akbari, 2016).

Urban forests also contribute significantly to climate change mitigation by sequestering carbon dioxide (CO2) from the atmosphere and acting as carbon sinks. Canadian urban forests already store significant amounts of carbon (Pasher et al., 2014; McGovern & Pasher, 2016; Steenberg et al., 2023). Moreover, urban forests, as a natural climate solution, have the potential to sequester and store more carbon if the existing trees are managed effectively and new trees are planted strategically (Drever et al., 2021). 

Carbon credit projects across Canada have promoted sustainable forest management, carbon sequestration, and biodiversity conservation. However, urban forest carbon credit programs are less prevalent in urban areas than in rural and boreal forest initiatives in Canada.  In urban forests, due to limited space for tree growth, soil compaction, and pollution, which impact tree longevity, tree mortality, and ongoing carbon maintenance, urban forest carbon sequestration and storage present unique challenges. Urban forests have a relatively small biomass for carbon sequestration compared to rural forests. Additionally, the diverse ownership of urban forests and the high cost of investments associated with certification further complicate the process of generating substantial carbon offset credits from urban trees. However, the importance of carbon sequestration has been recognized, and urban forest carbon programs offer an opportunity to enhance urban sustainability and contribute to climate change mitigation. The Sustainable Forestry Initiative’s Urban and Community Forest Sustainability Standard and the Forest Stewardship Council (FSC) Certification standard can assist municipalities in developing frameworks that incorporate carbon storage into their urban forest management plans (SFI, 2024; FSC, n.d.). The City of Mississauga attained FSC® certification for its woodlands in 2024. In Canada, there are also localized, voluntary, and self-evaluated carbon offset programs for urban forests. These programs promote tree preservation and planting and carbon credits on a voluntary basis (University of Toronto, 2019).

Mitigation and Management Strategies

Maximizing these benefits requires effective urban forest management of the existing trees and strategic planning and implementation of tree planting. One critical component of planning future urban forests is appropriate tree species selection, as different species vary in their ability to contribute to air pollution removal and withstand changing climate conditions. Using climate analogues and vulnerability metrics to inform tree species selection ensures that urban forests remain resilient under future climate scenarios (Esperon-Rodriguez et al., 2022). Furthermore, integration of urban forest management into municipal climate policies can enhance the effectiveness of climate adaptation strategies. Aligning municipal climate change and urban forestry policies in Canadian cities can lead to more cohesive and robust adaptation frameworks that benefit both urban forest and climate change mitigation (Cheng et al., 2021).

Several Canadian municipalities have successfully implemented urban forestry initiatives to combat air quality issues and climate change. In Kingston, the impact of urban forests was critical in mitigating the UHI effect, improving thermal comfort, and reducing energy consumption for cooling (Guilbault, 2016). In Vancouver, local climate zones have been analyzed to optimize tree planting locations, ensuring that urban trees contribute effectively to maintaining outdoor thermal comfort (Aminipouri et al., 2019). This approach highlights the importance of local and site-specific strategies in urban forest planning. Moreover, the City of Surrey has developed a conversation guide to engage residents in urban heat readiness, emphasizing the role of urban trees in mitigating heatwaves and improving community resilience (City of Surrey, 2021).

Municipalities and industry professionals consider several vital recommendations to maximize the benefits of urban forests, including those related to air quality. Sustaining healthy large-stature trees and promoting a diverse mix of long-lived and low-maintenance tree species enhances resilience against pests, diseases, and thus climate change while providing a more comprehensive range of ecosystem services (Wood & Dupras, 2021). Moreover, identifying priority areas for tree planting, such as heat-prone neighbourhoods and high-traffic areas, can maximize air quality improvements and thermal comfort (Chan et al., 2007). Additionally, implementing more efficient tree care, such as watering during droughts, can support the health of trees and enhance their role in mitigating climate change effects. These efforts can be supported by involving local communities in urban forestry initiatives through education and participation programs; engaged residents are more likely to support and care for urban trees, which can supplement municipal tree care efforts and ensure urban forestry’s long-term success and sustainability (Bourque et al., 2021).  

Finally, regular monitoring and maintenance programs are essential to ensure healthy and functional urban forests. Urban forests are indispensable assets for Canadian cities, offering significant benefits for improving air quality and mitigating climate change. By strategically managing and expanding urban tree cover, municipalities and industry professionals can enhance urban resilience, improve urban environments, and create more livable and resilient cities (Cheng et al., 2024).