Urban trees are a fundamental asset for cities. Trees sequester carbon, reduce energy use, remove air pollutants, filter stormwater and cool hot city streets by providing shade and releasing water vapor. Many of the world's major cities have implemented tree-planting programs based on the supposed environmental and social benefits of urban forests. Recent studies have increasingly proven these assumptions and provide empirical evidence of the contributions of tree-planting programs, as well as their feasibility and limits, to solving or mitigating urban environmental and social problems.
We propose that current evidence supports local cooling, stormwater uptake, and the health benefits of urban trees for local residents. However, the potential of urban trees to significantly mitigate greenhouse gas emissions and air pollution across a wide range of sites and environmental conditions is limited. Consequently, urban trees appear to hold more promise for climate and pollution adaptation strategies than mitigation strategies. This is largely due to space constraints that limit the extent of urban treetops relative to the current magnitude of emissions.
The most promising environmental and health impacts of urban trees are those that can be achieved with well-managed tree planting and localised on-site design interventions at the municipal level. Planting trees at these scales has documented local climate and health benefits, which can be maximized through site-specific design, followed by monitoring, adaptive management, and studies of long-term eco-evolutionary dynamics. A recent analysis by researchers at the Morton Arboretum in Lisle, Illinois, analyzed the effect of trees on temperature around the world and showed that trees contribute greatly to urban cooling, with a 16⁰F reduction in temperature. Urban trees provide relief from the heat of shade, but they also think about evapotranspiration by actively cooling the air in cities.
While this benefit makes our beautiful tree-lined city streets more comfortable to walk on, it also helps reduce energy costs for cooling, further reducing our carbon footprint. A systematic quantitative review of the benefits, costs and evaluation methods of urban trees in different cities of different climatic zones. The Effects of Urbanization on Runoff Contaminant Concentrations, Loads, and Their Seasonal Patterns in Cold Climates. A Hedonic Spatial Analysis of the Value of Urban Land Cover in the Los Angeles Multifamily Real Estate Market.
The benefit will vary depending on the density and extent of the canopy, but dense shade can offer up to 95% reduction in UV radiation. There is substantial literature indicating that trees provide benefits for municipalities and their residents, and this perception, in part, has motivated local, regional and global initiatives that promote urban tree planting (McDonald et al. A more diverse and mature urban forest has been shown to increase the environmental and economic benefits of trees. The health effects of trees can be a critically important aspect of the benefits of urban forests, even in dense cities with relatively limited space to support urban forests.
The public health impacts of urban trees have been particularly difficult to characterize because collaborative and interdisciplinary approaches are needed to determine the nature of these impacts. The urban heat island effect explains that urban areas tend to be substantially warmer than suburban or rural areas due to a lack of green space and the abundance of heat-reflecting surfaces, such as glass and asphalt. .