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. The MillionTrees NYC Initiative uses the model of experiments designed to engage environmentalists and designers in creating a large-scale, long-term urban green infrastructure research program. 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. Pollution and greenhouse gas (GHG) emissions in modern cities are disproportionately large relative to the extent of urban trees.
The Effects of Urbanization on Runoff Contaminant Concentrations, Loads, and Their Seasonal Patterns in Cold Climates. A systematic quantitative review of the benefits, costs and evaluation methods of urban trees in different cities of different climatic zones. Urban green spaces typified by permeable and unsealed soils can also absorb and store considerable amounts of rainwater in the ground. Impact of tree location and arrangement on outdoor microclimates and human thermal comfort in an urban residential environment.
For some ecosystem services, such as C sequestration, the ecological dynamics of C uptake by trees are well understood, but the direct impacts on atmospheric CO2 concentrations and climate depend on the spatial extent of urban trees, their demographic and population dynamics, and the interactions between productivity, heterotrophic respiration and C dynamics of the soil at large spatial and temporal scales. Temporal dynamics, including tree demographics, host-pathogen interactions, extinctions, and other population and evolutionary processes, also influence the extent to which tree-planting efforts influence urban conditions. .
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