Hannah Safford, Washington Office; Elizabeth Larry, Washington Office; E. Gregory McPherson, Pacific Southwest Research Station; David J. Nowak, Northern Research Station; Lynne M. Westphal, Northern Research Station.
An archived version of this topic paper is available.
The urban environment presents important considerations for global climate change. Over half of the world’s population lives in urban areas (1). Because cities are more dense and walkable (2), urban per capita emissions of greenhouse gases (GHGs) are almost always substantially lower than average per capita emissions for the countries in which they are located (3, 4). Urban areas are also more likely than non-urban areas to have adequate emergency services (5), and so may be better equipped to provide critical assistance to residents in the case of climate-related stress and events such as heat waves, floods, storms, and disease outbreaks. However, cities are still major sources of GHG emissions (6). Studies suggest that cities account for 40-70% of all GHG emissions worldwide due to resource consumption and energy, infrastructure, and transportation demands (7). Highly concentrated urban areas, especially in coastal regions and in developing countries, are disproportionately vulnerable to extreme weather and infectious disease.
Urban forests play an important role in climate change mitigation and adaptation. Active stewardship of a community’s forestry assets can strengthen local resilience to climate change while creating more sustainable and desirable places to live.
Benefits of Urban Forests
The term "urban forest" refers to all trees within a densely populated area, including trees in parks, on streetways, and on private property. Though the composition, health, age, extent, and costs of urban forests vary considerably among different cities, all urban forests offer some common environmental, economic, and social benefits. Trees in a community help to reduce air and water pollution, alter heating and cooling costs, and increase real estate values. Trees can improve physical and mental health, strengthen social connections, and are associated with reduced crime rates. Trees, community gardens, and other green spaces get people outside, helping to foster active living and neighborhood pride.
Carbon capture and energy savings
Urban forests-like any forest-help mitigate climate change by capturing and storing atmospheric carbon dioxide during photosynthesis, and by influencing energy needs for heating and cooling buildings; trees typically reduce cooling costs, but can increase or decrease winter heating use depending on their location around a building and whether they are evergreen or deciduous. In the contiguous United States alone, urban trees store over 708 million tons of carbon (approximately 12.6% of annual carbon dioxide emissions in the United States) and capture an additional 28.2 million tons of carbon (approximately 0.05% of annual emissions) per year (8, 9). The value of urban carbon sequestration is substantial: approximately $2 billion per year, with a total current carbon storage value of over $50 billion (8). Shading and reduction of wind speed by trees can help to reduce carbon emissions by reducing summer air conditioning and winter heating demand and, in turn, the level of emissions from supplying power plants (10). Shading can also extend the useful life of street pavement by as much as ten years, thereby reducing emissions associated with the petroleum-intensive materials and operation of heavy equipment required to repave roads and haul away waste (11). Establishing 100 million mature trees around residences in the United States would save an estimated $2 billion annually in reduced energy costs (12, 1). However, this level of tree planting would only offset less than 1% of United States emissions over a 50-year period (14).
Provision of usable goods
The sustainable use of wood, food, and other goods provided by the local urban forest may also help mitigate climate change by displacing imports associated with higher levels of carbon dioxide emitted during production and transport. Urban wood is a valuable and underutilized resource. At current utilization rates, forest products manufactured from felled urban trees are estimated to save several hundred million tons of CO2 over a 30-year period. Furthermore, wood chips made from low-quality urban wood may be combusted for heat and/or power to displace an additional 2.1 million tons of fossil fuel emissions per year (15).
Adaptation to climate and weather changes
Urban forests enable cities to better adapt to the effect of climate change on temperature patterns and weather events. Cities are generally warmer than their surroundings (typically by about 1-2°C, though this difference can be as high as 10°C under certain climactic conditions (16, 17)), meaning that average temperature increases caused by global warming are frequently amplified in urban areas. Urban forests help control this "heat island" effect by providing shade and by reducing urban albedo (the fraction of solar radiation reflected back into the environment), and through cooling evapotranspiration (4, 10, 16). Cities are also particularly susceptible to climate-related threats such as storms and flooding. Urban trees can help control runoff from these by catching rain in their canopies and increasing the infiltration rate of deposited precipitation. Reducing stormwater flow reduces stress on urban sewer systems by limiting the risk of hazardous combined sewer overflows (18). Furthermore, well-maintained urban forests help buffer high winds, control erosion, and reduce drought (10, 18, 19).
Increased community resilience
Urban forests provide critical social and cultural benefits that may strengthen community resilience to climate change. Street trees can hold spiritual value, promote social interaction, and contribute to a sense of place and family for local residents (21). Overall, forested urban areas appear to have potentially stronger and more stable communities (21). Community stability is essential to the development of effective long-term sustainable strategies for addressing climate change (22). For example, neighborhoods with stronger social networks are more likely to check on elderly and other vulnerable residents during heat waves and other emergencies (23).
Urban forests help control the causes and consequences of climate-related threats. However, forests may also be negatively impacted by climate change.
Although increased CO2 levels and warmer temperatures may initially promote urban tree growth by accelerating photosynthesis, too much warming in the absence of adequate water and nutrients stresses trees and retards future development (24). Warmer winter temperatures increase the likelihood of winter kill, in which trees, responding to their altered environment, prematurely begin to circulate water and nutrients in their vascular tissue. If rapid cooling follows these unnatural warm periods, tissues will freeze and trees will sustain injury or death.
Warmer winter temperatures favor many populations of tree pest and pathogen species normally kept at low levels by cold winter temperatures (24). Although climate change may reduce populations of some species, many others are better able than their arboreal hosts to adapt to changing environments due to their short lifecycles and rapid evolutionary capacity (19, 24). The consequences of these population changes are compounded by the fact that hot, dry environments enrich carbohydrate concentrations in tree foliage, making urban trees more attractive to pests and pathogens (24).
Climate change alters water cycles in ways that impact urban forests. Increased winter precipitation puts urban forests at greater risk from physical damage due to increased snow and ice loading (25). Increased summer evaporation and transpiration creates water shortages often exacerbated by urban soil compaction and impermeable surfaces. More frequent and intense extreme weather events increase the likelihood of severe flooding, which may uproot trees and cause injury or death to tree root systems if waterlogged soils persist for prolonged periods (25).
Especially cold regions may benefit from increased tourism, agricultural productivity, and ease of transport as a result of climate change (3, 4). However, the potential positive implications of climate change are far eclipsed by the negative (3.c). Rising temperatures, increased pest and pathogen activity, and water cycle changes impose physiological stresses on urban forests that compromise forest ability to deliver ecosystem services that protect against climate change. Climate change will also continue to alter species ranges and regeneration rates, further affecting the health and composition of urban forests (20, 26). Proactive management is necessary to protect urban forests against climate-related threats, and to sustain desired urban forest structures for future generations.
Options for Management
City "climate action plans" often incorporate urban forestry into climate change mitigation and adaptation strategies, recognizing that healthy trees and forests can strengthen a community’s ability to withstand and manage climate-related threats. Active urban forest management for climate change strengthens community resilience to climate change impacts (as well as other potential disasters), and creates more livable, desirable places to live, work and play.
Mitigation. Climate change mitigation in urban areas focuses primarily on reducing GHG emissions. Urban forest managers can help aid reductions efforts by preferentially allocating resources to trees that are more effective at mitigating emissions. Large-stature species with dense wood tend to store the most carbon (26), for instance, and trees of certain species may exhibit more desirable lifetime carbon capture-to-emissions ratios (27, 28). Maintaining tree canopy in perpetuity also sustains carbon storage within urban trees and forests and allows carbon to accumulate within urban soils. Urban soils in the United States are estimated to store approximately 1.9 billion metric tons of carbon (29).
Other effective mitigation strategies include strategically planting trees around buildings to promote energy efficiency, enlarging and improving planting sites to improve tree longevity and increase stormwater infiltration, and including trees in street improvement projects (28). Using wood in place of fossil-fuel intensive materials, such as steel and concrete, is also an important mitigation action. Wood, a renewable resource, sequesters atmospheric carbon as it grows; substituting wood products for fossil fuel-intensive alternatives in building construction thereby reduces net GHG emissions (30). Facilitating natural regeneration in cities where possible and working to reduce fossil fuel consumption associated with tree planting and maintenance also helps decrease emissions (27, 31, 32).
Adaptation. Incorporating climate resilience into tree planting and urban forest management plans helps improve the adaptive capacity of a community’s tree canopy. Planting a diverse mix of pest-tolerant, well-adapted, low-maintenance, long-lived, and drought-resistant trees ensures greater resilience (27, 28), while planting small groves of especially water-tolerant species in areas receiving peak volumes of stormwater runoff reduces flooding and pollutant transport (28). Establishing and adhering to a regular maintenance cycle can help protect cities from extreme weather events. Young trees must be pruned early and often to encourage development of strong branching structures that are less vulnerable to storm and wind damage, and hazardous or diseased trees must be removed (28). Although urban forests, like all other ecosystems, can never be totally invulnerable to climate change impacts, thoughtful management can improve resilience and help cities and communities better adapt to change.
Urban forest cover is a key mediating variable between climate change impacts and particularly vulnerable population demographics, such as the young, the elderly, and the poor. These populations often suffer disproportionate negative impacts from the multiple health hazards associated with climate change, especially when located near freeways, industry, rivers, landfills, and other areas with little green space. Developing a location-specific list of "climate smart" tree species and planting sites can serve as a useful first step towards increasing urban forest cover in these areas.
Local governance. Due to limited staff and budget resources, many cities rely on partnerships with private landowners, organized citizen groups, and nonprofit agencies in order to effectively manage urban ecosystems. In some areas, citizens participate in advisory commissions that provide input to local officials on policy and regulations governing urban forests. In others, partnerships promote innovative greening strategies that complement or augment existing programs (33, 34). Collaborative governance across traditional boundaries engages constituents, increases environmental and political awareness across generations, and enables communities to better address complex issues such as climate change (35, 36, 37).
Community stewardship. Volunteer-based urban forest initiatives may complement or augment city-run adaptation and mitigation strategies (38, 39). Community volunteers can gather data needed to develop informed urban forest management and climate action plans (35). Neighborhood workdays provide opportunities for residents to join together to restore, maintain, and/or expand the urban forest. Such citizen involvement improves urban forest health while strengthening community social ties, creating an environment conducive to cooperative adaptation to climate change (36, 40).