Urban Trees and Forests

Image credits: Gigi Ling, Unsplash

An urban forest is a collection of trees growing within a city. Urban forests can range from remnant native forest patches to trees planted throughout cities in streets, highways, and backyards.

Biodiversity role:

Trees provide vertical structure, nesting resources, cover, and fruit and floral resources. Large native forest patches function as biodiversity hubs, protecting more urban- sensitive species from urban stressors, and even non-native trees can provide structure or food that benefits native wildlife.

Human health role:

Urban forests have several mental and physical health benefits. Some include improved mental health, immune function, and BMI, and lower prevalence of lung cancer and preterm birth. Trees also provide shading and cooling, and high canopy cover can offset the urban heat island effect.

Key tensions and tradeoffs:

Increased tree canopy cover has been associated with increased prevalence of asthma and allergens although results are mixed. It has been linked to gentrification, so strong housing policies should accompany greening. Trees preferred for human benefits may be exotic species that do not provide the same biodiversity benefits as natives.

Protect large trees

Large trees can provide critical support for native wildlife communities, as well as shading and visual interest for people. Parks and larger open spaces offer the most opportunity for large trees.

Shade strategically

Shade heavily used areas, including transit stops and active transportation corridors. Plant deciduous trees along the south and west side of buildings to reduce summer heat.

Evaluate soil quality

Soil quality, including physical composition, nutrient structure, acidity, level of compaction, and porosity also strongly affects the health of urban forests. Match tree selection to the available soil volume and quality to support healthy trees.

Match tree species
selection to the site

Consider desired functions and site characteristics. Use native trees where possible, and target locally appropriate levels of diversity. Evaluate potential disservices, including high pollen production, susceptibility to disease, and maintenance issues.

Limit water needs

In arid regions, weigh the benefits of tree planting against the increased water use. In cities where trees were not present historically, identify near-native and drought-tolerant trees where possible.

Partner with community groups

Urban forest projects may contribute to gentrification of neighborhoods. Studies have found that urban trees are associated with increased property value. To mitigate potential impacts, it is recommended to partner with local nonprofit organizations and government agencies on tree planting efforts.

Encourage forest bathing

Consider adding features that encourage contemplation and calmness, including small paths, quiet spaces, and benches.

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