Factsheet: The role of tree diversity in strengthening urban forests

Are you looking for ways to make your urban forest more resilient? This factsheet explains what tree diversity means in the context of an urban forest and how tree diversity contributes to both climate resilience and urban biodiversity. It highlights approaches municipal staff, including urban foresters and other sustainability professionals can take to make their urban forests more resilient and biodiverse through tree planting and protection. 

The importance of tree species diversity   

Integrating diverse tree species into urban forest management is an important step towards creating a more resilient urban forest. Urban forests and the many benefits they provide are threatened by pests, disease and extreme weather. With extreme weather events like storms, floods and droughts on the rise, increasing tree diversity through strategic planting and management is vital for making urban forests more resilient. 

Tree species diversity measures the richness and evenness of tree species within an urban forest. Species richness refers to the total number of different tree species present, while species evenness indicates how evenly those species are represented. High species diversity is characterized by a variety of tree species represented evenly. 

At the scale of an entire municipality, many Canadian urban forests are species-rich. This is due to the variety of trees planted in private gardens, native trees that exist in patches of remnant or restored ecosystems, and publicly planted trees on city streets and in parks. However, at both the scale of an entire municipality and at a smaller scale (e.g., a street or city block), the species evenness of most urban forests is poor. This leads to urban forests with low diversity and lower resilience to environmental stressors, like pests and extreme weather events. 

How urban forests with high tree diversity are more resilient to pests and diseases

Major pests and disease outbreaks like Dutch elm disease and emerald ash borer infestations have decimated urban populations of American elm and green ash trees, showing the importance of species diversity in urban forests. At their peaks, these trees made up a large portion of the urban forest in cities and towns across North America. In many cases, they were planted unevenly, making up more than half the trees planted on city streets and in parks. The loss of these dominant species revealed urban forest vulnerabilities that resulted in substantial canopy loss.

Municipalities can avoid this type of devastating canopy loss in the future by planting tree species with an eye to increasing diversity. 

How species diversity contributes to climate resilience

Urban forests can “climate-proof” a city in various ways, including cooling streets during heatwaves and mitigating the effects of flooding.

Diverse urban forests are even more effective at climate-proofing, exhibiting greater resilience to extreme weather events like droughts and floods. A diverse urban forest is more resilient to drought, for example, because different tree species have varying levels of drought tolerance. Drought-tolerant species continue to grow normally, preserving the overall health of the urban forest as other, less tolerant species struggle during a drought.

As trees have long lives, consider selecting species that can thrive in future climatic conditions in your community. Select both native and non-native species adaptable to a wide range of hardiness zones, or those found at the southern edge of your municipality’s hardiness zone. These trees will be better equipped to handle extreme weather and rising temperatures. 

Cities like Vancouver, British Columbia have created urban tree lists that take climate resilience into account, providing a model for other municipalities that want to select suitable tree species.

How tree species diversity supports biodiversity

Diverse tree species also offer varied habitats and food sources for wildlife. For instance, eastern white cedars provide cover for small birds, while white oaks produce acorns for birds and mammals to eat. Increasing tree diversity in urban forests supports wildlife, which is crucial in urban ecosystems where food and habitat are often scarce. While native species typically offer better habitat for local wildlife, carefully chosen non-native species can also play a role, particularly in challenging urban conditions. Non-invasive, non-native trees can provide temporary corridors for wildlife, facilitating movement between higher-quality habitats. 

How your municipality can promote and advance species diversity 

1. Conduct tree and plantable space inventories: A tree inventory provides insights into species diversity within an urban forest. An urban forest’s vulnerabilities cannot be reduced if its species diversity is not known. Inventorying the trees located in publicly owned streets and parks is the best way to understand the tree diversity in your municipality.  

   Doing an inventory of spaces where trees could be planted will identify available locations for new plantings of diverse species in your municipality. Planting in these locations not only supports a more resilient urban forest but also promotes its growth. Tree and plantable space inventories can be conducted using software like i-Tree or geographic information systems (GIS).
    

2. Make enhancing tree diversity your goal when selecting species to plant: Based on your tree inventory, create a plan to prioritize (and de-prioritize) certain tree species for planting. With inventory data in hand, your municipality can develop a planting plan that will start to balance out species diversity. For example, your municipality may choose only to plant tree species that make up less than 10 percent of its urban forest. Whenever possible, prioritize native and larger trees for their ecological benefits, while considering non-native, non-invasive species in constrained urban environments.

   One guiding principle for minimizing urban forest vulnerabilities is the 10-20-30 rule. It suggests that there should be no more than 10 percent of a single species, 20 percent of a single genus and 30 percent of a single family in an urban tree population. Although challenging to implement, this guideline serves as a benchmark for enhancing species diversity and increasing urban forest resilience. 
    

3. Conserve existing tree diversity in your community: Maintaining and protecting existing trees, especially those that are large and mature, is important to promote species diversity and urban forest resiliency.

   Larger and more mature trees provide greater ecosystem services and biodiversity support than newly planted trees. Individually, they are also more resilient to extreme weather or biotic threats and have much lower mortality rates than newly planted trees. Large and mature trees need to be managed for long-term urban forest resilience and diversity goals to be met. They should be maintained and protected to support urban forest health and growth. 
    

4. Engage with the community to promote diverse tree planting and conservation: Education and engagement are great ways to connect residents with nature and raise awareness of urban forest management practices. Share the results of your inventories with residents and explain the importance of tree diversity and tree protection in creating a more biodiverse and resilient urban forest.  

   Connecting with residents can also lead to new opportunities for planting as homeowners and other landowners may have available plantable space on their properties. In many municipalities, tree diversity and ecosystem services can peak in residential neighbourhoods because these locations often have the space available to plant medium and large trees.  

   Consider creating lists of preferred species to plant based on tree diversity and available space. Share these lists publicly to guide residents and landowners in species selection and to advance municipal tree planting goals.  

Next steps

Explore how other communities have integrated tree species diversity into their urban forestry plans and tree planting strategies: 

• Vancouver Urban Forest Strategy – Learn how the City of Vancouver has integrated tree diversity data into its future planting goals (see pages 33–35).
• Halifax Urban Forest Management Plan (draft) – Explore the species and structural diversity of trees within the City of Halifax and see its plans to improve diversity. 
• Winnipeg Urban Forest Strategy – Review the City of Winnipeg’s public tree diversity plan, including its recommended species list and information on loss of elm and ash trees from pest and disease in the city.  
• Camden Tree Planting Strategy – Learn about the London Borough of Camden’s planting strategy, which includes information on how it will increase the quality and diversity of trees to support urban forest resilience.
• Urban Tree Diversity for Sustainable Cities – Review this policy brief by Nordic Forest Research summarizing the benefits that come with diverse urban tree populations and recommended actions to improve species diversity.  
• Strategic guide to increase canopy cover and urban forest resilience (in French only) – Review this guide that explores how to use a functional diversity approach to increase urban forest resilience. 

This resource was created in partnership by Tree Canada and FCM’s Green Municipal Fund for the Growing Canada’s Community Canopies initiative, which is delivered by the Federation of Canadian Municipalities and funded by the Government of Canada.