Tree Selection and Planting

Appropriate tree species selection should be based on environmental, ecological, social, infrastructural, and tree maintenance considerations. For example, tree selection should ensure an urban forest that is resilient and that can endure challenging urban environmental conditions, including air and soil pollution, soil compaction, drought, and road salt. Tree selection and planting are also site-specific and should consider soil properties, moisture availability, wind, frost, and light exposure (Nowak et al., 2010). Species-specific characteristics such as tree growth rate, shade tolerance, and visual appeal also play a critical role in species selection. In addition, considering maintenance needs is critical as this can determine long-term sustainability of urban forests and reduce management costs (Almas & Conway, 2016; LSRCA, 2018; Nowak, 2000; Conway & Vander Vecht, 2015). 

Tree Selection and Sites

Tree species selection should be based on site conditions, location, species compatibility with local climate and species hardiness zone, tree size, form, and aesthetic/public appeal. Other factors to consider are tree growth rate, environmental suitability (soil characteristics, pH, moisture regimes, salt tolerance), and the tree’s function within the space (shade provision, aesthetic, etc.). Well-considered criteria for tree species selection can result in secondary benefits for the larger urban forestry program. Some of these benefits include reducing long-term maintenance costs, building an urban forest resilient to climate change, providing manageable urban forest solutions to neighbourhoods and communities, and maintaining a diversity of tree species that benefit wildlife in cities and towns (Almas & Conway, 2016). 

In the design and planning stage of tree selection, trees should be visualized at full functional size. This helps to minimize later conflicts between trees and structures and reduces maintenance needs that may arise from unexpected issues. Selecting a tree that is well-suited to the soil conditions, light availability, pedestrian traffic, drainage, space, and microclimate of the specific site is essential to the tree’s long-term survival and to public safety (Vibrant Cities Lab, n.d.). 

To make the decision process easier, Canadian municipalities often list species that can thrive in their specific urban context based on the tree selection considerations and their experience (see City of Markham, 2009; City of Kelowna, 2020; City of Toronto, 2021, 2024; Metro Vancouver, 2017; Ville de Quebec, n.d.). 

Climate Change 

When selecting tree species, their long-term climatic suitability is increasingly important for successful planting decisions. As climate change brings warmer weather to many Canadian cities, individual species’ suitability is expected to change over time (Khan & Conway, 2020). The composition of urban forests needs to adapt and shift in anticipation of this trend. Suitable climate conditions for many tree species are shifting northward much faster than trees can naturally migrate (Metro Vancouver, 2017). As a result, tree species selection must be based on forethought and consideration of changing climate regimes, rising temperatures, and levels of tolerance that individual tree species have for rising temperatures and shifting conditions.  

To determine a species’ suitability in the context of climate change, a combination of historical range, current suitable habitat, and climate projection models can be used as a basis for analysis (LSRCA, 2018). This process helps determine if a species is retreating northward, continuing to persist, or becoming a new suitable possibility. Trees and other species that are becoming increasingly unsuited to their location may experience stunted growth and shorter lifespans. Species that persist in urban settings tend to have a wider ecological amplitude and climatic range and are likely to continue to thrive in the future (Das, Ossola & Beaumont, 2024; Liang & Huang, 2023).

For species that may become more suitable as climate change progresses, testing their survivability in the new environment before widespread implementation by planting limited amounts can prevent large-scale tree mortality in the future (LSRCA, 2018). These assessments should be updated as more data for different species and new climatic models become available. Flexibility in modifying planting programs can help mitigate the risk of implementing new species.  

Since urban forest planning is a long-term vision and young trees take years to grow, active engagement with nurseries can ensure that sufficient planting stock of specific species is available. Coordination with nurseries can also lead to the sharing of knowledge, which can be a hugely beneficial source of information in implementing new planting programs or testing of new tree species (Khan & Conway, 2020; LSRCA, 2018).

Species Diversity and Composition

The diversity of tree species is critical in building and maintaining a resilient urban forest. Having a large variety of tree species allows an urban forest to better absorb shocks brought by pests, diseases, and climate change. For instance, the spread of Emerald Ash Borer put around 20% of Montreal’s urban forest at risk, and thousands of trees were cut down between 1999-2020 (Canadian Institute for Climate Choices, 2021). Many of the trees that were impacted were mature with wide-spreading canopies, which provided various benefits like climate regulation, air quality, and stormwater management benefits that a newly planted sapling could not immediately replace. Ash trees across North American cities were impacted at similar or worse rates. This highlights the importance of supporting tree diversity to strengthen urban forest resilience to a changing climate and environmental threats.

In addition to considering urban forest diversity, tree species that are dioecious (male and female trees), tree species sex and overall tree diversity should be considered when selecting trees to plant. Historically, male trees have been preferably planted in urban settings as they are considered less “messy” and do not produce fruits or seeds (Nowak & Ogren, 2021). While female plants produce fruits and nuts and provide food for birds and other urban wildlife, they require higher maintenance and upkeep. As a result, they have not been favoured for planting to avoid fruit cleanup in high human traffic areas. Studies have shown how the planting of mostly male trees across cities in North America has led to increased urban pollen concentrations, which has been correlated with increased seasonal allergies. Favouring male trees has a long-term impact on urban forest and tree species composition, as well as resilience against pests and overall city biodiversity. It also contributes to perceived tree disservices as overproduction of pollen can impact human health (Katz, Robinson, Ellis & Nowak, 2024).

Furthermore, urban trees planted along streets, parks, in residential and institutional areas often include many cultivars selected for their specific traits, such as growth rate, canopy shape, leaf and flower colour, the absence of fruit, visual appeal, or other features. The predictability of their growth and other characteristics, and how these trees eventually grow, make these cultivars desirable for some urban environments and landscaping. There are hundreds of cultivars, of which some have been planted more than others. However, cultivars are clones that share identical genetic material, reduce genetic diversity, and increase vulnerability to pests, diseases, and environmental stressors. Heavily relying on cultivars can lead to portions of the urban forest becoming monocultures, but also having monocultures across cities (Avolio, 2023; Sacre, 2020; Lohr, Kendal & Dobbs, 2016).

Tree Planting

Tree planting material should be inspected for damage and disease before being purchased from nurseries to ensure successful tree establishment and growth. Some common issues of tree planting materials are damage to the trunk, broken branches, or injuries to the root system (International Society of Arboriculture, 2021). The three common types of nursery trees are ball and burlap, container, and bare root stock. Each has its advantages and limitations when planting and should be selected based on the needs of the site or project (International Society of Arboriculture, 2021; Natural Resources Canada, 2023). The Arbor Day Foundation has a useful guide on how to plant each type of nursery tree. Planting under specific circumstances may have different requirements, such as when planting under power lines or for hard-surface planting in intensely urbanized areas. 

Depending on how trees are packaged and sold, their planting methods differ. Young trees often come in containers, as ball and burlap, or as bare root trees. One of the main concerns for trees that come in containers is root girdling. Root balls should be loosened on the bottom and sides to ensure no roots grow to choke the plant (International Society of Arboriculture, 2021a; Tree Canada, 2023). For trees that come in ball and burlap, it is important to cut away the burlap and wire basket, or the packaging will slowly strangle the tree as it grows (Tree Canada, 2023). For bare root stock, the most important thing is to ensure that the root does not dry out prior to and during the planting process (Virginia Department of Forestry, n.d.). The ultimate goal of urban tree planting is for the trees to reach maturity and provide maximum benefits, such as carbon sequestration, shade, and biodiversity. Therefore, monitoring and maintaining young trees so that they continue growing and thriving is an imperative part of urban forest management.