Over the past few years, many cities have released climate change or climate adaptation plans which include targets for tree canopy cover or Urban canopy cover to mitigate the heat island effect. These targets are admirable; however, cities will face several challenges in reaching these targets due to various local and regional economic and social pressures.
Urban canopy cover or tree canopy cover is defined as the area or proportion of the ground covered by healthy tree crown, including leaves, branches and stems, when viewed from above (Doick et al., 2017; Hilbert et al., 2019a). Urban canopy cover is widely considered to be an environmental good or amenity, associated with the provision of many ecosystem services (Schwarz et al., 2015). These benefits include local cooling, flood mitigation, air pollutant reduction and increases to local biodiversity and human wellbeing (Kaspar et al., 2017) [1]
There is increasing tension between government organisations, think tanks/lobby groups, community groups and the general population over the climate crisis, cost of living, affordable housing and infill development. These are the most pressing issues that cities face, with an increasing need to solve them; however, it is becoming more challenging to solve one problem, let alone all these problems.
This op-ed intends not to address whether Tree Canopy Targets are a valid measure to mitigate climate change but a discussion around cities’ current issues to achieve these targets.
Growth Boundaries and Affordable Housing
Developers (and some politicians) are pressuring state(provincial) and municipal governments to release more land to increase affordability. However, these tracts of land are often within green corridors and the fringes of city urban growth boundaries. Developers are also lobbying to change planning controls (heights, colours, green space, lot size) as their costs increase and profit margins are squeezed.
These simultaneous pressures have a double impact, including reducing the tree canopy cover due to land clearing and reducing the possibility of land remaining or being allocated as public open space, thus reducing the chances of reaching the tree canopy cover target. The tree canopy targets often call for a minimum target for new growth areas, which are hard to achieve due to the smaller lot sizes and wide street profiles.
Infill Housing – reducing urban forest
As many cities are looking to reactive city centres and increase density to provide varying levels of housing affordability, there has been a push to increase infill housing (townhouses, apartments/condos) within the urban core. However, this is coming to the urban forest’s detriment as trees are removed to allow for infill developments. Although these developments include tree planting, they are often not of the same number or large tree species, thus reducing tree canopy cover.
Allowing enough space for trees
Trees need to be provided with enough soil, air, and water for tree canopies to reach their full potential and achieve the desired tree cover. Soil is a critical factor in the success of trees reaching maturity; however, it is often an afterthought with low-quality soil, and inadequate soil volumes only provided for trees. This is often due to either cost or lack of education and understanding by developers, governments and allied professions.
The required offsets for underground utilities and services often reduce the available space for trees, especially in streetscapes, due to trees not being valued by other professionals who see trees as a problem rather than a benefit.
Planning Regulations
In some cities, planning regulations (zoning ordinances) allow (promoting) residents to clear trees within bush/wildfire zones to ensure that they can defend their homes; however, this can lead to increased tree clearing, again reducing the tree canopy cover.
Perceived Visual Interference
When working with architects and retailers there, it is often challenging for landscape architects to convince either party that trees bring value. By planting trees, the benefits (cooling, green, visual, etc.) will outweigh the possible visual interference of the architecture or commercial signage.
Poor tree planting and maintenance practices
Trees are often planted and maintained by low-skill labour due to public and private procurement processes that usually look for the lowest-cost service rather than long term benefits. Thus many trees are often poorly planted or maintained (if at all), thus shortening the tree’s life.
Harsh Environments
Trees live in harsh and hostile urban environments due to vandalism, impervious surfaces and variable seasonal temperatures, including heat from buildings or utility lines. In cold climates (north of the Northern Hemisphere), a tree’s life is often shortened due to snow blowers and salt used to maintain streets and walking areas(sidewalks, footpaths) in cities.
Simplistic Approach
Often to reach these targets, cities can fall into the trap of taking the simplistic approach by planting large areas of land (parks, rivers, easements, etc.) to reach tree canopy targets. This reduces the heat island effect in the localised area but not across the whole city, thus lessening the impact of getting the tree canopy cover. There are still vast areas of the city that could have a small percentage of tree canopy cover (industrial areas, social housing, easements, new areas) but the city could still reach the target with concentrated areas of tree canopy cover. Tree canopy cover is more beneficial in reducing ambient temperatures when over impervious surfaces such as roads, plazas[2].
Lack of biodiversity
Often cities have the initiative to reach tree canopy targets through tree planting programs. However, these can sometimes be a lost opportunity for increasing biodiversity as they select a hardy species that is easy to propagate or easy to purchase from nurseries. Tree canopy targets are a great way to mitigate the heat island effect but also an opportunity to increase biodiversity.
City Budgets
Often cities don’t have the budgets for tree planting programs and often have tree budgets cut as they an everyday part of the city and often forgotten.
Lack of education
Tree canopy targets are a key initiative and objective for mitigating climate change; however, if there is no stakeholder education (public, professionals, developers, government), the associated tree planting programs can fail due to lack of understanding, inadequate funding, vandalism, and removal.
How can we help cities reach their targets?
After reading the above points, you are probably disheartened by the various challenges that cities face to reach their targets; however, as landscape architects, we can educate, design and improve the success of tree canopy targets and the associated programs. Whenever we work on projects(greenfield housing, infill, parks, education, etc.) we need to take every opportunity to assist cities in reaching their targets whilst increasing biodiversity and improving the quality of life for city residents & visitors.
Landscape architects can assist cities in reaching tree canopy cover targets by undertaking the following:
- Educating clients and stakeholders about the benefits of tree canopy cover in our presentations.
- Selecting trees (species, size, form, tree provenance, etc) suitable for the site.
- Create, support and promote the tree planting programs.
- Seek out city and community groups who are planting trees and get involved as part of company outreach/community programs.
- Write articles about the benefits of trees (including tree canopy cover)
There are numerous ways that we can assist in cities reaching their tree canopy targets; it all starts with every project that we work on and how we approach trees (soil, water, species selection, maintenance).
Written by Damian Holmes, Founder & Editor of World Landscape Architecture (WLA)
[1] Achieving tree canopy cover targets: A case study of Bristol, UK, Urban Forestry & Urban Greening, Volume 65, 2021 Max Walters, Danielle Sinnett,
https://doi.org/10.1016/j.ufug.2021.127296
[2] Scale-dependent interactions between tree canopy cover and impervious surfaces reduce daytime urban heat during summer Carly D. Ziter, Eric J. Pedersen, Christopher J. Kucharik, and Monica G. Turner https://doi.org/10.1073/pnas.1817561116