Greening the city - the role of urban trees

The potential of the 3-30-300 principle, a tree-based metric for assessing nature in cities

In my previous post in Policies for Places I discussed some of the issues which arise for cities in delivering climate change adaptation. I pointed to a range of possibilities for cities to help mitigate the effects of severe weather events and other climate-change related pressures from increased heat, air-quality and environmental pollution. These could include engineering-based solutions for flood prevention and protection from rising sea-levels, urban design features such as the promotion of walkability and active travel, and a range of nature-based solutions including accessible green spaces and water management schemes.

Here, I want to focus on the role of trees, now often a central feature in the urban climate adaptation process. I will explore the contributions of trees with respect to urban climate-change adaptation and also some issues about the spatial distribution of the benefits of tree cover and the potential inequalities which arise for city residents.

The role of trees

It has long been recognized that green space is vital for the promotion of public health, for encouraging social inclusion, reducing inequalities and fostering urban resilience. Spending time in parks, forests, or near bodies of water has been shown to help reduce stress, improve mental health, and encourage physical activity. It is desirable for the population to have access to spaces that allow physical activity and access to the outdoors in their living environment, close to home. However, it is clear that this access is not available for all individuals: inequalities in access to green spaces has the potential to re-enforcing social and spatial inequalities across the city.

Trees, which are often (but not always) prominent features of green spaces, have an important contribution to the livability and quality of life in urban areas. Their contributions can be broadly categorized into public health, environmental, and social benefits.

Public health benefits

· Stress Reduction: Trees and green spaces provide a natural setting that helps reduce stress and improve mental health.

· Physical Activity: Easy access to parks and tree-lined streets encourages physical activities such as walking, jogging, and cycling.

· Improved Air Quality: Trees act as natural air filters, absorbing pollutants and releasing oxygen, thus improving the air quality.

Environmental benefits

· Climate Change Mitigation: Urban trees help in reducing land surface temperatures, thereby combating the urban heat island effect.

· Biodiversity: Trees provide habitats for various species, thereby promoting urban biodiversity.

· Water Management: Trees assist in water management by reducing runoff and improving groundwater recharge.

Social benefits

· Social Inclusion: Green spaces foster community interactions and social inclusion, thereby reducing social inequalities.

· Aesthetic Value: Trees enhance the visual appeal of urban landscapes, making cities more attractive places to live.

· Property Value: Proximity to well-maintained green spaces and tree-lined streets can increase property values.

Assessing nature in urban environments: a tree-based metric

These roles underscore the importance of integrating trees into urban planning and development strategies. There is a need for a convenient tool for the assessment of the ‘greenness’ of cities. One such is the 3-30-300 concept which was first proposed by Cecil Konijnendijk in 2021and is now widely used. The index is based on 3 criteria which can be related to both the public health and the climate adaptation function of trees. These are:

- Every person should be able to see 3 trees from the windows of their home;

- Neighbourhoods should have a tree canopy of at least 30%: and

- Parks and green spaces should be no more than 300 meters from every house.

There is a rationale and an evidence base behind each of these criteria. Put at its most simple, the ‘3 trees’ criterion is a practical way of ensuring that there is enough nature around to generate the benefits to health and well-being which have been associated with the visibility of green space. The 30% criterion relates to research which shows that sufficient tree cover is essential to ensure the measurable benefits to air quality, temperature regulation and biodiversity in cities associated with climate change. The 300 meters criterion comes from planning research which shows that close proximity to parks or green space directly influences the frequency and duration of physical exercise adding to the positive effect on physical and mental well-being.

These metrics seek to ensure that everyone has easy visual and physical access to trees and green space. Since its launch the rule has been adopted by numerous international organization, cities, businesses, third sector bodies and citizen groups around the world.

Measuring and monitoring these metrics

Work continues to examine the applicability of the 3-30-300 metrics. Different methods have been developed for measuring and monitoring the three components of the rule. By way of example, a study by looks at its applicability in 5 medium-sized Polish municipalities. The study demonstrates the complexity of measurement, especially of the visibility criterion, but that all variables in the rule can be calculated based on publicly available data.

Another recently published study looks at the 3-30-300 rule for to evaluate green space accessibility and inequalities in Montreal. Using geospatial analysis, the study examines how well these criteria are met across Montreal’s neighbourhoods and investigates disparities linked to socio-economic factors. The study shows the wide variability in the distribution of green spaces across Montreal neighbourhoods, as measured by the 3-30-300 metric. Tree canopy coverage ranges from 0.8% to 84%, with a median of 25.7%, while distances to parks vary from adjacent to over 2.4 km. The number of trees around residences is highly skewed, ranging from 0 to 771. Spatial analysis highlights pronounced inequalities, with less than 20% of neighbourhoods meeting all three criteria. Hotspots of compliance are concentrated in peri-central and well-established residential areas in the West and East of the city, while central and peripheral neighbourhoods, especially in northeast Montreal, frequently fail to meet the standards. These findings demonstrate strong spatial disparities in urban green infrastructure, and inequitable access to green spaces.

What difference do trees make?

Reference has been made earlier to the evidence-base which is used to provide the rationale for the 3-30-300 rule, which mainly relates to the health and well-being benefits derived from the visibility of trees and accessibility of green spaces and woodlands, and we also have some studies which examine the distribution of trees across cities and highlight the spatial inequalities of green space within city landscapes.

More evidence is also becoming available on the effectiveness of trees in relation to the mitigation of climate change generally and more particularly on their effectiveness of mitigating urban heat in different climatic contexts and in comparison to treeless urban green spaces and in relation to rain capture.

On the first of these, Schwaab et al used high-resolution satellite land surface temperatures (LSTs) and land-cover data from 293 European cities to infer the potential of urban trees to reduce LSTs. They show that urban trees exhibit lower temperatures than urban fabric across most European cities in summer and during hot extremes. Compared to continuous urban fabric, LSTs observed for urban trees are on average 0-4 K lower in Southern European regions and 8-12 K lower in Central Europe. Treeless urban green spaces are overall less effective in reducing LSTs, and their cooling effect is approximately 2-4 times lower than the cooling induced by urban trees.

On the second, Pace et al applied a simulation model to the German city of Karlsruhe and its 27 districts with varying conditions of tree cover to analyze the potential for both heat mitigation and water run-off during dry and wet periods over a 5 year period. After analysing initial tree cover and drainage conditions, the model was used to simulate a green infrastructure scenario for each district with restored hydrology and tree cover at 30%. There are trade-offs between runoff and heat mitigation; the results confirm that dry soils before storm events lead to greater runoff reduction by 10%, and wet soils prior to heatwaves resulted in a greater evaporative cooling. Compared to current conditions, the green infrastructure scenarios resulted in decreasing the number of extreme heat hours (Heat Index > 31 °C) per year on average by 64.5%, and to reduce runoff in average by 58% across all city districts. Thus, the simulation results show that investing into a greener infrastructure, has positive impacts on microclimate and hydrology.

Utility of the rule

Climate change is increasing the frequency and intensity of urban heat islands and stormwater flooding. Climate adaptation strategies in cities are increasingly looking at green infrastructure including trees and urban forests to mitigate these threats. Strategically designed green infrastructure can mitigate runoff volume through tree canopies and redirect run-off from impervious surfaces. In addition, urban greens mitigate extreme heat via evapotranspiration and shade.

The studies referred to here have all made use of the 3-30-300 principle to a greater or lesser extent. The studies indicate that the principle is effective in assessing the distribution and accessibility of green space. Furthermore the application of the principle has revealed continental-scale patterns in the effect of trees and treeless spaces on urban land surface temperatures and has provided the basis for simulation of the effects of green infrastructure on mitigation of urban heat and flooding.

While the rule can serve as a foundation for future urban planning, it is not sufficient on its own. The studies point to the need for improved creation and management of urban green space to ensure benefits are shared equitably, and for complementary strategies to ensure long-term sustainability and access to green space.

It should be remembered too that the studies referred to here have been confined to particular places and urban conditions. There is plenty of scope for further investigation of the climate dependent effectiveness of such heat mitigation measures across the globe.