City life isn't always easy, but trees are proven to have surprising effects on the physical and psychological well-being of urban populations. Scientific research outlined in this article continues to prove overwhelming evidence of the numerous health benefits and advantages that mature trees offer. In this article, we discuss the ways that urban trees improve human health, and techniques that landscape architects and other related professionals can use to increase our urban tree populations.
The fact that trees can improve the quality of life for city residents and make a positive contribution to large-scale planning and infrastructure requirements in now beyond question. In fact, the establishment of healthy urban tree populations is quickly becoming a central component in strategies set out by urban planners and local authorities, due to the many proven urban tree health benefits - some of which we cover here.
1 – Carbon Reduction
One area of international concern in which trees are already playing a vital role to control is the creation of carbon offsets and the regulation of greenhouse gas emissions. Planting trees remains one of the most cost-effective means of drawing excess CO2 from the atmosphere, in light of global efforts to reduce climate change. A single tree can absorb as much as 48 lbs of carbon dioxide per year and can sequester 1 ton of carbon dioxide by the time it reaches 40 years old (Department of Horticultural Science, North Carolina State University). Although our tree populations currently absorb approximately 2% of total emissions, this could potentially be a lot more.
2 – Oxygen Creation
A single mature tree can release enough oxygen back into the atmosphere to support 2 human beings (Mike McAliney, 1993). Research showing that over a 50-year lifespan, a tree generates almost $32,000 worth of oxygen and provides $62,000 worth of air pollution control (Urban Forestry Network).
3 – Pollutant Removal
As one of their most important impacts, trees are capable of improving air quality by removing and storing a surprising amount of harmful pollutants such as:
- Sulphur Dioxide
- Nitrogen Oxides & Particulates
- Carbon Monoxide
- Cadmium
- Nickel
- Lead
4 – Youth Development
A study by Andrea Faber Taylor and Frances Kuo from the University of Illinois suggests that trees and other greenspace can have a therapeutic effect for children suffering from Attention Deficit & Hyperactivity Disorder (ADHD), improving both their attention levels and social functioning. The same research also showed that students with regular access the trees and greenspace performed better in tests.
5 – Illness Moderation
Due to the removal of harmful chemicals from our local ecosystems, trees decrease smog formation and have been shown to help reduce incidences of skin cancer, asthma, and stress-related hypertension illnesses (Tree Design Action Group).
A past study performed in the Netherlands indicates that every 10% increase in greenspace can postpone health complaints in communities by five years, while a separate US study suggests that hospital patients with a view of nature through hospital windows recover better after surgery. This type of data has prompted health authorities around the world to increase tree plantings on hospital properties.
6 – Cardio-Metabolic Health Improvement
While many studies over the years have revealed that exposure to greenspace can improve mental health, reduce blood pressure and stress levels, and increase physical activity; few studies have quantified the impacts that individual trees have on human health.
A new joint research project piloted by the Institute for Clinical Evaluative Sciences, the University of Chicago, and the Rotman Research Institute (amongst others), was conducted earlier this year in Toronto, Ontario. The study probed into the health benefits affiliated with street trees based on the assumption that trees are the most consistent green components in urban areas. The study focused on greenspace consisting of tree canopy only and not of bushes, grassland, or other "natural" settings. The research also went as far as to distinguish between trees along roads and streets versus those in private yards, parks, and other open areas; in hopes that such as a distinction would be helpful for future urban planning policies. The researchers hypothesized that street trees could have stronger beneficial associations with an individual's health than park trees and other greenspace because they are usually more accessible to all residents in a given neighborhood or inner-city location, as people are exposed to street trees in their daily activities and through views from windows when indoors.
7 – Health Perception
Released earlier this year, the study suggests that people who live in neighborhoods with a higher density of trees on their streets report significantly higher health perception and considerably less cardio-metabolic conditions, even when controlling for socio-economic and demographic factors such as age and income. This analysis shows that having an average of 10 more trees in a city block improves health perception in ways comparable to an increase in annual personal income of $10,000 and moving to a neighborhood with $10,000 higher median income, or being 7 years younger.
Results from multiple regressions and multivariate canonical correlation analyses indicate that people who live in areas that have more (and/or larger) trees on the streets have significantly fewer cardio-metabolic conditions, having recorded decreases of 0.04 units of cardio-metabolic conditions for every increase of 408 cm2 / m2 in tree density.
The study continues to annotate that 10 more trees in every block is about a 4% increase in street tree density in a dissemination area in Toronto. With most Toronto areas having between a 0.2% to 20.5% range of street tree density, this increase seems to be logistically feasible. According to the research findings, the resulting improvement in health perception and decrease of cardio-metabolic conditions by planting 10 more trees per city block is equivalent to increasing the income of every household in that city block by more than $10,000 – which far outweighs the cost of planting the additional 10 trees.
Additional Urban Tree Plantings Needed
The Toronto research project went on to explain that additional street tree plantings to reach the defined canopy density can be incorporated into various areas along roads in both residential suburban areas and downtown streets. The difficulty with street tree planting of course, is the fundamental conflict between maximizing uncompacted soil volume for root growth vs providing structural integrity required for roads and sidewalks.
Tree growth and fertility are strongly influenced by soil structure, as it affects the movement of air, water, and nutrients required for trees to flourish. Structural soil that has been compacted for load-bearing stability is resistant to root penetration and significantly reduces root growth. This stunts the maturity of a tree and in many cases leads to death of the tree.
A well-constructed soil functions like a reservoir, enabling a tree to accept, store and transmit water, nutrients, and energy; while providing room for roots to propagate. Good uncompacted soil allows the space required for life and the necessary biochemical exchanges for growth.
There is a way however to maintain the structural integrity of paved surfaces without compromising the soil requirements that urban trees need to thrive. Structural support modules (or soil cells) are modular units that assemble to form a skeletal matrix situated below paved surfaces. They support pavement loads, including vehicular traffic, while providing a large volume of uncompacted soil within the matrix for healthy tree root growth.
