We could discuss this topic to the ends of the earth, outlining both positives and negatives of tree presence within urban environments, though there is a mounting pile of evidence to suggest that trees really do have a (largely) beneficial impact upon humans in terms of health and well-being. However, most studies have looked at trees on a macro-environmental scale (such as exposure to general green spaces), and not looked at the linkages between individual trees and the impacts such trees have upon residents in the close locale. Therefore, the authors of this study sought to change this, by investigating the health benefits of trees to a “low granularity”, and instead of assessing such ‘greenery’ from the ground, they related their study to tree canopy sizes only – it was considered (by the authors) that tree canopy is the most important of all the ‘green’ available within urban areas. Thus, the study was planned, and the city of Toronto, Canada, was chosen as the location for the project.
The authors particularly sought to assess the relationship between street tree canopy density and its impacts upon: (1) overall health ‘perception’ (this was chosen as subjective perceptions of personal health has been found to influence mortality rates, and is considered a strong indicator of actual health of an individual); (2) cardio-metabolic conditions (including high blood pressure, obesity, high blood glucose, high cholesterol, and diabetes), and; (3) mental health problems (depression, anxiety, and addiction). Similarly, the benefits of trees in parks was also assessed, enabling for comparisons to be drawn between trees in differing locations.
In terms of the results found, I have segmented the three aforementioned categories into different sections. The manner in which the results section was written was rather disjointed by tables and seemingly tangential (but still contextually beneficial) points, though I pulled out the most important parts and included them below.
It was found that people who live on streets where there are more trees, and / or trees with larger canopies, report better health perceptions (after accounting for demographic factors: age, income, etc) – for every 400 square centimetre increase in ‘treed area’ per one metre squared of neighbourhood area, it was shown that health perceptions would rise by around 1% (around an extra 10 trees per block, of which there are 25 blocks per district area). In fact, this incremental increase in health perception as a result of there being more trees, or larger trees, was on par with an income increase of $10,200 per year per household, or an individual being 7 years younger. However, the authors note that trees alone do not majorly influence health perception (they only do so by a few percent) – there are many others factors that will impact upon an individual’s view of themselves.
Where trees are present within streets, the authors found that fewer cardio-metabolic issues were present in individuals. For every 11 trees within a city block, the occurrence of such health issues fell by around 0.5%, and is as impactful as an average increase of $20,200 in household income per year, or being 1.4 years younger. Again, tree presence isn’t the sole driver behind better cardio-metabolic health, though certainly plays a role (albeit, perhaps a rather small one – a few percent).
The authors found no statistical significance of tree presence upon the mental health of individuals, so will be undertaking further investigations to further analyse the relationship.
In light of the survey data, the authors conclude that an increase in tree presence along urban streets will have a beneficial impact upon an individual’s health perception, and to their cardio-metabolic health. With regards to their comparisons to annual income increases, the authors make an interesting point in that a household earning $10,200 more than another household, whilst on paper would be ‘better off’ in terms of health perception, may not actually be so – if the other family has over 10 additional trees in their block compared to the block where the higher income household is, the lower-income household may perceive themselves as healthier than the higher income household.
Therefore, as Toronto’s street tree density is on average between 0.2%-20.5% (including parks, it is around 28%), if tree planting rates increase (by as little as 10 more per block) – and canopy cover subsequently increases – then the health of residents will improve. However, the authors do note that Canada has a nationalised healthcare system, and therefore other countries where no such healthcare programme exists then the average income of a household may hold more weighting. This means that an increase in 10 trees per block in another country may equate to less of an increase in comparison to household income increase – 10 more trees may only equate to a $4,000 increase in household income, for example.
Curiously, the improved health of residents did not rank so significantly for trees within green spaces. This means, so the authors hypothesise, that it is the trees that are situated directly within the locality of a household (and within view) that will have the most beneficial impact on health, though the authors recognise that the other benefits of the street trees (such as reducing air pollution) will play a role. However, parks are still beneficial – that is not being disputed. The results simply underline the necessity for street trees – we cannot rely on parks exclusively to bolster the health of a populace.
It is also suggested that once tree density becomes significant (perhaps once over 20%) that the health benefits of trees becomes less significant (though are still evident). Additionally, an area with many evergreen trees does not mean residents who may view them have a better perception of their health – a great variety of trees, both deciduous and coniferous, are required to improve health perceptions of individuals impacted by their presence. However, residents living within areas occupied by many evergreen trees do report a better cardio-metabolic status.
Source: Kardan, O., Gozdyra, P., Misic, B., Moola, F., Palmer, L., Paus, T., & Berman, M. (2015) Neighborhood greenspace and health in a large urban center. Scientific Reports. 5 (11610). p1-14.
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