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Some of Britain’s city centers are full of trees and parks, while others have little vegetation to break up the bricks, asphalt and concrete. These differences are not just aesthetic: they affect whether animals can move and have implications for human health and social justice.
That’s why we wanted to properly assess how green these cities are. In our latest research, now published in the open access journal PLOS ONE, we looked at all 68 boroughs in the UK with at least 100,000 residents. Town centers were defined using the Consumer Data Research Center’s spatial datasets, which use comprehensive statistics to delineate retail boundaries. You can think of borders as similar to “central business districts”. For example, in Sheffield, the city center is the whole area within the central ring. London is a special case; because it is so large, it has several of these areas.
For each of these cities, we assessed three “greenness” metrics: 1) tree cover using an algorithm to randomly sample recent aerial photographs, 2) the presence of green areas using open source data from the Ordnance Survey (UK’s national mapping agency), and 3) the normalized difference vegetation index (NDVI), which uses satellite observations of light absorption and reflection to measure the vegetation cover in a given area.
Exeter greenest, Glasgow least green
By combining all three metrics into a single green score, we found that Exeter city center ranked highest, followed by Islington in London, Bristol, Bournemouth and Cambridge.
Glasgow city center is the least green, with Middlesbrough, Sheffield, Liverpool and Leeds also in the bottom five. Tree cover is probably the most reliable way to describe the differences: trees cover 12% of the total area of Exeter city center but only 2% of Glasgow.
Exeter, a small town in the remote south-west of England, has largely escaped the rapid industrial growth of better-connected cities. This is evident today in its leafy streets and meadows along the River Exe. In contrast, places like Glasgow and Sheffield were massive industrial powerhouses with considerable urban sprawl, although they still have wonderful parks outside the city centres.
North-South Green Divide
It is worth noting that the first five urban centers are all in the south of England, while the last five are former industrial areas in the north or in Scotland. In fact, only 25% of all northern cities in the top half of the table are green. Further analysis revealed a statistical association between lower green scores and higher levels of deprivation, as measured by risk of crime, health, economy, education and other related metrics. Additionally, areas with larger populations had lower tree cover and vegetation index scores.
Sheffield, where the study was conducted, is often considered to have the highest density of trees of any European city. But this is due to the huge swaths of trees in suburban areas and the surrounding fringes of the Peak District National Park. Our work shows that Sheffield actually has the lowest tree cover in the city center of all the cities included in the study.
The reasons for the split to the north side are complex and may depend on decisions made centuries ago and developments since then. It is clear that some urban centers have historically invested more in parks and tree lines than others. Industrial and war efforts then contributed to urban sprawl and reduced natural features in certain urban centres, particularly in northern England and Scotland.
In the 19th century, urban planners often incorporated street trees, especially in affluent areas. These decisions were influenced by the admiration of continental European boulevards and the affluence of “garden cities” and “spa towns”. This is exemplified by the “luxury effect”, where affluent neighborhoods experience higher biodiversity in cities around the world, often dictated by structural classism and racism.
Differences in historical development have therefore left us with urban centers full of greenery, such as Exeter, and others with much less greenery. The question remains why such an imbalance was not resolved over time.
This is an important question because about 70% of the world’s population will soon live in an urban environment. In the UK, 84% of people already do. This rapid increase in urbanization has led to a mismatch between humans and nature, as we often fail to create healthy and biodiverse spaces, especially in city centers.
But there is at least one reason for hope. Town centers in the UK and other areas of the world are changing, especially as digital shopping means many retail stores are closing. This is a problem in many ways, but it is also an opportunity. The decline of personal shopping gives us a chance to rethink and redevelop city centers to improve their green spaces.