Christchurch may not be as green and leafy as you think

Monday, 6 September 2021

The sun shines through autumn trees in the Botanic Gardens in Christchurch.

The Press and Stuff are supporting a campaign for Ōtautahi Christchurch to become recognised as a National Park City. This requires a commitment to protect and enhance nature within our urban area. So just how green is the Garden City? Reporter CHARLIE GATES takes a look.

Have a guess at how much of Christchurch is covered by tree canopy. What percentage of the whole city, excluding Banks Peninsula, is covered by leafy trees? Have you got a number in your head?

OK, I’ll give you some clues. Auckland, where debate has raged about mature trees being felled in their hundreds to make way for new housing, had a canopy coverage of 18.4 per cent between 2016 and 2018.

In Wellington, which doesn’t exactly brag about its leafy status, tree canopy covered about 30.6 per cent of the city and suburbs in 2019.

So, how about the Garden City?

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In 2015 and 2016, tree canopy cover in Christchurch, not including Banks Peninsula, stood at just 15.59 per cent. And, if you exclude large-scale plantation forests like Bottle Lake Forest, that number drops to 11.6 per cent.

An aerial view of Christchurch taken last year show the level of tree canopy coverage in the city centre.

Not a great result. Lower than Melbourne, which has 13 per cent, and far lower than Hobart in Tasmania, which has 59 per cent.

Christchurch also doesn’t compare well to other cities when it comes to parkland. About 16 per cent of land in Christchurch, not including Banks Peninsula, is made up of council-managed green space. That compares to 10 per cent in Melbourne and 46 per cent in Sydney.

But, how much tree canopy coverage should a city ideally have?

Riccarton Bush in west Christchurch is an example of the kind of forest that covered Christchurch many years ago.

Non-profit conservation group American Forests, which was established in 1875, recommends a 40 to 60 per cent target for cities in forested areas, 20 per cent for grassland cities and 15 per cent for cities in desert landscapes.

But what landscape category should we use for Christchurch? Before Europeans arrived it was swampy and grassy, with dunes in the east. But there are signs that earlier than that, Christchurch was forested.

University of Auckland associate professor Dr Margaret Stanley says Christchurch would have been heavily forested at one point in its history.

Thousands of volunteers have joined planting efforts across Christchurch. Henry Kircher, 3, joined in the planting of 3500 kahikatea in Cranford Basin in Mairehau in 2020.

“Riccarton Bush is an example of the forests that once flourished in the area where Christchurch city now sits.

“There would have been a lot of forest.”

Last year, workers building a flood mitigation scheme in Hoon Hay stumbled upon an ancient buried forest. The standing kahikatea stumps were thought to be about 2500 years old and were probably buried when the Waimakariri River changed course and covered them in silt.

In the 1840s, there were forests in Riccarton, Papanui, and on the northwestern slopes of the Port Hills.

“Forest bathing”, or spending time amongst trees, can generate a sense of well-being.

This leafy history means Christchurch could be forested again. The 40 to 60 per cent canopy target recommended for cities in forested areas could be feasible for Christchurch.

But why would we bother? What are the benefits of a leafy city?

The benefits of more urban trees range from improved mental health, cooler neighbourhoods and protection against climate change impacts like flooding.

Spending time amongst trees, such as this jogger in Hagley Park, is as good for mental health as physical wellbeing.

Mental health benefits

Academic research into the impact of urban trees on mental health has discovered some astonishing benefits.

A study published last year found that living within 100 metres of a tree was associated with using fewer antidepressants.

De Montfort University psychology lecturer Dr Melissa Marselle studied the drug prescriptions of about 10,000 people living in the German city of Leipzig and compared that to the levels of urban greenery in their neighbourhoods.

Bottle Lake Forest in the east of Christchurch is a large area of urban forest.

She concluded that planting more street trees could improve the mental health of a city’s residents.

Stanley says the mental health benefits of urban trees are found in many studies.

“You can measure the wellbeing of people where there are trees and where there are no trees.

“It shows really direct measures of reducing blood pressure and stress levels.”

A lack of trees in parts of Baltimore creates heat islands in some of the poorest neighbourhoods.

Ecological scientist Colin Muerk says an urban forest could replicate the Japanese idea of “forest bathing’’, where people spend regular time in forests to soak up the mental health benefits.

“There is a sense of wellbeing that comes from walking in the woods.

“Being present in an urban forest is akin to forest bathing.”

A view of Christchurch looking towards the Port Hills shows tree coverage in the south of the city.

Stanley says there is also evidence that urban trees mitigate against the higher risk of floods due to climate change by sucking up water and reducing runoff.

Tree-less streets hotter

Trees also play a major role in counteracting another major impact of climate change – extreme heat in cities.

A study by Portland State University urban studies and planning professor Vivek Shandas found city areas with fewer trees got much hotter on summer days.

Heat mapping surveys in Baltimore found a temperature range of between 30 degrees Celsius and 39C on one of the hottest days of the summer.

Protesters fought to save several native trees from being felled on Canal Rd in Auckland last year.

The study, published in the New York Times, found the area with the lowest temperature was one of the leafiest suburbs, while the hottest had very few trees. The study called these hotter parts of the city “heat islands”.

The ‘leafy’ suburbs

The research also exposed how the benefits of urban trees are rarely distributed evenly across cities. The cooler and leafier suburbs in Baltimore were also the richer ones.

This trend can be seen in Christchurch. Some of the richest suburbs have the highest canopy coverage.

Fendalton has tree canopy coverage of 23 per cent, excluding plantation forests, Riccarton has 17 per cent and Cashmere has 19 per cent. Meanwhile, eastern coastal suburbs have 11 per cent coverage and Hornby has 7 per cent.

The study on links between mental health and urban trees concluded that planting more foliage could also help resolve the health gap between rich and poor.

“Street trees should be planted equally throughout a city to ensure those who are socially disadvantaged have equal access to nearby nature, thereby safeguarding urban health equity and preventing green gentrification.”

An urban forest has been planted in Woolston as part of a flood mitigation scheme.

So, given the clear benefits that urban trees bring to a city, how do we make Christchurch a greener, leafier place?

Stanley says the simple act of planting more street trees would make a difference. “For some people, they are the only trees they interact with. They are important for people.”

Schools could also make a difference by planting a tree rather than erecting a shade sail for the summer.

A view of east Christchurch and the residential red zone show the level of tree coverage in the east of the city.

“Schools spend large amounts of money on shade sails when a tree provides much greater shading and is much more effective.”

Fringes of sports grounds and golf courses could be used for trees. Stanley says only a third of golf courses are used for the sport and there are 900 hectares of golf courses in Auckland.

A “treebate” scheme in the US city of Portland offers ratepayers a discount of up to $50 off their rates bill for each tree planted on their property.

Tree Council secretary Mels Barton, who has been campaigning to save trees from the chainsaw in Auckland, says the Government needs to put protection of trees on private land back into national planning legislation.

Many urban trees have been felled in backyards across the country to make way for more housing after national planning protections were removed in 2009. The law change came into effect in 2012.

She says reinstating those protections is critical to retaining trees in cities.

“You don’t have sufficient public land in Auckland to plant enough trees to make up for what we are losing on private land.''

Christchurch has a similar problem. The 2017 council report that calculated Christchurch’s tree canopy coverage also estimated the theoretical upper limits for coverage in Christchurch.

If council-owned land was planted with trees it would bring the coverage up to 24 per cent and if all grass covered or bare land was planted then it would rise to 68 per cent.

There are also opportunities to plant more trees emerging from Christchurch’s earthquake recovery.

A 2.75-hectare urban forest is being planted on Linwood College’s earthquake-damaged grounds in Woolston.

The former residential red zone also presents a huge opportunity for reforestation.

A council spokesman says parts of the 630-hectare Ōtākaro Avon River Corridor, or Ōtākaro Park, would be planted with native trees.

The city council has been developing an urban forest strategy since at least 2017.

The plan will be released for public consultation in the first quarter of 2022. It will include canopy coverage targets, deadlines for tree planting and measures to address the disparity in tree coverage between rich and poor suburbs, a council spokesman says.

Such a plan could help Christchurch meet requirements to be recognised as a National Park City by the international National Park City Foundation. The foundation is campaigning globally for cities to be “greener, healthier, wilder and fairer places to live” on the belief that nature is not a “nice to have” but “critical for our wellbeing”.

And if the council’s canopy targets are ambitious enough and are carefully implemented over many years, Christchurch could become a much leafier place.

If Christchurch hit the 60 per cent mark for tree canopy coverage, it would mean most people could enjoy the shade of a tree and its many benefits.

Christchurch could become a city of dappled sun and truly live up to its title as the Garden City.