Our cities are losing their leaves

Green spaces in the cities of Aotearoa New Zealand are declining. That has consequences for the environmental services they provide.

Parliamentary Commissioner for the Environment | Te Kaitiaki Taiao a Te Whare Pāremata

 March 2023 

New Zealand cities have plenty of green space but it is rapidly declining according to a  report  released by the Parliamentary Commissioner for the Environment.

New Zealand’s cities are growing. Urban intensification will help to address the country’s housing supply shortage – without the increase in transport emissions that would likely accompany growth outwards. Cities that sprawl less will also reduce pressure on productive soils close to the urban fringe.

The benefits of urban green space can be grouped into three broad categories. The Commissioner's report focuses on the benefits provided by environmental services.

But not all intensification is the same. The style of infill townhouse development that is currently playing out within our cities comes with particular risks for the existing network of urban green space and the environmental services it provides. These changes are not easy to undo.

Why is green space important?

The benefits of urban green space can be grouped into three broad categories:

  • visual amenity and placemaking
  • recreation, health and wellbeing benefits
  • environmental services.

While visual amenity and wellbeing benefits are important,  the report  focuses on the benefits provided by environmental services. These include temperature regulation, carbon sequestration, flood mitigation, erosion control, food provision, air and water filtration and habitat for biodiversity.

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Stormwater regulation The surfaces created by most buildings, roads, footpaths and driveways are impervious – they block rainwater from soaking into the ground. Conventional stormwater systems are designed to cope with rain to specific levels. But they can be overwhelmed by large storm events, increased pressure from new developments or lack of maintenance, which leads to blockages.

Green spaces act like giant sponges, slowing the flow of rainwater and trapping and filtering pollutants. When rain falls on a patch of bush, or even a lawn, some of the water is caught by the vegetation. Evaporation from these plant surfaces means that some rainwater never reaches the ground.

Plants also help by increasing the volume of soil and by loosening compacted ground, which allows water to soak into the soil more quickly and deeply – reducing runoff. The roots of trees and wetland plants such as sedges and rushes can be particularly helpful for this.

It has been estimated that urban vegetation can help soak up a third of the water resulting from extreme rainfall events.

Temperature regulation The air temperature in heavily built-up urban areas is often several degrees higher than surrounding rural areas. This is known as the urban heat island effect. High urban air temperatures add to energy demands, place stress on infrastructure and ecosystems, and cause undesirable health outcomes.

Green spaces in cities can cool their immediate surroundings, providing a vital service to cities facing problems from excess heat. Studies show that street trees can cool the air between 0.4 and 4.5 °C by a combination of shading and evapotranspiration. Most of this cooling happens within 30 metres of the trees. Large parks over ten hectares can cool much larger areas, with drops of up to 1.2 °C out to 350 metres from the edge of the park.

Most of New Zealand’s cities are located in coastal areas and are heavily influenced by the ocean. So while vegetation will still provide some cooling, the extent and location of this benefit will be influenced by wind direction, topography and the structure of the city.

Air filtration Air quality in New Zealand cities is generally considered to be good in comparison with many other parts of the world. But it is declining. A 2022 study found that air pollution was 10% worse in 2016 than it had been a decade earlier. The study indicates most of the harm from air pollution here is from vehicle emissions. The pollution is worst where there are lots of vehicles moving slowly and stopping and starting often.

Plants can filter pollution from the air – by either trapping the particulate matter on their leaves (which is later washed off by rain) or by absorbing it into the plant itself.

Trees in Auckland have been estimated to remove 1,230 tonnes of nitrogen dioxide, 1,990 tonnes of ozone and 1,320 tonnes of particulate matter annually. A  2016 study of 245 cities worldwide,  found that most reductions in particulate matter occur within 30 metres of vegetation.

Biodiversity The green spaces alongside roads, rivers and coasts, town green belts, wetlands, estuaries and gullies all link to form a wider ecological network that supports biodiversity. Around 86% of New Zealand’s population lives in cities. While urban areas make up a very small proportion of Aotearoa, they still contain a very high number of nationally important, rare and threatened environments.

From a te ao Māori perspective, biodiversity and mauri are tightly connected. Mauri indicates the life generating and supporting capacity of an environment, and relates to the health and wellbeing of the people and organisms living in it. A place with a rich diversity of species will have more mauri than a basic monoculture of grass.

Protecting any remnant of an important native ecosystem in urban areas by avoiding further fragmentation and degradation is crucial. The ecological value of an existing patch of indigenous habitat is hard to overstate.

Soil Healthy soils are the unseen engine room of any green space. All the environmental benefits green spaces provide rely on healthy soil. An adequate volume of healthy soil not only provides water, nutrients and oxygen to plants but also allows plants’ root systems to anchor down, which helps them to withstand extreme weather events like drought and wind.

Despite that, urban soils are under considerable pressure. Most new residential subdivisions begin with earthworks that remove almost all the topsoil from the area. Roads, footpaths, building foundations and other essential services such as waste and stormwater systems are installed, and then, when building is nearing completion, green spaces are created by adding a thin layer of topsoil and planting. During construction, soils may also be heavily compacted by heavy machinery or to meet engineering requirements.

This means that soil on private lots and alongside roads may be too compacted or not deep enough for healthy trees to grow.

How New Zealand cities have grown

The report,  Are we building harder, hotter cities? The vital importance of urban green spaces , presents new data on how public and private green space in Auckland, Hamilton and Greater Wellington has evolved over the decades. The bulk of this research was done by analysing aerial photographs of the three cities in the 1940s, 1980s and 2010s.

Auckland, Hamilton and Greater Wellington have expanded considerably over the last 80 years, consuming around 60,000 hectares of land. In all three cities, our data suggest most of that growth happened between 1940 and 1980.

Since 1980, population density in Auckland, Hamilton and, to some extent, Greater Wellington has increased because of infill development, including the increasing popularity of townhouses and apartments. This trend is likely to continue.

Despite that, average city-wide population densities in Auckland (2,500 people per square kilometre) and Hamilton (2,000 people per square kilometre) are low by international standards. Cities such as Fukuoka in Japan, Bristol in the United Kingdom and Malaga in Spain – none of them among the densest cities in their countries – have  average densities of over 4,000 people per square kilometre. 

Auckland's urban boundary (red) in 1942, 1980 and 2017. Like most New Zealand cities Auckland rapidly grew outwards from the 1940s to 1980s (9,500 hectares to 38,800 hectares). In the last few decades, most of the growth has been in the existing urban footprint (which in 2017 was 52,400 hectares).

What is green space like in our cities today?

Although each is different, Auckland, Hamilton and Wellington are well-endowed with green space

Today, Auckland, Hamilton and Greater Wellington have very different endowments of urban green space.  Our research  found that in 2021, 45% of Hamilton’s urban area was green space of one kind or other. The equivalent figure in Wellington was 64% – or 72% if the large network of reserves surrounding the city (such as the outer green belt) is included.

The imagery available for Auckland is older. Our research found that in 2011, 55% of Auckland’s urban area was green space. An analysis of land-use data provided by Auckland Council indicates that in 2016/17, green space accounted for 47% of the urban area. The ongoing intensification since then suggests that today’s value is likely to be lower again. 

Residential yards and gardens account for slightly more than half of available green space in all three cities. Public parks and reserves are another third, with vegetated berms in the transport corridor making up the remainder.

Below is an interactive map displaying public and private green space in  Hamilton in 2021  based on our analysis. View a  map for Wellington , and a  map for Auckland .

In 2021, Hamilton City had about 45% green space. Dark green indicates public green space, light green indicates private green space.

Quality and composition of green space

This image, captured with an infrared camera, reveals the surface (not air) temperature on a sunny summer afternoon in January on a treelined street in central Wellington. The yellow areas are hotter surfaces, and the blue/black areas are cooler. The air temperature was 23 °C. The surface temperatures at two locations are shown at the top left of the image. The pavement in full sun is 47.7 °C whereas the ground under the tree is 20.3 °C.

The type of urban green space – bare land, grass, shrubs, wetlands, trees – matters for the extent of environmental services they can provide. A wetland or a space covered in a variety of shrubs will provide more cooling, stormwater management, air filtration and biodiversity than a lawn of grass will. The combined environmental benefits will be greatest if the area is covered in forest.

One proxy for looking at the quality of green space in terms of the environmental services it provides is to look at tree canopy cover. Our data shows that in Hamilton, grass accounts for around two thirds of total green space. That ratio is reversed in Greater Wellington, with woody vegetation accounting for around two thirds of the overall total. In Auckland, it’s about half and half.

Other studies have found that on average, Wellington has 31% tree canopy cover, Auckland 18%, Hamilton 15% and Christchurch 14%. These averages can partially be explained by topography: vegetation clearance and infill development is much easier on Hamilton’s floodplain than on Wellington’s hillsides. The age of a suburb, development style and zoning rules also play a role.

There is also signification variation within cities.

When it comes to tree cover, a study has found that in Wellington City, the flat suburb of Rongotai has almost no trees, but the hilly suburb of Highbury has around 70%. A similar Hamilton City Council study found Hamilton also has wide variation between suburbs, with tree cover ranging from 3.5% in Frankton South to 29% in Chedworth.

In Auckland, tree cover ranges from 8% in Māngere/Ōtāhuhu to 30% in Kaipatiki. It also varies depending on who owns the land. Studies have found canopy cover in Auckland public parks and reserves is 29%, but only 14% on private land.

Vegetation maps for three Auckland suburbs: East Tāmaki Heights (left/top), Māngere (middle), Freemans Bay (right/bottom). Light green indicates grass, dark green indicates other vegetation, and black indicates impervious surfaces like building, roads and concrete.

Our research shows green space has been declining

If you look at how urban green space has changed through time, there has been a noticeable decline in green space in Auckland and Hamilton.

While there is considerable uncertainty involved, our data suggest that private green space per person in Auckland declined by around 30% between 1980 and 2016. The equivalent figure in Hamilton was around 20%.

This graph shows the change of green space over time in Auckland, Hamilton and Greater Wellington. The yellow dot represents analysis of more recent Auckland Council data suggesting green space accounted for 47% of the urban area in 2016/17.

Limitations and uncertainty with the through-time analysis

The trends shown in this graph very likely underestimate the decline of green space in each the cities shown. There are two reasons for that. The first is that the through-time analysis does a poor job of capturing driveways and off-street carparks, which have become more common over time. The second is that the analysis concludes in 2016/17, a time at which construction activity in our cities was just beginning to ramp up.

This decline is mostly due to changes on private land

The loss of private green space in Auckland and Hamilton appears to have been driven by two main factors: infill development in brownfield areas and a denser built form in new greenfield subdivisions.

Infill development has converted pre-existing yards and gardens into houses and driveways

Change in green space Hamilton East subdivision from the 1940s (left) to 2016 (right).

Change in housing density in the same Hamilton East subdivision from the 1940s (left) to 2016 (right). Light grey represents buildings.

Newer developments on the urban fringe tend to favour larger houses on smaller sections

Changing development styles: Suburbs in Auckland (left/top two): Mt Roskill, post-war development, and Flat Bush, built in the 2010s. Suburbs in Wellington (right/bottom two), Trentham developments in 1950s, and 2010s.

Public green space is not compensating for this decline in private green space

Local authorities keep close track of the green space they own or look after, and they regularly publish statistics about it. Data on current green space are often used to inform how much public green space is needed in new developments on the city fringe.

To date, many councils have been struggling to provide additional parks and reserves to compensate for the reduced size of private yards and gardens. A survey undertaken for this report highlights that a number of Tier 1 councils – including Hamilton, Tauranga and Hutt City – have provided no more than a few square metres of new parkland for each additional resident since 2016.

Much of the green space being provided tends to be small. Auckland, Wellington City and Christchurch are the only cities to have set aside areas of open space larger than 10 hectares in recent years. In Christchurch’s case, this was partly driven by the earthquakes rendering significant areas uninhabitable.

What does the future hold?

Urban green space is likely to continue to decline

The development trends shown here were already playing out before recent Government moves to encourage additional housing supply through intensification in New Zealand’s largest cities.

The National Policy Statement on Urban Development will provide additional incentives for development ‘upwards’ in areas close to existing centres and public transport nodes. This will enable buildings of at least six storeys.

The Medium Density Residential Standards will allow more medium-density infill development across significant swathes of the urban area. While the policy requires at least 20% of all development sites to be kept as landscaped areas, the standards will place continued pressure on privately owned green spaces within our cities. How much pressure will depend on how it is implemented.

The Medium Density Residential Standards in particular are likely to place continued pressure on private green space in years to come.

Green space as a proportion of private residential land: city-wide averages versus post-2016 developments. The Medium Density Residential Standards (MDRS) require at least 20% of all development sites be kept as landscaped areas, a lower proportion compared to current averages.

What happens to existing public spaces within these urban areas remains to seen, but both policies allow councils to exclude public open space from being developed.

While the National Policy Statement on Urban Development and Medium Density Residential Standards both identify accessibility to natural spaces and open spaces as a key element of “well-functioning urban environments”, neither provides any guidance, tools or additional funding sources to help councils achieve that.

There will be more demand for green space

Looking forward, as our cities grow, they will also increasingly feel the effects of climate change. That will mean increased heat and more frequent extreme rainfall events. Parks, yards, vegetated berms and trees can help to mitigate the impacts of both, but only if they are healthy and functioning.

The estimated increase in average air temperatures in an Auckland suburb 20 years from now due to climate change will be compounded by further loss of green space as intensification occurs. Research suggests that a 10% loss of green space (as a proportion of the total area) could add 0.3 °C of warming on top of the 0.5-1.0 °C already caused by climate change.

Increasing how much land is covered by impermeable surfaces will considerably change how this water moves through cities. For a rainfall event that delivers 24 millimetres of rain in 24 hours, a section with 65% of the land covered by an impervious surface will have 18% more runoff than a section with 55% of the land covered by an impervious surface.

At the same time, the ongoing shift towards townhouse and apartment living – meaning smaller sections and yards – will make the recreational, amenity, health and wellbeing offering provided by public parks and reserves even more sought after.

The reality is that at the very time urban green space is likely to be in greater demand (both socially and environmentally) there is likely to be less of it.

What can we do about it?

The difficulty of re-establishing green space once lost should cause us to think very carefully about how we shape our future cities. The  report  makes several recommendations to ensure that the contribution green space can make to urban environments is fully accounted for in future urban design.

Design image for Te Ara Awataha, Northcote’s new greenway.

Planning for and providing urban green spaces should not be optional

Territorial authorities are not required to plan for or provide public green space in the same way they are required to for roads or three waters infrastructure.

This 'optionality' means that green space is given a lower priority than other forms of infrastructure. One recommendation is to amend the Local Government Act 2002 and the National Policy Statement on Urban Development to ensure that urban green space is one of the mandatory things councils must plan and provide for.

Councils should consider buying land for future public green space earlier in the planning process

The difficulty of retrofitting green space into existing urban areas highlights the importance of adequately providing for it from the outset in new subdivisions on the outskirts. When land is being considered for development, councils need to identify areas that could become public green space early in the planning process. Central government could help this through legislation and tax avenues.

Barry Curtis Park, Flat Bush, Manukau City is one of the largest urban parks to be built in New Zealand in recent decades. The park was created on dairy farming land identified by Manukau City Council in the late 1990s. The first stage of the park opened in 2009, with completion due in 2022.

Regular monitoring and reporting is necessary

Regular monitoring and reporting is necessary if we are to know how green space is changing in our cities. Without that understanding, councils will be able to say little about whether the various services that green spaces provide are being maintained. We also need more New Zealand data on the environmental services green space provides as the current evidence base is mostly international.

We need to improve the quality and quantity of green space in existing suburbs

Councils and government agencies could improve public green spaces like schools, parks and other ‘forgotten’ corners of public land, such as along road corridors. In the short term, this could be done by adding patches of larger shrubs and trees. Redesigning the layout of an entire park or street to better reflect the needs of the nearby community is at the other end of the spectrum.

Green spaces of all types form ecological corridors across our urban landscapes and perform important environmental services.

Ensuring that the area of private land that is not built on continues to be able to provide environmental services should also be a priority. Auckland Council, Hamilton City Council, and Christchurch City Council are trying to address that issue by including rules on minimum tree provision in the plan changes that implement the Medium Density Residential Standards.

There is also scope to expand financial incentives that encourage keeping (and planting) shrubs and trees on private land. Auckland Council, for example, adjusts the stormwater contribution it charges developers according to how much of the site is sealed. Similarly, Christchurch City Council has proposed charging developers who retain existing trees less.

Having shrubs and trees on private land does not need to stifle housing supply. Medium-density developments currently account for more than 60% of the new dwellings being built in New Zealand cities. Multi-storey apartments, by contrast, account for less than 10%. Encouraging more development ‘upwards’ could allow more private green space to be retained if it is carefully planned for from the outset.

For more information on our urban green spaces and other environmental investigations visit  www.pce.parliament.nz 

The benefits of urban green space can be grouped into three broad categories. The Commissioner's report focuses on the benefits provided by environmental services.

This image, captured with an infrared camera, reveals the surface (not air) temperature on a sunny summer afternoon in January on a treelined street in central Wellington. The yellow areas are hotter surfaces, and the blue/black areas are cooler. The air temperature was 23 °C. The surface temperatures at two locations are shown at the top left of the image. The pavement in full sun is 47.7 °C whereas the ground under the tree is 20.3 °C.

Change in green space Hamilton East subdivision from the 1940s (left) to 2016 (right).

Change in housing density in the same Hamilton East subdivision from the 1940s (left) to 2016 (right). Light grey represents buildings.

Green space as a proportion of private residential land: city-wide averages versus post-2016 developments. The Medium Density Residential Standards (MDRS) require at least 20% of all development sites be kept as landscaped areas, a lower proportion compared to current averages.

The estimated increase in average air temperatures in an Auckland suburb 20 years from now due to climate change will be compounded by further loss of green space as intensification occurs. Research suggests that a 10% loss of green space (as a proportion of the total area) could add 0.3 °C of warming on top of the 0.5-1.0 °C already caused by climate change.

Increasing how much land is covered by impermeable surfaces will considerably change how this water moves through cities. For a rainfall event that delivers 24 millimetres of rain in 24 hours, a section with 65% of the land covered by an impervious surface will have 18% more runoff than a section with 55% of the land covered by an impervious surface.

Design image for Te Ara Awataha, Northcote’s new greenway.

Barry Curtis Park, Flat Bush, Manukau City is one of the largest urban parks to be built in New Zealand in recent decades. The park was created on dairy farming land identified by Manukau City Council in the late 1990s. The first stage of the park opened in 2009, with completion due in 2022.