A new report from Environment commissioner Simon Upton, Urban Ground Truths, says that green spaces provide environmental services that will become more important as climate change progresses.

As well as providing flood protection by absorbing rain from extreme weather events, soil is vital to support trees that provide shading and cooling. Trees can reduce temperatures between 0.4°C and 4.5°C, depending on their size.

“Even a single tree can provide 3°C of cooling.”

New Zealand hasn’t experienced heatwaves in the same way as other countries - yet. But with human-induced planet heating now locked in, Upton says we will in future. “Heatwaves are coming. It’s a wake up call. Now is the time to take action.”

But limiting topsoil depth and compacting subsoil are routine practices in urban development, which can have irreversible consequences, limiting the type and size of tree that can grow. “Compaction of subsoil will forever preclude anything of substantial size being grown.”

Soil is treated as the ‘poor cousin’ of environmental management, Upton says. “It’s a precious resource, it’s a living resource, and it's a finite resource. We treat it very casually compared to water.”

Perceptions around efficiency are a key driver of subsoil stripping, with bulk stripping using large machinery to prepare sites for concrete slab construction often the default choice of developers, the report says.

While earthworks are usually necessary during residential developments to meet geotechnical requirements or remove contaminated soil, the report found there are market and regulatory drivers which incentivise removing more soil than is necessary. 

“By the time excavators arrive at a subdivision or infill site, key design and construction choices have been made that influence the volume and extent of soil to be removed from or retained on site, with enduring consequences.”

Research by Manaaki Whenua Landcare Research found that councils across New Zealand have variable rules around the depth and volume of landscaped soils. It also found some specifications were inadequate, restricting the capacity of soils in newly developed sites to soak up excess rainwater and support the growth of larger shady trees.

National guidance

The commissioner wants national guidance to provide clarity and consistency on the depth of landscaped soils for grass, green areas and shade-bearing trees within urban developments, as well as desirable canopy coverage. 

Upton also wants to see the recommended topsoil depth for lawns increased to improve rainfall storage capacity and soil specifications for trees to include both subsoil and topsoil. 

The report also recommends greater protections for urban soil during the development process. These include creating national guidance to support councils’ management of urban soils, clarifying the roles and responsibilities of regional councils in relation to soil conservation, and implementing incentives for developers to encourage soil protection and conservation.

It also suggests that territorial authorities tools such as integrated catchment management plans to better coordinate stormwater and soil management with urban growth demands.

A financial win?

Rather than adding to costs, the commissioner says protecting soil should be a financial win for councils. Green spaces can do some of the heavy lifting for stormwater that councils would otherwise need to provide for with big pipes and other infrastructure.

More green space and tree canopy also provides a better living environment - a selling-point for developers.

The report follows on from the commissioner’s report released last year, Are we building harder, hotter cities? The vital importance of urban green spaces.