Although adding trees to buildings can have real value, at Thomas Heatherwick's recently opened 1,000 Trees, the carbon costs outweigh the environmental benefit, says Philip Oldfield.

Almost a decade ago, journalist and researcher Tim De Chant published a piece entitled Can we please stop drawing trees on top of skyscrapers? It quickly went viral.

In it, he chastises architects for adding arbitrary vegetation to buildings giving a perception of sustainability. "It's just not realistic. I get it why architects draw them on their buildings. Really, I do. But can we please stop?"

In the years that have followed, architects haven't listened. A plethora of verdant buildings and towers have been realised, and no multi-storey section is complete without some form of lush hanging greenery, or over-sized trees growing from dubiously shallow soil depths.

Project descriptions often talk of accommodating forests, headlands, farms and more, all at height above the city. The latest to embrace this trend is Heatherwick's 1,000 Trees, the first phase of which has recently opened in Shanghai.

The potential of nature is diminished in this quest for a self-indulgent architectural concept

1,000 Trees creates an architectural topography; two stepped mounds, 60 and 100 metres in height (the latter of which is set to be built in the second phase, over the next few years).

Sited adjacent to a public park that runs along the Suzhou river, the design seeks to visually extend this green space up the building using trees that sprout out of its matrix of concrete columns, creating what the architects describe as "two tree-covered mountains".

However, while the intention is clear, the resultant architecture limits trees – one of the most wonderous forces of our natural world – to a decorative flourish atop columns. The potential of nature is diminished in this quest for a self-indulgent architectural concept.

Trees and greenery can provide so much value to our cities. Greenery can help mitigate the urban heat island effect by lowering ambient temperatures, shade public spaces and buildings and absorb pollutants from the air.

Through photosynthesis, trees absorb carbon dioxide and release oxygen. They provide biodiversity, and house birds and insects. There are psychological benefits too, with access to greenery reducing stress and improving mental health and concentration.

The trees are a one-liner, contributing to a sense of urban drama

With benefits aplenty, it is unfortunate so few seem to have been harnessed or considered in 1,000 Trees. Instead, nature is relegated to superficial ornament. The trees are a one-liner, contributing to a sense of urban drama, but with little substance as to how they might provide tangible benefits to the environment, or the community.

Using vegetation to extend a park up a building is nothing new. In Fukuoka, Japan, in the late 1980s, the proposed siting of a new government building on Tenjin park caused public uproar.

In response, Emilio Ambasz designed the ACROS Centre with lush garden terraces (publicly accessible), water features and some 50,000 plants. In this way, any green space lost from the park in the footprint of the new building would be returned to the public.

Where Ambasz's design differs to 1,000 Trees is not only the extent of the vertical greenery (a lush carpet of planting providing a clear continuity of the park in Fukuoka, versus scattered planters in Shanghai) but in environmental rationale.

The ACROS Centre sits at the north end of the park, meaning its green terraces face directly south, shading the office building from the sun. At 1,000 Trees, most of the greenery is on the northern facade, with the southern elevation a mixture of glazing and murals.

What is the carbon impact of accommodating hundreds of trees on a building of this scale?

At 1,000 Trees care has been taken to select appropriate plant species to thrive at varying altitudes. While its verdant qualities will no doubt increase as the trees and plants grow over time, at the moment, it's the ribbed concrete planters that dominate, rising out of the columns like giant designer bathroom sinks.

This raises the question – what is the carbon impact of accommodating hundreds of trees on a building of this scale?

Measuring from drawings, I estimate a typical planter and column top contains around 14 tonnes of reinforced concrete. Each kilogram of reinforced concrete releases 0.111 kilograms of carbon dioxide in its production (according to the Inventory of Carbon and Energy).

This means a single planter would have an embodied carbon of 1,554 kilogrammes carbon dioxide equivalent (CO₂e). The amount of carbon dioxide a tree absorbs is dynamic, and depends on a variety of factors, but can be estimated as 10 kilogrammes per year for the first 20 years of its life.

This means the carbon dioxide absorbed by a tree would take around 155 years to offset that emitted in the production of the concrete planter (these are conservative figures, and exclude emissions from transportation, construction, steel balustrades, maintenance, et cetera).

It's clear the carbon cost outweighs any environmental benefit

There are other environmental benefits the trees could provide which would shorten this timescale, but they are not considered in the design. Their location isn't informed by sun, wind or climate, but by an arbitrary structural grid. In any case, it's clear the carbon cost outweighs any environmental benefit.

While we should be careful to not reduce nature to mere numbers, these are exactly the kind of questions our profession should be asking itself in a climate crisis.

Other studies have also sought to quantify the benefits of vertical greenery elsewhere. Stefano Boeri's Bosco Verticale includes 13,000 plants and 700 trees in planters across 27 storeys.

Research by the Council on Tall Buildings and Urban Habitat (CTBUH) has shown the additional shade they provide could reduce the energy needs of the building by 7.5 per cent. Would this be enough to offset the embodied carbon in the concrete planters? Probably not, but it would at least go some way.

However, there is a human benefit to vegetation beyond all this. Personally, I see the real value of Bosco Verticale, and of Boeri's latest iteration, the Trudo Vertical Forest social housing tower in Eindhoven, as making vertical living more attractive and pleasurable through the green verandas they provide, at odds to barren balconies in much high-rise housing.

In Singapore, research I have contributed to explored residents’ experiences of sky gardens and found they can contribute to feelings of peace and escapism (though residents were also frustrated by the stringent rules that governed gardens at height).

Could 1,000 Trees provide a similar human benefit, an escapism from the intense concrete jungle of Shanghai? Visualisations and photographs of the project show walkways and terraces with greenery and blossoms hanging from above.

However, with this first stage accommodating mainly retail and food and beverage outlets, who will have access to these spaces? Will they be truly public, where anyone can come along and enjoy sitting beneath a tree?

There are opportunities for architecture to engage better with nature

Our public and green spaces are being increasingly privatised, ticketed and controlled. There are legitimate questions about whether green spaces at height can provide benefits to the broader community, or if they're simply elitist eco-bling, accessible only to a few for a hedonistic selfie or an expensive cocktail.

At 1,000 Trees, I'd guess the ground-level park along the Suzhou river will provide a much greater human benefit to Shanghai than trees dotted across the building.

This is not to reject entirely the idea of vertical greenery. I still believe there are opportunities for architecture to engage better with nature and vegetation in our dense urban realms. But, we must do better than 1,000 Trees.

The photography is by Qingyan Zhu.

Philip Oldfield is Head of School of the Built Environment, UNSW Sydney. He is the author of the Sustainable Tall Building: A Design Primer (2019).

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