architects, engineers and developers are creating increasingly greener structures

and doing it in a more literal way than ever before.

This is what happens when trees meet buildings.

Buildings with trees are actually nothing new. The mythical Hanging Gardens of Babylon

are often imagined as a stepped palace of terraces containing numerous trees, shrubs

and exotic flowers.

Although no such building was found during excavations in Mesopotamia, and its existence

has been subject to much debate, artists have kept this imagery alive in their paintings

throughout the centuries.

The current revival of green architecture began in the 1970s, when the energy crisis,

coupled with growing awareness of humankind's impact on the environment, propelled architects

and engineers to think more carefully about sustainable development.

Although there are many different ways to approach sustainable building design,

an increasing number of architects and engineers began to incorporate green roofs and other

energy saving measures into their projects.

A prominent example of this is the Willis Faber and Dumas Headquarters in Ipswich in

the United Kingdom, designed by Sir Norman Foster and completed in 1975.

The building features a reflective double-leaf facade and a grass-covered roof, which could

be used as breakout space for the firm's employees.

Other approaches advocate integrating trees and plants within buildings, often to soften

the psychological impact of living in modern cities.

One of the most idiosyncratic proponents of this approach

was Austrian artist Friedensreich Hundertwasser, who designed an apartment building

and a number of hotels throughout the 1980s and 90s incorporating mature trees

in Vienna and elsewhere across his native country.

Since then, research has demonstrated that green space can have a substantial physical

and psychological impact on our urban environments.

Studies have shown that even a small park can reduce local surface temperatures

by as much as 7 degrees Celsius, while urban trees planted along streets

can reduce temperatures by up to 3.9 degrees Celsius.

Green roofs also perform well

their temperatures can be as much as 4.4 degrees Celsius cooler than conventional roof finishes

and - if used across an urban area - could reduce temperatures by as much as 7 degrees Celsius.

The benefits of temperature regulation can also be found in deciduous trees, which offer

passive solar shading in the summer, while allowing the sun's rays to penetrate deep

into a building's floor plan during winter months.

Trees can also reduce the amount of exhaust gases and particles in the air, with some

studies claiming that they help reduce local concentrations of nitrogen oxide by up to 57%.

Aside from their physical impact, there is powerful evidence that the inclusion of plants

within our built environment can deliver psychological benefits too.

Hospital patients with views of green spaces have had faster recovery times and plants

have been shown to reduce stress amongst office workers, which in turn increases their productivity.

This can often lead to extensive planting being added to existing structures, either

as a way to replace lost habitats or decrease urban island heat effect, or to create green

spaces for the benefit of people living and working in dense urban areas.

Projects of this kind include Chicago's green roofs project launched in 2001,

as well as linear parks, such as the 1993 Promenade Plantee in Paris

This was the precursor to the hugely popular High Line

that now occupies a disused elevated freight railway in Manhattan, which first opened in 2009

and Thomas Heatherwick's unrealised Garden Bridge in London, proposed in 2013.

Arguably more exciting is the growing trend of integrating entire trees into built structures

- leading to increasingly ambitious projects with deep, strategically placed planters that

provide enough space for tree roots to develop.

One of the most impressive examples of this is ACROS, a cultural centre in Fukuoka, Japan.

First opened to public back in 1995, this 14-storey building designed by Emilio Ambasz

steps down to meet the city's Tenjin Central Park.

The building contains deep planters alongs the edges of its terraces that allow trees to take root and grow.

Its 5,400 square metres (or 58,000 square feet) of greenery

contained some 37,000 plants representing 76 different species when it first completed.

Today, it contains more than 120 varieties of plant and over 50,000 individual specimens.

Architectural office MVRDV took a similar approach when designing the Netherlands'

pavilion for the 2000 Expo in Hanover, Germany.

Here, the entire fifth floor of the structure was given over to a 'forest' in which

deep planters accommodated mature trees.

Although the structure has since fallen into disrepair, with the mature trees dying off,

a new generation of shrubs and trees has since colonised the structure

proving the soundness of the principle

Later in the decade, architecture and engineering firm WOHA designed the Royal Pickering Hotel in Singapore,

which included extensively planted terraces at four storey intervals featuring

tropical shrubs, vines and palm trees.

This building completed in 2013, just a year before the twin towers of “Bosco Verticale”

or the “Vertical Forest” in Milan.

Conceived by Stefano Boeri Architects, these residential towers -

standing 112 metres and 76 metres tall respectively in central Milan

- are the first successful example of tall buildings fully covered in trees.

Standing between 3 and 6 metres tall when they were first planted, the trees will be

allowed to reach maximum height of 9 metres before being pruned.

In order to ensure their stability in the wind, the trees are tethered to the building using steel wires

Structurally, buildings that incorporate trees will of course require special consideration

and additional strengthening and reinforcement in areas.

In the case of Bosco Verticale the additional cost of this reinforcement was clearly offset

by benefits and did not prevent the project proceeding.

There are other issues to consider when mixing trees with buildings too - such as the embodied

carbon of the steel used in reinforcement or building superstructures and the increased

demands placed on maintenance.

In this example, the project team were able to use Milan's excess underground water

to cultivate the trees, minimising the towers' ecological footprint.

Another challenge posed by this new building type, with large amounts of potentially flammable

organic matter attached to facades, is fire.

Principally, engineers use established guidelines for green facades and green roofs to ensure

their buildings meet fire safety regulations.

Regular pruning and irrigation also help to mitigate the risk of fire, as does ensuring

that the soil contains a relatively low percentage of organic matter.

Learning from his work in Milan, Stefano Boeri is currently working on a number of similar

projects in places such as Switzerland, China and the Netherlands.

The concept of incorporating trees into buildings is also employed across South-East Asia and

features heavily in the work of Vietnamese studio Vo Trong Nghia - who have incorporated

trees into buildings as diverse as private residences through to a university office in Hanoi

Currently under construction in Taipei, Belgian architect Vincent Callebaut's Agora Tower,

based on the form of a DNA double helix, will feature extensively planted balconies.

Meanwhile in Shanghai, British designer Thomas Heatherwick's 1,000 Trees, a mixed-use building

that features trees and shrubs on its structural columns is nearing completion

...and in the UK, Heatherwick's Maggie's Center in Leeds uses its roofs to create

a stepped garden, complete with trees and lush vegetation.

In Singapore, WOHA continue to design green architecture, including the Oasia Hotel Downtown,

a 27-storey tower that features a trellis-like facade and three large sky terraces with swimming pools and palms

This innovative structure was named "Best Tall Building Worldwide" in 2018

by the Council on Tall Buildings and Urban Habitat.

The designers and engineers working on the latest generation of treescrapers

have also decided to tackle the issue of embodied carbon

and the considerable amounts of energy consumed when manufacturing steel and concrete.

Increasingly, carbon-negative materials such as Cross Laminated Timber (CLT) or glue laminated

timber, commonly known as glulam, are being used.

High rise structures such as the proposed 18-storey Tree Tower Toronto by Penda

or this 350 metre tall theoretical proposal for Sumitomo Forestry's headquarters in Tokyo by Nikken Sekkei

demonstrate the continually growing trend of incorporating trees into buildings

and the work of today's architects and engineers to push this concept to its very limits

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