How we did it ... Walkway delivers lofty message about climate

Huw Morris, Planning, 13 June 2008

BACKGROUND: As part of the gardens' celebration of the Year of the Tree, the scheme follows a successful but smaller temporary walkway installed several years ago.

WHO IS BEHIND IT? Marks Barfield Architects and the Royal Botanic Gardens, Kew.

PROJECT AIMS: Visitors will gain an up-close perspective of trees and gain understanding of their role in helping wildlife and the climate.

SKILLS INVOLVED: Design, modern construction methods, sustainability.

Of all the celebrations marking 2008 as the Year of the Tree, one of the more lasting monuments was unveiled at the Royal Botanic Gardens at Kew in south-west London last month. Xstrata is the ultimate walk on the wild side and visitors will need a head for heights.

A £3 million steel structure that aims to show the importance of trees to wildlife and the climate runs through woodland designed by the iconic 18th century landscape gardener Capability Brown. The treetop walkway takes visitors 18m high into the tree canopy for a bird's-eye view of Kew while giving them the chance to see the rich biodiversity of the forest canopy.

Sweet chestnut, lime, broad-leaved oak and a pine are among the sights, while 14 new trees are being planted to enhance the experience. Visitors may even catch a glimpse of the purple hair streak butterfly - rarely seen because it lives its entire life high up in oak woodland - as well as the nuthatches, woodpeckers, insects, lichens and fungi that rely on them.

The walkway is complemented by another attraction, the Rhizotron. Entered through an apparent crack in the ground, this is the UK's only walk-in viewing area dedicated to illustrating the rich world of tree root biology.

The scheme, designed by London Eye architects Marks Barfield, was inspired by the nature that surrounds the site. The firm's principal Julia Barfield says: "We wanted the walkway to be a visually light, discreet presence, at ease in its natural surroundings while at the same time being unashamedly man-made."

At 200m long, the walkway comprises a string of 12 modular trusses connected by ten circular platforms. A larger, classroom-sized platform at the mid-point allows space for school groups of up to 35. There is also a bench to enable visitors to rest and enjoy longer views towards the gardens' Palm House. Access to the walkway is by a main stair tower after exiting the Rhizotron. This tower also has a single-stage hydraulic lift with a glazed panoramic car.

One of the major challenges facing the Marks Barfield design team was to strike a balance between enabling visitors to get as close as possible to the tree canopies while being mindful of the potential damage to the complex root system below ground. A radar survey was carried out to gauge the extent of the roots at the pylon and pile foundation locations. With this information, the design team strategically placed the 12m to 18m concrete piles between the major roots.

Most of the walkway was prefabricated off-site to save time and minimise disruption by avoiding on-site welding in this extremely sensitive environment. Kew's tree specialists also had to manipulate branches during the construction to help the positioning of the walkway's components. In total, more than 400 tonnes of steel was used in the scheme.

"We want to tell the story of a tree's role in diversity in this planet and as a key contributor in capturing carbon to help in the fight against climate change," says Kew's arboretum and project manager Tony Kirkham. "As a trained arborist, I have had the privilege of experiencing trees, birds and other wildlife from a completely different perspective. It is fantastic that we are able to give visitors the same experience and we hope that it will make them realise that trees matter and we need to nurture them."