Radical Nature attempts to save the planet

There is a birch tree sawn into metre lengths by Anya Gallaccio, and reassembled with the help of steel bolts, and tropical trees, complete with roots and soil, growing horizontally. There is a pontoon island of rhododendrons, designed to float in a Scottish lake. There are fruit and vegetables growing under lights.

This is Radical Nature: Art and Architecture for a Changing Planet 1969-2009, an exhibition which (to paraphrase the verbose official description) is something to do with the relations of people, art and nature to each other. Here contemporary artists such as Simon Starling and Tomas Saraceno hold hands across the decades with pioneer, hippy-tinged, makers of Land Art in the Seventies. One exhibit from the earlier period is the "Dolphin Embassy", where humans might greet their intelligent flippered friends.

It’s sometimes a bit blurry, whether we’re looking at symbols or tools. Are the exhibits meant to make us reflect on man’s inhumanity to nature, or are they practical proposals for saving the planet? When Agnes Denes planted a wheatfield in lower Manhattan, was this for the spiritual enlightenment of New Yorkers, or was it a prototype of urban agriculture? More the former, I suspect. Saving the planet is more to do with the Chinese changing the way they build power stations, or Americans changing the way they make cars, than anything an artist can do.

There are moments of charm, such as the piece called "I am sorry, goodbye", a rough-hewn timber igloo, or Pantheon, parked on the Barbican’s waterside terrace, the experience of which may or may not be improved by attendants randomly offering free cups of hibiscus tea.

There are also moments of beauty, including the photographs of Denes’ 1982 wheatfield. But, given the underlying seriousness of its subject, Radical Nature comes across as a little whimsical. The overall sense is one of gentle melancholy or resignation, rather than anger, or belief that anything can be changed.

Until 18 October. Information: 020 7638 4141, www.barbican.org.uk

Radical Nature
Barbican Art Gallery
Silk Street, EC2 8DS