“Radical Nature” is an earth summit in the form of an art exhibition. Green thoughts abound, albeit not in a green shade but in the brutalist concrete bunker of the Barbican Art Gallery – an intermittently apt space, as it turns out, for these assembled shards of eco-polemic, organic architecture and Land Art experiment.

While Gallaccio’s dead tree might stand for an entire micro-system, Henrik Hakansson’s Fallen Forest actually is a displaced microsystem, or at least a fragment of one, alienated to the same bare Barbican mezzanine. The piece consists of a 16-square-metre fragment of rainforest, kept alive for the duration of the show by ingenious means of irrigation, but flung weirdly on its side. Protruding obscenely from a panel of suspended earth, leaves and tendrils trail on the floor like the outflung arms of a fallen body. This visibly wilting cross-section of a disappearing world is another prompt to morbid contemplation.

Nearby, Newton Harrison and Helen Mayer Harrison’s Full Farm flashes suddenly back to a more innocent era of eco-optimism. Originally created in the early 1970s, the work is a kind of allotment reared within frames and forms consciously intended to evoke the bare structures of Minimal Art. Tomato plants flourish amid tubes of neon. The work was originally intended as a funky, counter-cultural riposte to the prevailing formalism of New York art in the Sixties – the Harrisons live and work in California – and each time it is recreated it revives, however briefly, the grow-your-own flavour of the period that brought it into being. A clarion call to self-sustainability, it comes with the proviso that it must always be donated to a local school at the end of any given exhibition.

“Radical Nature” is something of a schizophrenic exhibition, torn between doomy prophesy and various forms of agitprop optimism and eco- experiment. The show contains a number of green-tinged works of art and architecture from the past, although many of these come in the form of relics and remains. Joseph Beuys’ Honeypump at the Workplace, created for Documenta 6, in Kassel, in 1977, was originally designed as a kind of benevolent parody of a machine – a closed circuit containing two tons of honey, continuously pumped around a network of transparent tubes by two huge ships motors lubricated with margarine. Like the Harrisons’ Full Farm, albeit more antic and carnivalesque, Beuys’ mechanised installation was an eccentric model for a self-sustaining community. But in “Radical Nature” it survives only as an assemblage of disused apparatus, the melancholy aftermath of a Happening that can never happen again.

Several other ghosts from the past haunt the exhibition. The late Buckminster Fuller is preserved in an infectiously watchable film shown on endless loop in a replica of one of his signature geodesic domes. A charismatic phantom, he expatiates on the primary structures of nature and their potential for the forms of human architecture. Another of the show’s several cubicles and modules contains a film documenting the making of Robert Smythson’s long-lost, pioneering work of Land Art, Spiral Jetty. Created in 1970, the piece was a huge earthwork in the form of a spiral path extending into the Great Salt Lake in Utah. Vast quantities of basalt were trucked into place to form a quixotic track, leading nowhere, set down in the middle of nowhere. The work became a pilgrimage site for nature-worshippers, a Romantic invitation to embark on a sense-disorientating, spinningly circular voyage through an utterly remote part of the great American wilderness. Long submerged by the rising waters of the lake, Smythson’s piece now survives only in the form of grainy colour film footage. Mementoes of Smythson’s transcendentalist enterprise standing in for the transcendentalist experience itself.

“Radical Nature” is a thought-provoking, sensitively assembled exhibition, although it cannot quite overcome the limitations imposed by the tangled history that it sets out to recount. The more exuberant manifestations of land-inspired art, such as Smythson’s Spiral Jetty, or Agnes Denes’ equally extraordinary Wheatfield Confrontation of 1982 – which involved the planting of an entire wheatfield in a brownfield site in downtown Manhattan – have taken place outside the confines of the art gallery. They can only ever be present as phantoms of themselves, in the form of filmic or photographic records. So inevitably, the gloomier manifestations of modern eco-eschatology – dead or dying plant-life, juxtaposed with the grey walls of the Barbican itself – assume a greater weight and consequence than perhaps they should. The harshly urban qualities of the gallery space, together with the very nature of the traditions which the show sets out to explore, have the inevitable effect of skewing “Radical Nature” towards a chastened mood of melancholy.