Across an oeuvre of 30 books, including eight novels, he's established himself as one of the foremost living wilderness writers. His commitment to preserving the natural world and native populations against capitalist greed has yielded books on New Guinea, Africa, South America, Antarctica, Alaska, Siberia, Nepal and the Caribbean.

Nearer to home, he's championed the rights of native Americans, indigenous Long Island fishermen and migrant farm labourers. He's also ventured out on spiritual quests, as a pioneering advocate of hallucinogenic drugs and exponent of Zen Buddhism.

We pass the graveyard where his second wife, poet Deborah Love, is buried. Matthiessen mourned her death from cancer through the Himalayan journey recounted in The Snow Leopard (1978), in which he travelled with zoologist George Schaller to observe bharal, or blue sheep, and the elusive wild cat of the book's title. It won the National Book Award for non-fiction in 1979, but Matthiessen got labelled a nature writer when he saw fiction as his major work.

While he writes non-fiction quickly his novels are vast undertakings. His trilogy about swampland outlaw Edgar Watson - Killing Mister Watson (1990), Lost Man's River (1997) and Bone by Bone (1999), now reworked as a single 900-page novel, Shadow Country:

A New Rendering of the Watson Legend - was a 30-year endeavour.

His fifth novel Far Tortuga (1975), a paean to Caribbean green turtle fishermen, took eight years. It was the novel he says he most enjoyed writing because he experimented with a minimalist style that mirrored Zen teachings. 'I cut out all the adjectives and adverbs. I just give the bare facts. So you have the 'hereness' of everything.'

His non-fiction work Men's Lives (1987) elegised the dying traditions of Long Island baymen. Walking on the beach the day he finished the book, Matthiessen found the 1.5-metre-tall whale skull that now sits forbiddingly on his front porch.

Despite a lifetime of battling political treacheries, Matthiessen says he's 'past bitter'. Perhaps his equanimity comes from Zen - a 'roshi', or Zen teacher, Matthiessen leads a group of meditators every morning in the converted stable adjacent to his house. Although born in New York City, he grew up mostly in wild areas of New York State and Connecticut, where he kept copperhead snakes in cages.

An English literature major at Yale, he also took courses in botany, zoology and ornithology.

Aged 13, he volunteered at a charity camp for children from the Connecticut slums. The indignity of economic inequality first hit him the night he helped put on a banquet for the kids. 'They were looking over their shoulder every two seconds, which would suggest they never had enough, that they always had some bigger brother or uncle who took their food away. They all violently overate and were violently sick.' After learning that his family - his father was a wealthy military architect - was listed in the Social Register (an annual directory of elite society), Matthiessen removed his name at age 15. His emerging social conscience led to him being thrown out of his home at 17, after which he spent a year in the navy - an experience drawn on for his third novel, Raditzer (1960). Depicting the friendship between the scion of a rich family and an orphan in the navy, it was the novel in which Matthiessen feels he was 'starting to come into my own'.

He moved to Paris after college to study at the Sorbonne, where he co-founded the Paris Review in 1952. His first novel, Race Rock, appeared in 1954, but upon returning to America he worked as a commercial fisherman to support his young family. 'It was extremely hard work, but it was interesting. I loved the fish and the sea birds. And it kept me in very, very, good shape. The better shape I'm in the better I write.'

By the late 1950s, regular commissions from the New Yorker magazine enabled him to write fulltime. 'I wanted to go explore the last wild places before they were ruined. In those days, they didn't cover anything wild. The furthest afield they would go was to Europe.' His itinerant lifestyle meant he was rarely home, which he admits was 'tough on my family'.

Matthiessen suggests his novels are 'too demanding for the general reader' and offer little to 'your average woman reader who wants a romance or something' so his critical acclaim was never quite matched by mass sales. East coast reviewers have been cool to his fiction, he says. 'My kind of people just don't interest them very much - very tough, hardy people. Of course, people in the city have the same problems - the same heartbreak, the same grief and the same love. But their lives have been so well chronicled by practically all our modern writers.'

For Shadow Country, he recreated the harsh world of farmers and fishermen around the turn of the last century, focusing in particular on the buccaneering entrepreneur Edgar Watson. The novel's origins date back to the 40s, when his father took him on a boat trip up the west coast of the Florida Everglades - a labyrinth of 10,000 mostly mangrove islands.

'He showed me this river that came down into the Gulf of Mexico, and he said that about three miles up that river there's the only house in the Everglades, and it belonged to a man named Watson who was killed by his neighbours. That detail of the man killed by his neighbours in this very lonely river stuck in my brain. They would have been very small-time farmers or fishermen. They're not murderers. So what happened? What did this guy do?

It was either a lynching or self-defence - they said he shot first.'

Matthiessen planned to write a novel about the devastation of the Florida environment and the dispossession of its native tribes, but the Watson saga took over. 'I interviewed everybody over 90 years old in southwest Florida. This whole incredible American legend opened up. He was supposed to have killed over 50 people - not true at all. He probably did kill seven or eight people - workers in his fields. The rumour was that when payday came around he shot them instead.'

As Matthiessen dug deeper into Watson's life, the bogeyman sugar cane baron of myth became more complex. 'The wives all admired him. His kids did too, with the exception of one. He was very likeable, very personable, very charming, very smart, a very good farmer - he just had a murderous temper, and he drank too much. He had children with other ladies too, unofficially. He was a rascal. I had great fun with Watson. I gave him a very, very quick wit, and a sardonic take on life.'

Though originally written as a single novel, Matthiessen divided it into three volumes when the manuscript ran to more than 1,500 pages - a decision he regretted. 'The three parts were like movements of a symphony. When you separate it you lose that architecture of the whole novel.' After the triptych was published, he decided to rework it as a single novel, expecting it would be a one-year project; it ended up taking six years. 'My first notes went back to 1978. This was my fiction over the whole period, except two novellas. That's half my writing life on this project.'

Tacked to the wall of his office - a cottage across from his house - is a photo of Watson: a shaggy-haired man with a drooping handlebar moustache. There are photos of his British-born wife, Maria Eckhart, from her modelling years, and a picture of Australian-born art critic and author Robert Hughes, his former travel-mate and deep-sea fishing companion.

Matthiessen says his 'first real novel' was At Play in the Fields of the Lord (1965), depicting the collision between Amazon Indians and missionaries. Made into a film by Hector Babenco in 1991, the novel inspired psychedelic drug culture icon Timothy Leary to send Matthiessen a letter saying it was the best account of tripping he'd read.

Matthiessen turned to Zen in 1969 for a way of seeing holistically without chemicals - a journey chronicled in Nine-Headed Dragon River (1986). But he feels he greatly benefited from LSD. 'Every patient has some block, and it takes them sometimes years to ferret that out through conventional therapy.

LSD goes right to the heart of the trouble. It really does bust through to what your hang-up is.'

Throughout his interior journeys, social advocacy continued to drive him. From his commitment to native Americans came In the Spirit of Crazy Horse (1983) - a 650-page polemic about a shoot-out in 1975 between American Indian Movement activists and the Federal Bureau of Investigation. Matthiessen argued that Leonard Peltier, a native American given two life sentences for allegedly murdering two FBI agents, was convicted on trumped-up charges.

A former FBI agent and the former governor of South Dakota sued Matthiessen and his publisher, Viking, for US$50 million in libel damages. Though ultimately decided in Matthiessen's favour,

the suits lasted for nine years.

Then president Bill Clinton read the book and in 2000 was reportedly planning to pardon Peltier, but demurred under FBI pressure. 'We thought we had nailed it down. I was very angry at the time.' Matthiessen remains in contact with Peltier. 'He's a very brave man. I've never heard a whine out of him. He's supposed to be a cop killer, but even his guards like him.'

Matthiessen has recently written articles about Alaskan Inuit and global warming, which he plans to turn into a book. 'The whole Arctic world is melting out from under their feet.'

As he pulls out of his driveway to take me to the bus, he points to an old giant willow that fell in a storm a few nights earlier.

The tree collapsed with geometric precision in the narrow space between his house and meditation garden, even missing the surrounding wires. On the turf of this environmentalist visionary, it seems like no accident.

Shadow Country: A New Rendering of the Watson Legend (The Modern Library, HK$320, in hardback)