New Zealand's soil is being washed away at almost 10 times the world's average rate. Science reporter SIMON COLLINS begins a six-part series on the problem and the solutions.
A massive slip at Tarndale, on the East Cape, is carving its way through the region - and its people.
For 27 generations, the Ngariki Kaiputahi whanau claims to have lived in the steep, mist-covered hill country inland from Gisborne known as the Mangatu blocks.
A few years ago, says Kupu Lloyd, the land supported 18 large farm stations, each employing several workers.
Today, there are just three or four stations. And up the Tarndale Rd, in the heart of the Mangatu blocks, just two farmhouses are still permanently inhabited: Arowhana Station, which Mr Lloyd manages for an owner who lives at Pukekohe, and a house down the road used by a casual farm worker, James Green.
This year the road was closed as the huge Tarndale slip, which has been chiselling its way up the gully since about 1915, finally undermined the narrow ridge that separates it from the Mangatu slip in the next gully over.
Now Mr Green can no longer drive out to work on other farms, except by taking a long way around down a rough forestry track and across a ford.
"I'm lucky Kupu's here," he says. Arowhana provides about the only casual employment around.
Mr Green's son Kane, aged 8, is "sort of on correspondence". The school bus run that used to go down Tarndale Rd to Waipaoa School has ended. The school is down to just 14 pupils.
Mr Lloyd hopes that a Ngariki Kaiputahi claim to the Waitangi Tribunal, heard recently as part of the tribunal's combined Gisborne hearings, may result in land being restored to the whanau so that it can become self-sufficient again.
"It would be good to see people back on the land," he says.
But for now, rural depopulation is more common, not just in the Mangatu blocks but throughout the slip-scarred East Coast.
The social decline reflects the loss of the soil which sustains life - a thin, biologically and chemically rich layer of the earth's surface which is as vital as water and air.
It takes between 100 and 400 years for nature to create a single centimetre of topsoil, and between 3000 and 12,000 years to develop enough soil to form productive land over the hard rock that underlies most soils.
But in a Hawkes Bay catchment just south of Gisborne, Landcare scientists estimate that about a quarter of the soil that was present when Europeans started farming the land in the 1870s has now been washed into the rivers.
On intensively farmed land at Pukekohe, south of Auckland, large areas have lost the equivalent of all their original topsoil. Their present topsoil has been created out of the subsoil with heavy use of fertiliser which is leaching into local groundwater.
Landcare's Dr Aleksey Sidorchuk says the 210 million tonnes of soil pouring into New Zealand's lakes and oceans each year represent 1.5 per cent of the world's total erosion from just 0.18 per cent of its land area - giving this country an erosion rate almost 10 times the world average.
A vicious circle is under way, as shallow soils cannot absorb as much water as deeper soils, causing increased surface runoff with more risk of gullying.
The organic material in the soil that is washed into the sea is roughly as big a source of carbon as the carbon dioxide emissions from New Zealand's cars and industry, helping to trap the sun's heat and causing global warming.
Warmer temperatures, in turn, are expected to bring more cyclonic storms such as Cyclone Bola, which devastated the East Coast in 1988.
"Under such conditions, erosion rates will increase," says another Landcare scientist, Dr Mike Page.
The heroic era when European settlers hacked and burned the bush off more than half of New Zealand's land area in the years before World War I has long since ended in tears.
Jeremy Williams, the great-grandson of a nephew of the missionary Samuel Williams who came to the East Coast in the 1870s, said his ancestors didn't realise how fragile the soil was in the valley west of Ruatoria where he still farms.
"They went to the land that was the most fertile. They made millions," he says.
"They felled the biggest trees, split them for fenceposts and buried them in pits. Then they cut the undergrowth, cut a wedge in the trees to cut them down ... burned the forest.
"They had a million years of humus. They had unbelievable stocking levels - and back then one bale of wool would buy you a new truck!"
Soil consultants Dr Doug Hicks and Dr Frank Gibbons say signs of trouble appeared as early as the 1880s, when rabbits began eating the sparse remaining cover on the South Island's tussock grasslands. High-country sheep numbers fell by 20 to 50 per cent in many districts in the last 20 years before 1900.
In the North Island, farmers walked off some of the worst eroding hills in inland Taranaki in the Depression of the 1930s.
There was another boom on the remaining farmland after World War II, when fertiliser topdressing allowed hill-country farmers to boost livestock numbers from typically three to five a hectare to 10 or more.
But again, many over-reached themselves and their land began to slip. Livestock numbers plummeted again when subsidies were axed in the 1980s, and many hill-country farmers have had to come to terms with the inevitable: trees.
Jeremy Williams has now sold 2000ha of the land his ancestors cleared to investors who have planted it in pine trees. His farm is reduced to 850ha, and even on that he has planted 340ha in forests.
The Gisborne-East Cape region is changing more than any other because it accounts for a full third of all the NZ soil washed into the sea.
The South Island's steep, rain-drenched West Coast accounts for another third, but it is largely natural erosion on land which is still in native bush.
By contrast, the Waikato River delivers less than 1 per cent of our soil losses, Auckland-Northland 2 per cent and the Bay of Plenty-Coromandel 7 per cent.
Gisborne soil conservator Trevor Freeman says the irony is that the East Coast land most prone to slide is not the steepest, rock-studded backbone range that is closest to the eastern Bay of Plenty coast and still mostly in bush.
"The land that was cleared for farming is the rotten hill country, because of the [less steep] topography and because of the naturally high fertility," he says. "The rock itself is fertile, not just the soil."
But the fertile rock is also soft, making whole hillsides liable to slip into the gullies - a form of deep-seated mass movement, or slump, which is quite different from the more obvious surface scars or slips.
In the Gisborne district, Dr Hicks has calculated that 41 per cent of the farmland is susceptible to slump and 68 per cent to slips - on both counts almost twice the proportion of erodible land in any other region.
The Waiapu River, draining the northern part of the district, dumps 20,520 tonnes of soil, gravel and weathered rock into the sea every year for every square kilometre of its catchment. That ranks as one of the highest erosion rates ever recorded anywhere in the world.
And that is only part of the problem, because much of the soil and gravel that slides off the hills has not yet reached the sea, but has simply built up in the gullies.
Below the Tarndale and Mangatu slips in the upper reaches of the Waipaoa catchment, local boys ride horses across a flat expanse of gravel, several metres above what used to be the steep valley floor. Mr Lloyd points to the site of a former marae, now buried.
In the Waiapu catchment, rubble has piled up under bridges, and eventually on top of them. Higher replacement bridges are now being buried too.
The entire lower length of the Waipaoa from Te Karaka to the coast near Gisborne has had to be stopbanked to prevent the raised riverbed from overflowing on to the vineyards, market gardens and fertile pastures of the Poverty Bay flats.
Landcare's Gisborne scientist, Dr Mike Marden, says: "We have so much sediment stored within our stream systems already that that material is going to be coming down the rivers for 100 years-plus."
But practices are slowly changing. The Forest Service began planting trees to stabilise the hills in 1960 and had planted 30,000ha in the district by the time it was disbanded in 1987. Following the market theories of the time, the Labour Government walked away from further subsidies.
But just a few months later, Cyclone Bola hit. Labour relented and agreed to subsidise a five-year conservation forestry scheme run by the Gisborne District Council at the southern end of the district only.
In 1992, the new National Government took over the scheme, renamed it the East Coast Forestry Project and extended it to the whole district. The Government now gives out $2 million to $3 million a year in partial subsidies to landowners with the most severely erodible "target land".
Manager Randolph Hambling says the project has funded 30,000ha of new forests in its first 10 years, but a study in 2000 found 60,000ha of target land were still unprotected.
In total, exotic forests now cover 153,000ha, or about 17 per cent of the region.
About the same amount is still in native bush, and 11 per cent has reverted to scrub. But just over half of the region is still in pasture.
In the wake of Bola, Dr Hicks studied 465 sites in the Waipaoa catchment in 1989 and estimated that 80 per cent of the hill country in the Waihora sub-catchment, and 100 per cent of the Waimata sub-catchment, needed soil conservation measures.
Only 49 per cent of Waihora and 52 per cent of Waimata actually had partial or complete forest protection.
"In the Waimata catchment right up in the headwaters, virtually nothing is stable," Dr Hicks says.
"That particular area was fast being converted to forestry at the time of the survey. If you go back there today, you'd find that most of that 48 per cent where planting was absent has since gone into trees."
New Zealand now has 1.8 million hectares of exotic forests, most of it planted for commercial rather than conservation reasons.
By contrast, Dr Hicks says, Japan has planted 10 million hectares in exotic forests since the 1940s, covering most of its erodible hills.
"What has gone on in New Zealand pales into insignificance compared with the extent of the planting Japan has done," he says.
"But I think some recognition needs to be given to the fact that for the past 50 years or so, some of us have been reasonably active planting up those deforested hillsides."
The progress is impressive. But for big gullies like the Tarndale, says Dr Marden, it is too late.
"There's no way in the world that this hole can be fixed."
* Monday: The invading sea.