Slip slidin' away

SCARRED EARTH: “Cyclone Bola was a real eye-opener, and really showed the vulnerability of land on the East Coast,” says Dr Mike Marden, who ran Landcare Research’s Gisborne office for 32 years solo.
This main image shows landslide damage in the Tauwhareparae ranges, inland from Tolaga Bay, caused by the 1988 cyclone. Picture supplied
MEASURING ROOTS: Dr Mike Marden and Dr Jagath Ekanayake measuring the dimensions of a kanuka root system in 1996. Picture supplied
SEDIMENT SOURCE: Wairongomai Gully, also known as Barton’s Gully, in the Tapuaeroa Catchment inland of Ruatoria, is an example of a gully that has been left untreated for too long. This gully now covers an area of 90 hectares and is the largest gully in this region, three times larger than Tarndale Gully.

THE East Coast is world famous for its erosion issues, and nobody can explain the reasons why better than Dr Mike Marden.

Any story, research project or community meeting on the issue, and his name is likely to come up. And with good reason.

For the past 32 years, Mike has been the solo helmsman of Landcare Research’s Gisborne office with the occasional technician for support. Mike retired last week, and the Awapuni Road office is going to close.

During Mike’s years here he has produced reports identifying the region’s erosion hot-spots, and has assessed the effectiveness of reforestation in “closing” gullies and preventing the initiation of shallow landslides.

A landmark study following Cyclone Bola in 1988 found that forested areas, whether exotic, native or mature scrub, had 90 percent fewer landslides compared to adjacent land in pasture. Their research also showed that where trees formed a closed canopy, this acted like an umbrella to keep about 30 percent of the moisture off the ground, while tree roots were able to hold the soil and soak up another 30 percent of the water — which meant soils under a forest cover were dryer for longer periods, and therefore were less likely to fail than those in farmland areas.

This was huge information for a region that had burned and chopped down almost all its native vegetation in the late 19th to early 20th centuries, and ever since has helplessly watched as invaluable topsoil washed into the sea.

Geologists visiting the region in the 1940s warned of the consequences if the soft, fine soil on jagged hills was left exposed, but it was not until the 1960s that the government, through the Forestry Service, began to fund the planting of radiata pine as a means of restoring stability to the severely-eroding hillsides.

Fifty years later, Mike says we have yet to get on top of the erosion issues facing this region and by now we should have done a lot more than we have.

“I would like to see everybody get on with the job, and treat the remaining areas known to be generating sediment.”

In 1992, after Cyclone Bola had devastated the region — destroying up to 60 percent of the pasture on some farms — the Government stepped up its reforestation efforts to prevent further soil loss on the most severely damaged parts of the region.

Mapping the land

By this stage Mike and his small crew had mapped out the most severely erosion-prone land and identified the location of 3160 gullies to target with planting.

There is a whopping $20-$30 million available through the Erosion Control Funding Programme to deal with this region’s erosion issues but it has to be spent in the next four years, before the project runs out.

Gisborne District Council has identified the “worst of the worst” eroding land as Land Overlay 3a and is encouraging landowners to take advantage of the funding available, but it is up to the landowners to do their bit.

“There has always been sufficient government money to get the job done, but it has never been fully utilised.”

While many landowners have taken advantage of the funding, Mike says the uptake has been slow and that is why there is still so much to be achieved.

“It is a real shame,” he says.

As of 1997, an estimated 2150 gullies remained untreated. In the meantime they had grown in size and continued to pump out more sediment than ever before.

“With each passing storm, the owner’s property becomes less productive.”

If left untreated for too long some gullies could end up the size of the world-famous Tarndale Slip, “a lost cause”.

Cyclone Bola should have been lesson enough for landowners with vulnerable land, he says. Some farms were so badly damaged they had to sell up to forestry.

While that was a one-in-70-year storm, with climate change the number of high-intensity storms is predicted to increase.

“There does not seem to be any long-term appreciation that cumulative damage to farmland by successive storms is a serious problem. The erosion issues and solutions to prevent it worsening were identified decades ago, and yet we are still dealing with the problem. How can that be?”

Northern Irish roots

Mike was born in Northern Ireland. His family made its way to New Zealand when he was five, with a haphazard stop over in Pakistan delaying his schooling two years.

They lived firstly in Taranaki, then Whangarei and finally settled in Whakatane, where he did most of his schooling.

He completed his geology and masters degrees at the University of Canterbury, before undertaking his PhD at Massey University in Palmerston North — studying erosion in the Ruahine Ranges.

In 1985 he landed a job at Landcare Research’s Gisborne office, then run by the Forest Research Institute, the science division of the New Zealand Forest Service, as its first appointed scientist.

He has never looked back.

Mike started researching earthflows, and gullies in particular, with the view to better understanding how and why forestry was so effective at closing them down.

Then Cyclone Bola hit in March 1988.

“That was a real eye-opener, and really showed the vulnerability of land on the East Coast.”

Bola provided lots of opportunities to gather new data, and before long international interest grew as well, for Bola was no ordinary storm and the East Coast geology is far from ordinary itself.

“The scale of the damage that occurred, from a single storm, across such a wide area, was likely unprecedented in New Zealand.”

On the first day, the rainfall was light and soaked into the soil. But by the fourth and fifth days it started to bucket down.

“The storm stalled over the Tauwhareparae area, and just sat there.”

Soon the land without vegetation was unable to soak up any more moisture, and large chunks of hillsides failed.

Many farms were absolutely devastated.

Afterwards, researchers from all around the world came to look at the East Coast’s erosion.

The Tarndale and Mangatu slips in the Waipaoa catchment, and Bartons’ gully in the Waiapu catchment, which generate significant amounts of sediment, became household names to scientists. There was even a big international study, funded by the US government, to understand where in a catchment sediment was generated and where in the ocean it ended up.

Important work

During this time Gisborne’s Landcare Research office produced some of its most important work, focused on understanding the rainfall that led to the initiation of shallow landslides, which occurred immediately during a storm, and earthflows, which reactivated almost a month later.

“Once the entire soil profile was saturated, the earthflows began to move slowly, like glaciers. We measured the rate of earthflow movement before and after planting with trees to see if the trees stopped these features moving, and in time they did. So, this proved to be an effective treatment for these features.

“Despite the success of forestry in restoring stability to once eroding hillsides, a lot of people are not happy with the current forestry situation — mainly because of the impacts harvesting has had on the environment. The public has been particularly concerned about the downstream impacts on stream ecology, and have complained about the copious amounts of forestry slash coming down the rivers and ending up on the beaches.”

While the root of the erosion problem is natural, it increased dramatically following deforestation to create more farmland.

“In hindsight we made a mistake cutting down all of the indigenous forest,” Mike says. “At that time there was no realisation that removing the forest cover was going to have such a disastrous effect.”

Not only was the land slipping away but the rivers were filling in with sediment.

“Up the Waipaoa River there used to be quite a narrow channel with large boulders and clean streams.”

By the 1940s the character of the river bed changed as sediment worked its way from the upper catchment downstream to the Poverty Bay Flats, and the river bed became choked with fine-grained silt and sand. In later years this prompted the building of stopbanks along the lower reaches of the Waipaoa River. Meanwhile, erosion in the headwaters was getting worse.

Not until the 1960s did the Forestry Service begin planting Mangatu Forest in an attempt to hold the sediment back.

“Replanting those steep, highly-erodible slopes with radiata pine was done for the right reasons.”

Areas planted early had a dramatic effect in closing-down the numerous small-sized gullies, while those gullies that had not been planted became bigger.

When it came to harvest the earliest of the planted forests, there were fears the land would again fall apart.

Mike’s team found there was a short period of about eight years following harvesting during which cutover was at greatest risk of falling apart — that is, until the new trees become well-established and had developed an extensive root system.

“So, much to our surprise, while there was some reactivation of old eroded sites, it didn’t fall apart. However, there are still a lot of problems associated with the harvest and post-harvest periods of short rotation species such as radiata pine.”

Tackling erosion

To tackle erosion, Mike believes there is still a long way to go before we match the right land type with the correct land use that is sustainable in the long term.

“A focus of current research is looking at changing land use, where its current use is considered to be unsustainable. Some of the options being considered include reversion to a native vegetation cover, planting manuka and kanuka, or changing from radiata pine to some other exotic like eucalyptus or redwood, which have longer lifecycles, so we are only disturbing the soil once every 60 to 70 years as opposed to once every 27 years.

“Already some of the more extremely-eroded forested areas are being retired and are reverting to native forest.

“But it is probably going to take another 100 to 200 years before we get the right mix of land uses on the right pieces of land.

“While the planting of manuka and kanuka to speed-up reversion is currently favoured for land marginally sustainable as forestry or farmland, we don’t really know how many years it will take before these plantings become an effective erosion control treatment.

“Is there a risk of increased erosion given that . . . the ‘window of vulnerability’ is going to be even longer than between successive rotations of pines? For the land’s sake, perhaps that is a risk we should be prepared to take.

“Also, how are we going to get kahikatea, rimu, totara back into the mix of plants? Are they going to come away naturally, or are we going to have to plant them?”

The seed source for many of these native species would still be in the soil but they can’t germinate because the thick mat of pine needles excluded light which they need to sprout.

“But during harvesting the needle duff gets broken up and bingo, the native plants will come up like hair on a dog’s back.”

Some of the badly-eroded sites will take longer to revert naturally, mainly because there is very little soil left. In other areas where there is still a good soil cover, reversion could be sped up by planting native species in the gaps.

Making a change

Planting manuka, for example, gets the land back into production for honey rather than producing trees.

“Change will take ages, but it will happen.

“There has also been a shift to looking at catchment well-being as a whole, which includes addressing social, ecological and economic issues, not just erosion.”

A Landcare Research team will be carrying out a research project on this very issue over the next three years, while also taking into account climate change impacts.

A similar project is under way in the Waiapu catchment.

“We need to start to see some big changes in land use in a reasonably short period of time. But when it comes down to it, it is the landowners who make the final decision.

“We don’t have organisations with the power to dictate what people can and can’t do with their parcel of land, even though the current occupiers are only custodians for a very short period of time.”

While Mike is retiring, he is not going anywhere so in the meantime will continue to assist his Landcare Research colleagues with ongoing contract work, and complete the writing-up of several research papers.

If the opportunity arises he would relish a chance to remap all the remaining untreated gullies, 20 years after his last study in 1997, to see how much progress has been made in targeting them for reforestation, and to see how many still require treatment.

“The custodians of this region need to see that these untreated gullies are treated within the next four years before the funding runs out.”

THE East Coast is world famous for its erosion issues, and nobody can explain the reasons why better than Dr Mike Marden.

Any story, research project or community meeting on the issue, and his name is likely to come up. And with good reason.

For the past 32 years, Mike has been the solo helmsman of Landcare Research’s Gisborne office with the occasional technician for support. Mike retired last week, and the Awapuni Road office is going to close.

During Mike’s years here he has produced reports identifying the region’s erosion hot-spots, and has assessed the effectiveness of reforestation in “closing” gullies and preventing the initiation of shallow landslides.

A landmark study following Cyclone Bola in 1988 found that forested areas, whether exotic, native or mature scrub, had 90 percent fewer landslides compared to adjacent land in pasture. Their research also showed that where trees formed a closed canopy, this acted like an umbrella to keep about 30 percent of the moisture off the ground, while tree roots were able to hold the soil and soak up another 30 percent of the water — which meant soils under a forest cover were dryer for longer periods, and therefore were less likely to fail than those in farmland areas.

This was huge information for a region that had burned and chopped down almost all its native vegetation in the late 19th to early 20th centuries, and ever since has helplessly watched as invaluable topsoil washed into the sea.

Geologists visiting the region in the 1940s warned of the consequences if the soft, fine soil on jagged hills was left exposed, but it was not until the 1960s that the government, through the Forestry Service, began to fund the planting of radiata pine as a means of restoring stability to the severely-eroding hillsides.

Fifty years later, Mike says we have yet to get on top of the erosion issues facing this region and by now we should have done a lot more than we have.

“I would like to see everybody get on with the job, and treat the remaining areas known to be generating sediment.”

In 1992, after Cyclone Bola had devastated the region — destroying up to 60 percent of the pasture on some farms — the Government stepped up its reforestation efforts to prevent further soil loss on the most severely damaged parts of the region.

Mapping the land

By this stage Mike and his small crew had mapped out the most severely erosion-prone land and identified the location of 3160 gullies to target with planting.

There is a whopping $20-$30 million available through the Erosion Control Funding Programme to deal with this region’s erosion issues but it has to be spent in the next four years, before the project runs out.

Gisborne District Council has identified the “worst of the worst” eroding land as Land Overlay 3a and is encouraging landowners to take advantage of the funding available, but it is up to the landowners to do their bit.

“There has always been sufficient government money to get the job done, but it has never been fully utilised.”

While many landowners have taken advantage of the funding, Mike says the uptake has been slow and that is why there is still so much to be achieved.

“It is a real shame,” he says.

As of 1997, an estimated 2150 gullies remained untreated. In the meantime they had grown in size and continued to pump out more sediment than ever before.

“With each passing storm, the owner’s property becomes less productive.”

If left untreated for too long some gullies could end up the size of the world-famous Tarndale Slip, “a lost cause”.

Cyclone Bola should have been lesson enough for landowners with vulnerable land, he says. Some farms were so badly damaged they had to sell up to forestry.

While that was a one-in-70-year storm, with climate change the number of high-intensity storms is predicted to increase.

“There does not seem to be any long-term appreciation that cumulative damage to farmland by successive storms is a serious problem. The erosion issues and solutions to prevent it worsening were identified decades ago, and yet we are still dealing with the problem. How can that be?”

Northern Irish roots

Mike was born in Northern Ireland. His family made its way to New Zealand when he was five, with a haphazard stop over in Pakistan delaying his schooling two years.

They lived firstly in Taranaki, then Whangarei and finally settled in Whakatane, where he did most of his schooling.

He completed his geology and masters degrees at the University of Canterbury, before undertaking his PhD at Massey University in Palmerston North — studying erosion in the Ruahine Ranges.

In 1985 he landed a job at Landcare Research’s Gisborne office, then run by the Forest Research Institute, the science division of the New Zealand Forest Service, as its first appointed scientist.

He has never looked back.

Mike started researching earthflows, and gullies in particular, with the view to better understanding how and why forestry was so effective at closing them down.

Then Cyclone Bola hit in March 1988.

“That was a real eye-opener, and really showed the vulnerability of land on the East Coast.”

Bola provided lots of opportunities to gather new data, and before long international interest grew as well, for Bola was no ordinary storm and the East Coast geology is far from ordinary itself.

“The scale of the damage that occurred, from a single storm, across such a wide area, was likely unprecedented in New Zealand.”

On the first day, the rainfall was light and soaked into the soil. But by the fourth and fifth days it started to bucket down.

“The storm stalled over the Tauwhareparae area, and just sat there.”

Soon the land without vegetation was unable to soak up any more moisture, and large chunks of hillsides failed.

Many farms were absolutely devastated.

Afterwards, researchers from all around the world came to look at the East Coast’s erosion.

The Tarndale and Mangatu slips in the Waipaoa catchment, and Bartons’ gully in the Waiapu catchment, which generate significant amounts of sediment, became household names to scientists. There was even a big international study, funded by the US government, to understand where in a catchment sediment was generated and where in the ocean it ended up.

Important work

During this time Gisborne’s Landcare Research office produced some of its most important work, focused on understanding the rainfall that led to the initiation of shallow landslides, which occurred immediately during a storm, and earthflows, which reactivated almost a month later.

“Once the entire soil profile was saturated, the earthflows began to move slowly, like glaciers. We measured the rate of earthflow movement before and after planting with trees to see if the trees stopped these features moving, and in time they did. So, this proved to be an effective treatment for these features.

“Despite the success of forestry in restoring stability to once eroding hillsides, a lot of people are not happy with the current forestry situation — mainly because of the impacts harvesting has had on the environment. The public has been particularly concerned about the downstream impacts on stream ecology, and have complained about the copious amounts of forestry slash coming down the rivers and ending up on the beaches.”

While the root of the erosion problem is natural, it increased dramatically following deforestation to create more farmland.

“In hindsight we made a mistake cutting down all of the indigenous forest,” Mike says. “At that time there was no realisation that removing the forest cover was going to have such a disastrous effect.”

Not only was the land slipping away but the rivers were filling in with sediment.

“Up the Waipaoa River there used to be quite a narrow channel with large boulders and clean streams.”

By the 1940s the character of the river bed changed as sediment worked its way from the upper catchment downstream to the Poverty Bay Flats, and the river bed became choked with fine-grained silt and sand. In later years this prompted the building of stopbanks along the lower reaches of the Waipaoa River. Meanwhile, erosion in the headwaters was getting worse.

Not until the 1960s did the Forestry Service begin planting Mangatu Forest in an attempt to hold the sediment back.

“Replanting those steep, highly-erodible slopes with radiata pine was done for the right reasons.”

Areas planted early had a dramatic effect in closing-down the numerous small-sized gullies, while those gullies that had not been planted became bigger.

When it came to harvest the earliest of the planted forests, there were fears the land would again fall apart.

Mike’s team found there was a short period of about eight years following harvesting during which cutover was at greatest risk of falling apart — that is, until the new trees become well-established and had developed an extensive root system.

“So, much to our surprise, while there was some reactivation of old eroded sites, it didn’t fall apart. However, there are still a lot of problems associated with the harvest and post-harvest periods of short rotation species such as radiata pine.”

Tackling erosion

To tackle erosion, Mike believes there is still a long way to go before we match the right land type with the correct land use that is sustainable in the long term.

“A focus of current research is looking at changing land use, where its current use is considered to be unsustainable. Some of the options being considered include reversion to a native vegetation cover, planting manuka and kanuka, or changing from radiata pine to some other exotic like eucalyptus or redwood, which have longer lifecycles, so we are only disturbing the soil once every 60 to 70 years as opposed to once every 27 years.

“Already some of the more extremely-eroded forested areas are being retired and are reverting to native forest.

“But it is probably going to take another 100 to 200 years before we get the right mix of land uses on the right pieces of land.

“While the planting of manuka and kanuka to speed-up reversion is currently favoured for land marginally sustainable as forestry or farmland, we don’t really know how many years it will take before these plantings become an effective erosion control treatment.

“Is there a risk of increased erosion given that . . . the ‘window of vulnerability’ is going to be even longer than between successive rotations of pines? For the land’s sake, perhaps that is a risk we should be prepared to take.

“Also, how are we going to get kahikatea, rimu, totara back into the mix of plants? Are they going to come away naturally, or are we going to have to plant them?”

The seed source for many of these native species would still be in the soil but they can’t germinate because the thick mat of pine needles excluded light which they need to sprout.

“But during harvesting the needle duff gets broken up and bingo, the native plants will come up like hair on a dog’s back.”

Some of the badly-eroded sites will take longer to revert naturally, mainly because there is very little soil left. In other areas where there is still a good soil cover, reversion could be sped up by planting native species in the gaps.

Making a change

Planting manuka, for example, gets the land back into production for honey rather than producing trees.

“Change will take ages, but it will happen.

“There has also been a shift to looking at catchment well-being as a whole, which includes addressing social, ecological and economic issues, not just erosion.”

A Landcare Research team will be carrying out a research project on this very issue over the next three years, while also taking into account climate change impacts.

A similar project is under way in the Waiapu catchment.

“We need to start to see some big changes in land use in a reasonably short period of time. But when it comes down to it, it is the landowners who make the final decision.

“We don’t have organisations with the power to dictate what people can and can’t do with their parcel of land, even though the current occupiers are only custodians for a very short period of time.”

While Mike is retiring, he is not going anywhere so in the meantime will continue to assist his Landcare Research colleagues with ongoing contract work, and complete the writing-up of several research papers.

If the opportunity arises he would relish a chance to remap all the remaining untreated gullies, 20 years after his last study in 1997, to see how much progress has been made in targeting them for reforestation, and to see how many still require treatment.

“The custodians of this region need to see that these untreated gullies are treated within the next four years before the funding runs out.”

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