Thursday, December 7, 2023

The land most likely to collapse under floods

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Report compares how pasture, indigenous planting and pine plantations stood up to Cyclone Gabrielle.
Floods and landslides displaced 300 million tonnes of soil, estimated to be worth $1.5 billion. The impact was most severe on land with pasture and exotic forest plantations. Photo: Gisborne District Council.
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Eyewatering metrics in a Manaaki Whenua Landcare Research report on Cyclone Gabrielle’s impact reveal the different impacts pasture, exotic plantation and native forestry planting had on land resilience in Hawke’s Bay and Tairāwhiti .

Just released, the rapid assessment report uses satellite imagery of land before and after the event, determining that the regions experienced 300,000 landslides, with an average landslide containing 1000t of soil.

The resulting 300 million tonnes of soil lost is estimated to be worth $1.5 billion. The impact was most severe on land with pasture and exotic forest plantations.

The report found a 90% reduced likelihood of land collapsing when covered in woody vegetation. 

But the outcome did depend upon the region being studied and type of vegetation.

In southern Hawke’s Bay, the likelihood of land collapse was reduced by 90% for indigenous forested area, and 80% for land covered with exotic forest. 

However, in northern Hawke’s Bay exotic forestry proved less effective, with a 60% reduction in collapse, against indigenous reducing risk by 90%.

Further north in the Tairāwhiti  region, however, the ability of exotic forestry to protect the land was even less at only 50% effective. 

Across northern Hawke’s Bay and Tairāwhiti, the report says, forestry management practices such as non-thinning of trees, thin soils, and multiple forestry rotations all contributed to a failure to hold land in place. 

In the Ngātapa-upper Wairoa catchment area, the likelihood of landslides in harvest forest was determined to be twice as likely as any other land cover. 

But the report’s findings were not limited to failings in forestry plantings alone, with the likelihood of landslides on 20-degree slopes greater on pastureland than on exotic forests in the Hawke’s Bay region. 

Further north in Gisborne, high producing grassland and exotic forest had similar likelihoods for land slippage. 

The report authors note that high producing grassland tended to be more susceptible than exotic forests to landslides, except in Gisborne coastal hill country.

The report’s authors have recommended more detailed field investigations to determine specific causes of forest plantation failure. 

But landslides in Tairāwhiti were found to be linked to land where plantations had been harvested, with those plantations being almost five times more likely to experience landslides compared to indigenous forest. That increased to more than 10 times the rate after the plantation had been harvested.

Of the 300 million tonnes of soil released, Esk Valley is estimated to have received about 6 million tonnes, with half of that dumped into waterways and 1.5 million tonnes laid on the river valley flood plain to an average depth of 80cm. 

According to the scientists’ models, if mature indigenous forest had been on that catchment’s highly erodible land areas, the mass of eroded soil would have reduced from 5.7 million tonnes to 4 million tonnes, with an average depth of sediment reduced by 30%, to 50cm.

The report only looked at satellite imagery that applied to land classes six and seven, which represent 70% of the land area in the affected regions.

Manu Caddie, spokesperson for Mana Taiao Tairāwhiti in Gisborne, said the research is a nail in the coffin for all unsustainable land use in the region, and highlights how specific Tairāwhiti-northern Hawke’s Bay  issues are.

“Crown policies supported land clearance over the past 150 years, now we need public policy that supports rapid restoration of permanent indigenous forest, with associated jobs and income to landowners.”

He said the results also challenge the current claims about transition forestry, where exotic plantings provide an initial income and cover for emerging native forests.

“The problems with that idea are that pine performs poorly compared to indigenous forests in terms of keeping the land in place. We can’t wait another 30 years, let alone 70-80 years, as some companies are proposing.”

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