Are pine plantations in peril?

Erosion solutions sought: There were 300,000 landslides during and after Cyclone Gabrielle. Pictured is one blocking Tiniroto Road. File picture

As the Government prepares to respond to a raft of recommended changes to land use and forestry in this region by the end of this month, one Ruatōria-based environmental group says a new report published this week could prove a decisive factor . . .

A new report on the performance of different land cover during Cyclone Gabrielle is being hailed

by sustainable land use advocates Mana Taiao Tairāwhiti as the nail in the coffin for pine plantations and pasture on most land in Tairāwhiti.

Manaaki Whenua Landcare Research, the Crown Research Institute focused on land use and biodiversity, this week released the report revealing significant differences across Tairāwhiti and Hawke’s Bay. Commissioned by the Ministry for the Environment, the research has been welcomed by Mana Taiao Tairāwhiti.

“Indigenous forest — including broadleaved hardwoods along with mānuka and kānuka — held the land together at such a rate it barely compares with standing pine, let alone harvested plantations and bare pasture,” said spokesperson Manu Caddie.

More than 300,000 landslides were recorded by the research using satellite imagery from before and after Cyclone Gabrielle. The average landslide contained a thousand tonnes of soil.

The report authors note that: “This proliferation of landsliding has removed productive soil from farms and deposited sediment on floodplains. The total mass of landslides is estimated at 300 million tonnes, with an economic cost of approximately

$1.5 billion (conservatively estimated at

$5 per tonne of eroded soil).”

Exotic plantations were disproportionately low as a source of landslides in the five southern districts but were disproportionately high in the two northern districts (Wairoa and Tairāwhiti). Only minor amounts of landsliding occurred on areas covered with indigenous forest.

The report suggests possible causes for the low effectiveness of exotic forestry for reducing landslide probability include: forestry management such as non-thinning; multiple rotations of forestry; and thin soils caused by a long erosion history.

Landslides in Tairāwhiti were most prevalent on sites where pine plantations had been harvested, suggesting the shallow roots of pine trees on thin soils are a poor combination, Mr Caddie says.

Pine plantations in the region had around five times the probability of landslides compared to indigenous forest, and that increased to more than 10 times the rate after the plantation had been harvested.

“These results highlight the resistance of indigenous forest to landsliding and the comparative susceptibility of harvested forest across all slope types. High producing grassland tended to be more susceptible than exotic forest except in Gisborne coastal hill country.”

Mr Caddie says while the research has some technical limitations, overall it provides some of the best evidence to date that erosion-prone land needs to be urgently recloaked in permanent indigenous forest.

“The research really seems like the nail in the coffin for the unsustainable land use in the region — it highlights how specific the issues are to Tairāwhiti and the northern Hawke’s Bay regions, and why changes at a massive scale are urgently required.

“Crown policies supported land clearances for farming and pine plantations 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.

“Some pine promoters are doing deals for carbon credits off pine plantations with promises that these will eventually transition to diverse indigenous forest. 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.”

Mana Taiao Tairāwhiti is currently working on a submission to the Emissions Trading Scheme Review and the ETS Permanent Forestry Category Redesign. The group will make its submission available on its website by the end of July, for the consultation deadline of August 11.

“These are really important policies that are driving some crazy behaviour by businesses trying to make a lot of money from carbon farming. The ETS was supposed to help us reduce emissions; instead it is encouraging businesses and consumers to continue emitting while the country gets covered in pine trees.

“Diverse indigenous forest sequesters more carbon in the long term and should be prioritised along with actually reducing gross emissions.”

Titled “Rapid Assessment of Land Damage — Cyclone Gabrielle”, the report presented several possible causes for the low effectiveness of exotic forestry for reducing landslide probability in northern Hawke’s Bay and Gisborne.

Those comprised:

■  Forestry management such as

non- thinning

■  Multiple rotations of forestry

■  Thin soils caused by a long erosion history.

However, the report recommended more detailed field investigations to determine specific causes.

“The physical mechanism for landslide initiation is well understood,” the report stated.

“Intense rainfall increases the pore water pressure in the soil, which reduces the effective weight of soil at the failure plane between soil and regolith. On steep hill slopes this often results in shear stress exceeding shear strength, causing slope failure. If there is woody vegetation growing on the soil, then roots growing through the soil-regolith boundary will increase the shear strength and reduce the probability of failure (8-12).

“These mechanisms are generally sufficient to explain the spatial distribution of landslides in Cyclone Gabrielle; that is, landslides occur mostly where intense rainfall has fallen on steep land without protective forest cover.

“The reduction in landslide probability

by woody vegetation is modelled at

90 percent by commonly used regional

soil erosion models (5,6). In southern Hawke’s Bay-northern Wairarapa hill country, this expected reduction was

largely observed for both indigenous forest (90 percent reduction) and exotic forest (80 percent reduction).

“However, in northern Hawke’s Bay, exotic forestry was less effective than expected (60 percent), while indigenous forest maintained normal reduction (90 percent). In the Gisborne coastal hill country exotic forestry was ineffective at reducing landslide probability, with indigenous forest having only a moderate reduction (50 percent).”

The report concluded:

“In the Esk River catchment, approximately 5.7 million tonnes of soil was eroded by landslides. Half of that was delivered to waterways, of which approximately 1.5 million tonnes was deposited on the flood plain of the Esk River valley at an average depth of 80cm.

“If soil conservation plans had been implemented on 50 percent of pastoral farms in the Esk catchment, the soil eroded by landslides would have been less, at

5.3 million tonnes. If soil conservation plans had been implemented on 80 percent of pastoral farms in the Esk catchment, the soil eroded by landslides would have been even less at 4.7 million tonnes, and the sediment deposited on the Esk valley floodplain would have been less at 60cm average depth (these scenario estimates assume that currently 30 percent of farms have implemented soil conservation plans).”