Pohutukawa – Holding It All Together

Eastern Bay of Plenty residents can thank the pohutukawa in their area for reducing slips along the coastal and harbour cliffs, according to a recent study into landslides in the area.

Project Crimson, a charitable trust established 13 years ago to protect pohutukawa and rata in New Zealand, invited its science advisor and Trustee Dr. Gordon Hosking, to assess the relationship between slips and pohutukawa, following the July 2004 floods in the eastern Bay of Plenty.

The objective of the study was to clarify the association, if any, between pohutukawa and the numerous slips along coastal and harbour cliffs from Matata to Te Kaha, by comparing vegetation on slip areas with the vegetation on non slip areas either side. The study followed community concerns about the safety of pohutukawa along this popular coastline.

The findings show pohutukawa are much more common on the areas that were not affected by slips and are in fact the best mechanism for protecting New Zealand coastlines from slips. The most common factor associated with the slips was lack of coastal pohutukawa forest and change in surrounding land use.

Where land was modified for pasture or residential purposes, slips were more likely than if the land was not modified. In these modified areas the vegetation had been altered or removed altogether, and there was evidence of water runoff from above.

“Hopefully what will come of this study is a view from the local people that pohutukawa are essential in reducing landslides in the district”, says Gordon Hosking, forestry health consultant and science advisor for the Project Crimson Trust.

“Pohutukawa act as stabilisers of the soil and are very useful in a fragile geological area such as the eastern Bay of Plenty. Planting pohutukawa on colluvial slopes and along cliff edges would almost certainly reduce slippage.

“For centuries these trees have held this fragile coastline together. It is only with the change in land use that landslides in coastal areas have increased.

“The greatest thing to come from this study is that we can do something useful to limit slips in the future. Being aware of what contributes to landslides and what reduces the likelihood of them, will help us manage these coastal cliffs in the future.

Slip assessments were grouped in five general areas:

Coastal cliffs between Otamarakau and Matata (27 sites)

Cliff above Ohope west (1 site)

Ohiwa Harbour edge (7 sites)

Waiotahi Beach (15 sites)

Opotiki north to te Kaha (13 sites)

A full copy of the study is available on http://www.projectcrimson.org.nz

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