Erosion

Cause or Contribution

I recall when I was at school we were taught about erosion. That was a particular problem in the Gisborne and Hawkes Bay areas. Much of the problem was exacerbated by the removal of ground cover and of trees which lent a certain stability to the land.

Tree roots mechanically reinforce soil, increasing its shear strength (“root cohesion”) and helping soil layers act more like a single, stronger mass. Large roots can anchor through the soil into weathered rock, tying the shallow soil mantle to more stable material beneath and across potential slip planes.

Foliage and litter intercept rainfall and soften raindrop impact, which otherwise breaks up soil aggregates and starts surface erosion. Roots and organic matter improve soil structure and porosity, which increases infiltration and reduces rapid surface runoff that can trigger shallow failures.

Once trees are cut, root systems begin to decay over several years; as root biomass and strength decline, the extra shear strength they provided drops markedly.

Studies of clear-felled forests show that steep slopes can become significantly less stable as root strength is lost, with higher rates of shallow landslides and soil slips.

Loss of canopy and litter exposes soil directly to rain, increasing surface sealing, runoff, and hillslope erosion, which can evolve into rills, gullies, and local slope failures.

There are factors which impact upon or modify that effect.

Slope angle and geology: Steep slopes, weak bedrock, or thick loose colluvium are more sensitive to tree removal than gentle slopes on strong rock.

Soil depth: Where the failure surface is shallow (about 1–1.5 m), tree roots often penetrate to that depth and their loss has a larger impact on stability.

Rainfall regime: High or intense rainfall after deforestation increases pore-water pressures and runoff, which, without root reinforcement, can trigger landslides.

Time since removal: Immediately after felling, some stabilising effect remains; the most critical period is typically a few years later, when roots have substantially decayed but the slope has not yet adjusted.

In summary, tree removal from hillsides usually decreases ground stability over the medium term, primarily because decaying roots no longer reinforce the soil and the bare surface becomes more vulnerable to erosion and landslides.

This then leads to an issue that lawyers love. Is tree removal causative of erosion or is it contributory to erosion? And the lawyer’s answer to that is “it depend.”

Tree removal can be both causative and contributory, depending on the context, but in most hillslope cases it is better described as a contributory factor that can become effectively causative on already marginally stable ground.

From a causative perspective landslide usually results from several necessary conditions together: weak material, slope angle, and water pressures; tree removal does not create these but can tip them past a threshold.

After clear‑felling, the rainfall intensity and duration needed to trigger landslides can drop to about half of that required in forested areas, so events that would not previously fail the slope now do so; in this sense vegetation removal is a proximate causal change in the system.

The argument for contributory is clearer. Roots add measurable “apparent cohesion” and tensile reinforcement to the soil mantle; when trees are removed and roots decay, that reinforcement declines, increasing susceptibility but not guaranteeing failure.

National and international syntheses (including New Zealand work) describe vegetation removal as increasing landslide susceptibility for years to decades after clearing, making slopes more likely to fail under storms that they might otherwise have withstood.

Context and circumstances are everything. On a robust, gently sloping site, removing trees may only modestly increase risk, so it is contributory at most and rarely determinative.

On steep or weak hill country close to a stability threshold, the loss of root reinforcement and hydrological buffering after tree removal can be the decisive factor that allows a particular rainfall or earthquake to produce failure, and is therefore reasonably described as causative in relation to that event.

The recent tragedy at Mount Maunganui has generated a considerable amount of discussion especially around the issue of tree removal around the site of the landslip. The loss of life has been tragic and sympathy and condolences must go out to those who have lost loved ones.

But at the same there is a place for a discussion about how the landslip happened. Some of the discussion has revolved around decisions of the local authority and the part played by iwi in developing a policy surrounding tree removal.

It is a fact that there is a body of opinion that would like to see exotic or introduced trees removed from certain areas – a sort of vegetative decolonization process. This is happening in a number of areas including where I holidayed over summer. Some council employed arborists sallied forth into a nearby bush (on a steep sandstone slope) to remove exotic trees.

Irrespective of whether or not tree removal is part of a vegetative decolonization process at the behest of iwi representatives on local authorities or whether it is part of a vegetation management programme it is clear that tree removal potentially may, in certain circumstances, be causative of erosion and there is little doubt that it is contributory to erosion and slips.

Thus it ill-behoves the Prime Minister to try and shut down the discussion around the causes of the landslide by characterizing some of the views expressed as “misinformation.”

The news reports have him saying:

“I’m aware there’s a lot of misinformation and stuff going on out there, there always is in circumstances like this.

Some members of the public have speculated whether the removal of trees on Mauao in recent years for the protection of culturally significant sites and for restoration purposes contributed to last week’s slip. Mauao is co-managed by the local council and iwi groups…

The people on the margins with their rhetoric, they just need to, frankly, keep it to themselves.”

The problem here is that Prime Minister Luxon is falling into the generalizing trap that was so amply demonstrated by his predecessor, Jacinda Ardern. He was critical of those spreading “misinformation” as he called it but failed to define precisely what the misinformation actually was.

Of course misinformation in the current milieu is a buzzword that captures a number of sins and often it is necessary to say exactly what is encompassed by the term. But what the PM was trying to do was shut down debate and in doing so was interfering with freedom of expression. Clearly he was buying time before an “official narrative” had developed.

It may have been better for him to have been concerned about the “speculation” that was going on about the cause of the slip. There can be no doubt that in the minds of many causation and contributory have a tendency to merge and that can create difficulties. And the PM did refer to speculation but it would have been better for him to emphasise that rather than lean heavily on the misinformation angle and attempt to shut down debate

But the matter develops further – the narrative becomes clearer - and in an article in the Herald under the headline “Mount Maunganui ‘misinformation’: Did cutting down trees really cause the landslide?” a disdainful tone is adopted. Dr Tom Robinson suggests in the article that the slope was going to fail regardless of whether trees had been removed from the mountain.

The article states

“In the wake of last Thursday’s devastating landslip – which saw a fast-moving pile of dirt and other debris bury six campers – a succession of theories have been explored and spread on social media claiming the slope failed because numerous large exotic trees had been removed from the slopes of Mauao.

In 2023, the Tauranga City Council (TCC) removed eight of them – all located on the same side of the mountain as the campground. It said they were “deteriorating, damaging” significant archaeological terraces.

A year earlier, several other exotics were also removed.”

Then the disdain creeps in

“Critics – most without geological science backgrounds – have claimed the actions of TCC and local iwi who own Mauao are now partly responsible for the deadly slip that has now occurred.

And Robinson – whose work involves a broad range of techniques including geospatial modelling, scenario modelling, risk analysis and statistical analyses – told the Herald that any slip site should be considered susceptible to more.”

But the problem lies in the headline – causation.

It is unlikely that the removal of the trees caused the slip. But it is not beyond the realm of probability that the removal of the trees contributed to the slip. Certainly the land had experienced similar problems in the past but that is not to say that the presence of trees may not have mitigated to extent and scope of the erosion.

No doubt further details will come to hand as investigations proceed. But the Prime Minister should not have adopted the Ardern default position of describing discussion as misinformation. And it displayed a woeful ignorance of the fact that erosion and landslips have a number of factors present. The removal of trees may be one of those contributing factors. But more importantly the PM’s attitude demonstrated a level of arrogance directed towards those exercising their right to freedom of expression. And that must be a concern.