Clarky's Comment - May 2011

The storms that swept the North Island on 27th April caused extensive damage to both forests and farmland. Windthrow in forests just north of Taupo is being harvested now. Trees were both uprooted and snapped off over a period of just a few hours intense wind. This would be the worst windthrow in that region since Cyclone Bola in 1988. But I suspect that damage was small relative to the loss of pasture from Hawke's Bay sheep and beef farms. That represents a loss that is much more difficult to recover from for the affected landowners, and will have long-term effects on soil productivity and farm profitability.

The current volatile weather, attributed to the Southern Oscillation (La Nina) has highlighted the exposure of New Zealand's steep country to erosion. But should we be surprised when we see slips and erosion taking place? Our country exists because we sit on the edge of the Australian and Antarctic Plates that are colliding with Pacific Plate. New Zealand's geological history can be divided into three main periods of sedimentation and three periods of mountain building dating back to about 500 million years ago. We exist in a constant state of erosion, sedimentation and uplift under plate collision. There is not much we can do about that.

What we can do is be aware of the risks and seek to mitigate or avoid acceleration of this natural process. We will not completely prevent erosion taking place.

There are three main negative effects of erosion that need to be recognised and managed. The first is the direct loss of fertile soil and the subsequent loss of productivity. The second is the sporadic degradation of waterways, wetland or estuary habitat. I say sporadic because much of New Zealand's ecology is well adapted to cope with occasional severe disturbances. Left to their own, such habitats can, and do, recover, sometimes quite quickly. We have evidence of fish life being largely unaffected within a few months of heavy sediment and debris loadings during intense rainstorms in the Eastern Bay of Plenty. If anyone doubts that a more complete ecological recovery can take place, including native bush on damaged stream banks, take a trip up the Kauaeranga Valley at the back of Thames. This river was the subjected to regular floods and sediment during the days of the Kauri logging dams.

A much more debilitating hazard for stream wildlife is continuously elevated nutrient and fine sediment loadings as often occurs from pastoral grazing of steep lands. By definition this creates environmental change. The Manawatu River is the best example of the aggregation of these effects over a large catchment.

The third effect is the build up of stream bed sediment and destruction of productive pasture and crops, and in some cases even buildings, that have been established on land that was formed as a result of previous erosion and sedimentation (i.e. most of New Zealand's lowlands).

Forestry has been blamed for depositing debris and mud on neighbouring lowlands in several regions of New Zealand over the past 12 months following intense rainstorms. But it is not forests nor logging that is the cause of the inherently unstable soil and rock geology we have in New Zealand. The soil and logging debris is certainly more at risk of movement soon after a logging operation, but growing forests and logging them has been shown in the Hawke's Bay Pakuratahi catchment study to provide better overall erosion protection than attempting to farm such lands.

Letting the land regenerate to native vegetation might sound attractive from an erosion perspective, but while this may reduce the probabilities, it will not eliminate the occurrence of damaging landslides and debris flows. Retiring land often results in the initial invasion of exotic weeds, including gorse, which offers limited erosion protection until native takes over in the longer term. It also begs the question of whether a small, mostly steep country that relies so heavily on its primary products for its income and standard of living can afford to completely abandon vast tracts of land.

Regional and district council planners must now be contemplating how they react to public concerns about sediment and debris emanating from steep exotic forest lands. To react too strongly will result in forestry becoming unattractive as a land use. With pastoral farming the only other practical productive use of such land and given its propensity for long-term erosion and soil productivity loss, it seems disingenuous, to me, to discourage forestry.

I put it to council planners that a better policy might be to encourage forestry on steep land, subject to operations being carried out in accordance with best practice guidelines. They should incorporate into their planning frameworks proper recognition of, and hazard management for, zones downstream of high energy steep land streams. Such responses, are needed to minimise inappropriate development and activities in high-risk catchments that are inevitably going to suffer periodic impact and, if the climatologists are right, at an increasing frequency.