Poplars: The Choice for Land Stability

Major weather events have shown that tree planting significantly reduces slipping damage. Poplars, with their incredible root networks, are a logical choice for farmers managing erosion-prone land.

You’d be hard pressed to find a better reason to protect pastoral soils than Cyclone Gabrielle.

Hundreds of millimetres of rain in only 24 hours doused Gisborne and Hawke’s Bay hill country in February 2023, with hillsides sliding into oblivion.

Where trees had been left to their own devices and not managed – say wilding trees on steep slopes above rivers – there was a huge amount of damage. 

And there’s no denying that tree debris from these river corridors made flooding much, much worse.

But looking outside of those river zones, on pastoral landscapes in medium to steeper hill country, the slipping damage was significantly reduced where trees had been planted and managed. 

Land slipped outside the planted areas – often above – but in the majority of cases erosion was significantly reduced by the presence of trees. 

A major research project published in 2008 by AgResearch found poplars, willows and eucalypts reduced slips by an average of 95% compared with slips on unplanted sites. This work was on slopes of around 27 degrees.

“Spaced trees dramatically reduced the incidence and severity of soil slippage on erodible slopes, and they were even more effective when their diameter at breast height exceeded 30cm,” the researchers found.

The researchers looked at 65 sites in Manawatū and Wairarapa after storms in February 2004 and July 2006.  Most of these sites (53) had poplars planted, while six had willows and six had eucalypts. 

They recommended a 13-18m spacing as being very effective.  That’s 30 to 60 stems a hectare.

If this spacing is reduced to a spacing of 10m apart, the carbon box is also ticked with poplars.

Another study, by Hicks and Crippen, looked at vegetation after the Manawatū-Rangitikei storm in February 2004. This data was used later to show that spaced trees possibly reduced erosion by 34% compared with under open pasture.

It’s the incredible root network that poplars grow which is the key to increasing slope stability.

A recent Gisborne field trial showed a three-year-old Veronese poplar with a root system about 8m wide and 5-6m deep. The scientists said out on the hills it would take eight to 10 years to reach this size. 

Another paired catchment study at Pakuratahi, just north of Napier, looked at sediment loss in a pastoral landscape, and in a forested landscape. This work, which is about to be repeated, and another Tutira study showed sediment loss is 1.5 to 17 times greater on land without trees. 

With Hawke’s Bay alone having 250,000 hectares of land identified as erosion prone, poplars are a logical choice for farmers who want to keep farming.

They also have a range of other benefits: providing shade and shelter, and as a fodder source during dry periods. 

An AgResearch trial in central Hawke’s Bay measuring temperatures and grazing activity of Angus cows found they grazed for another 30 minutes a day when offered shade. Their body temperatures reduced and their condition improved if shade was present in their paddocks. 

The key to successfully integrating poplars into pastoral landscapes is to protect them well from cattle when the trees are young. Cattle damage is a main reason why newly planted poplars die.

Traditionally the main method of planting poplars has been to ram in large heavy poplar poles and cover them in a sleeve.

However, rooted cuttings are a good alternative, particularly on slightly drier sites, and where hotwires can be used to exclude stock. 

Rooted cuttings are lighter to move around the hills, easier to plant, and they cost less than poles. They can also be planted over a wider period of time than poles.

A combination of sleeves and hotwires is a good way to protect these trees. 

Well-managed poplars are a cost-effective way to keep pastoral soils on hill country and enable farming to continue. 

The study finding a 95% reduction in slips was an AgResearch Client Report from June 2008, The Effectiveness of Space-Planted Trees for Controlling Soil Slippage on Pastoral Hill Country. It was written by Douglas, McIvor, Manderson, Todd, Braaksma and Gray.  It is available on the NZ Poplar and Willow Research Trust website.