The majority of poplar and willow are not planted in locations that stop landslide erosion

Blog post.

Tom Mackay-Smith and Raphael Spiekermann

7/4/20242 min read

This graph from Raphael's research is really interesting.

After spatially mapping trees and landslides over an 84,000 hectare area in the Wairarapa, the graph indicates most landslides occur on slopes between 25° to 40°. However, most poplar and willows are growing on slopes below 25°.

Graph indicating the degree of slop where most landslide occur in Wairarapa.
Graph indicating the degree of slop where most landslide occur in Wairarapa.

Many of these trees will be poplars and willows planted on creeks, gullies and riverbanks, as well as willow and poplars planted on less steep slopes to increase survival.

But many will also be poplars and willows planted in the wrong place for reducing sediment delivery to streams.

The images below show this effect:

The first image shows the landslide susceptibility where the stabilising effect of trees has been removed. The second shows a landslide susceptibility prediction with the influence of individual trees. The third is the underlying map of the trees.

Despite the study finding poplar and willows to be highly effective at reducing landslide erosion, many of the trees planted in this image are doing very little for landslide erosion because they are located where landslides are highly unlikely to occur – with or without trees present. Except the eucalyptus grove on the right.

Three images demonstrating the land stabilizing effect of trees
Three images demonstrating the land stabilizing effect of trees

We have produced a couple of videos explaining the research in more detail for those interested:

- One of these videos quantifies the effect of silvopasture trees on landslide erosion.

- Another describes a sediment connectivity model and discusses the concept of targeted erosion and sediment control.

Contact info

0225905367

tom@verdantiaresearch.co.nz