This fact sheet is based on research from Scion. It looks at the benefits of planting trees on unstable land.
Why plant trees on erodible land?
Plantation forestry can potentially reduce soil erosion by up to 95%.
Benefits of planting trees on vulnerable and unstable landscapes include:
reductions in soil erosion
storing carbon dioxide from the atmosphere (carbon sequestration)
environmental benefits, like cleaner water and increased biodiversity
reduced damage to downstream land and infrastructure.
Trees protect landscapes from erosion, especially during severe storms, by:
providing a canopy that intercepts rainfall and reduces the amount of water in the soil
having roots that bind the soil together, making it less likely to wash away.
Types of erosion you might see on your land
Rill erosion
Rill erosion happens when water from rainfall doesn’t soak into the soil, but runs across it instead.
Gully erosion
Gully erosion is caused by flowing surface water.
Sheet erosion
Sheet erosion happens when rain falls on bare or sparsely covered soil, loosening fine particles that are carried downhill in surface runoff.
Soil slip
Soil slip – shallow landslides after heavy rain.
Earthflow erosion
Earthflow - large slow-moving slips.
Land type and forestry
As land gets steeper it tends to have:
decreasing versatility
increasing vulnerability to erosion
declining soil quality
fewer ways to help prevent erosion.
Climate conditions at higher altitudes become harsher and less suitable for growing grass or trees.
Types of erodible land and options for use
Land with less limitations
Type of erodible land:
Alluvial valleys or terraces (where loose clay, silt, sand or gravel has been deposited by running water), fertile, lower altitude.
Generally rolling to steep land on hard geology, fertile & lower altitude.
Rolling to steep land, prone to some forms of sheet erosion, rill or gully erosion.
Potential for forestry:
For these types of land erosion and sedimentation are relatively low.
Forestry is likely possible to protect or improve water quality through modifications to existing land use practices, with or without the support of small-scale plantings of a range of species.
Larger scales of planting would come down to cost and landowner objectives.
Land with some limitations
Type of erodible land:
Land with various features which has some climate, altitude and erosion limitations.
Moderate to steep landforms that are prone to soil slip or sheet and gully erosion under pasture.
Limited productivity under grazing on steeper parts, and prone to gully erosion.
Potential for forestry:
These types of land are increasingly limited by:
steepness climate
lower soil fertility
increased vulnerability to erosion.
There are fewer species options (commercial and semi-commercial) as the site gets steeper. You would need to match species to conditions like soil depth.
Some commercial forestry is possible but is limited by the constraints of land.
Land with more limitations
Type of erodible land:
Generally steep uplands subject to high rates of natural or induced erosion.
Potential for forestry:
Land highly likely to erode away, especially under grass cover.
Soils are deeper, and erosion is slow moving, so depending on other land constraints, wide scale tree planting options can work in these areas.
Type of erodible land:
Limited productivity under grazing, with skeletal (thin) soils on steep land.
Vulnerable to debris flow/debris avalanche.
Potential for forestry:
Generally unsuited to large scale conversion for timber harvesting because it's:
highly likely to fail
highly vulnerable to debris flow (where water-laden masses of soil and bits of rock rush down mountainsides and funnel into stream channels) and generation of debris floods (water carrying debris like rocks, soil and trees)
these may happen more often with climate change.
Reversion, honey or native vegetation enhancement should be seriously considered in much of this group, except outwash slopes (left over from glaciers).
Type of erodible land:
Generally moderate to rolling hill country subject to deep seated mass earthflow erosion (slow moving landslides).
Potential for forestry:
Broadly non-productive, high altitude and highly eroding landscapes.
Good to consider adding native vegetation cover where practical.
Limitations of planting erodible land
After harvest, there’s a 6-8 year timeframe where land is vulnerable to erosion. This happens until a planted crop grows up and the canopy (the branches and leaves you see when you look up in a forest) closes.
Some vulnerable landscapes at risk from extreme and/or frequent storms will always struggle to maintain commercial plantations. This is because of:
rainfall intensity and runoff
the amount of moisture already in the ground
slope and slope length
soil type and geology
vegetation cover (how many plants and trees are already on the land)
previous extreme storms
whether the land faces the direction of frequent intense storms.
Further reading
This fact sheet is based on research from Scion funded by the One Billion Trees Partnership Fund, Te Uru Rākau - New Zealand Forest Service/Ministry for Primary Industries.
