How tree planting prevents erosion

Plants and trees need land to grow on and the constantly changing climate is providing a number of challenges including erosion. One of the many benefits of tree planting is that it can help minimise erosion! This article will discuss the science behind how tree planting helps prevent erosion….

Soil erosion costs New Zealand around $100-200 million per year through loss of productive capacity, damage to infrastructure and degradation of waterways. Tree cover (as opposed to pasture) helps to mitigate erosion through two main functions;

Removing water from the soil profile. Up to 60% of the water that falls in a water catchment is not transported downslope if tree canopy cover is in place. 30% is removed by tree cover (leaves of tree) intercepting the rain. Another 30% is used by the tree making best use of the water from its roots and passing out into the air as vapour. Removing this amount of water from the soil profile particularly during storm events significantly reduces the probability of slope failure (erosion).

Tree roots in the soil holds the soil together. Roots in soil provide structural integrity (similar to reinforcing steel in concrete). Trees have deeper, stronger, more extensive root systems than pasture plants, which helps to reduce mass movement erosion.

The Ministry for Primary Industries (MPI) runs a number of programmes to encourage landowners to plant trees on erodible land. These programmes include the Afforestation Grant Scheme, Sustainable Land Management Hill Country Erosion Fund and the Erosion Control Funding Programme.

These programmes are designed to ensure that crown investment achieves the best result from taxpayer dollars. They consider;

• all erosion types present and their severity (slump, earthflow, landslide, etc.)
• that prevention is better than cure (early treatment of potential erosion, can prevent intense treatment later on)
• whether the treatment area is sufficient (mid slope boundaries to be avoided)
• suitable species to grow (will they grow, how quickly, what are their roots & canopy like)
• the right amount of trees in given area (want to achieve rapid root reinforcement and canopy cover/rain interception)
• risk of site failing prior to successful establishment of control (high risk sites require faster establishment results, or risk of failure is high)
• risk of treatment not establishing well (failures are expensive, consider all options)
• potential pest issues, including farm animals
• ongoing maintenance/management
• time (all afforestation treatments take time before they become effective, some much longer than others)
• longer term effects (harvesting on steep land, effective riparian setbacks)

Typically Poplar and Willow are used on less severe erosion or erodible features which allow pastoral farming to continue. These trees are very quick and easy to grow in varying conditions. For more aggressively eroding or erodible sites trees such as Radiata Pine, Eucalyptus, Manuka and Douglas fir are used. Native reversion (replanting with native species) is also an effective treatment option for which funding is available.

So if you are suffering from erosion on your land considering tree planting as a eco-friendly alternative. You’ll also enjoy the numerous, health and wellbeing, social, environmental and economic benefits.

First published in Kiwi Gardener issue 443. Written by NGINZ. Reproduced with permission of Kiwi Gardener.