Clearfelling

Clearfelling is the most common type of harvesting in New Zealand and involves cutting down an entire stand of trees. Trees are extracted using methods suited to the land, access, forest size and the surrounding environment. Many clear-fell operations take place on steep terrain, where cable hauling is the most common extraction method used.

Why does clearfell harvesting create a “window of vulnerability”?

Window of Vulnerability

Any forest cover, indigenous or exotic plantation, reduces soil erosion on steep, erosion-susceptible lands due to:

The soil reinforcing effect of the forest’s root network (“root reinforcement”). Compared with pasture and other short vegetation, forests have deep woody roots that help to bind soils together and anchor them to the underlying rock formation.
Forest soils are generally drier and therefore stronger than soils under short vegetation such as pastures. This is due to the “umbrella effect”— tall rough forest canopies intercept more rainfall and allow less “throughfall” than short vegetation cover, such as pastures. In addition, the forest canopy and litter layer can protect soils from damage by rainfall and water running over the surface of the soil.

However, Marden (2004) noted that reductions in erosion depend on the size and spacing of the trees in the forest—they are least during forest establishment when trees are small and increase with stand age due to increasing volume of tree roots and canopy cover. This process is shown below, where the decay curve for net root reinforcement after harvesting is because the harvesting of the mature trees also kills their roots and they begin to decay (the exceptions are “coppicing” species such as redwoods and some hardwoods, where the roots and stump remain alive after the stem is harvested, and fresh shoots grow from the stump.)

Radiata pine roots decay quite rapidly after the tree is cut down, and as they do so their reinforcement effect declines to zero. The replanted trees take time to grow enough roots to fully occupy and reinforce the soil and this results in a period of reduced root reinforcement of between 6-8 years post-harvest, depending on the initial spacing and growth of the replanted tree crop.

At the same time, the “umbrella effect” from the tree canopy is also lost due to harvesting and takes time to recover as the young tree crowns grow larger and begin to cover the site and achieve canopy closure.

The window of vulnerability — Typical changes in forest vegetation root strength or root reinforcement after timber harvesting showing the ‘window of vulnerability’, estimated to be in the period 1-6 years after harvesting, but this is species and density dependent. Source: Phillips et al. (2024).

Visualisation

Where and when does the window of vulnerability occur?

Foresters usually harvest forests when they reach the age of 25-30 years, so if you know the current age of a plantation forest, it is possible to predict when it will be harvested. The FCP uses region-wide maps of plantation forest ages (developed by Indufor) to predict where and when clearfell harvesting is likely to happen.

For the Forestry Catchment Planner application, a 0-5 year period is used for the visualisation of the window of vulnerability.

If we can predict where and when forest land will be in the window of vulnerability, we can use region-wide maps of erosion susceptibility (developed by GNS Science) to see if this land is highly erosion-susceptible, and therefore more likely to generate landslides and other erosion processes. So, we can visualise the future risk of sediment (from erosion) and harvesting slash reaching our rivers, by assessing the erosion susceptibility of the harvested land.