Plantation Forests

New Zealand's forestry industry is largely based around sustainably-managed plantation forests. About 90% of New Zealand's forests are radiata pine (Pinus radiata). The remainder are Douglas fir (Pseudotsuga menziesii), which amounts to 6% of total plantation area, along with 4% made up of other softwood and hardwood species.

Radiata Pine

Radiata pine's 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.

Harvest Cycles

The key difference between a plantation forest and an indigenous forest, at least in NZ, is that the plantation forest will most likely be harvested once it is mature. For radiata pine plantations, this is done at an average age of 28 years, although this can be earlier or later depending on the forest owners' objectives.

With reference to forestry harvesting cycles, we can determine the age of the plantation with the aim of showing where harvesting is predicted to happen at any time in the future. In this way, we can show how much land will be within the window of vulnerability and where it will be located.

Clearfell Harvesting

It is important to recognise that outside of the window of vulnerability, erosion and landslides can still occur on steep, erosion-susceptible land, even under a tall forest. But these landslides are less likely to happen because tall trees reinforce and protect the soil.

Therefore, when clearfell harvesting plantation forests on steep, erosion-susceptible land, we need to recognise that there is a period with higher susceptibility to landslides and erosion, and manage the location and size of the clearfells to minimise any increase in landslide sediment and associated slash entering our river networks.