Forest plantations in Aotearoa can be found on flat, gently rolling, or very steep terrain.
Establishment of plantations
Forestry plantations require careful preparation of the land prior to planting. Existing vegetation is cleared by slashing, scrub cutting, spraying, and burning. Cleared land may be drained and earthworks done to prepare the planting area. All can potentially cause increased surface run off carrying high sediment loads and nutrients, especially following rain.
Once planted, seedlings must be ‘released’ to ensure aggressive weed growth is controlled. Usually a selective herbicide is used to control weeds immediately around the newly planted seedlings. Herbicides and pesticides used on or around trees to control weeds and insect pests are chemical contaminants that have the potential to leach through the soil and pollute nearby waters. Fertiliser application in plantations is often not necessary and inorganic nutrients such as nitrogen, phosphorus, and boron may only be applied in areas where soil nutrients are deficient.
Nutrients, herbicides, and pesticides are less likely to contaminate waterways when riparian vegetation is maintained as a buffer between the land and the adjacent stream, river, or estuary.
Harvesting operations and access
When trees are harvested, an area has to be carefully assessed and an effective plan put in place that mitigates for any environmental impacts. The extensive network of roads and tracks established throughout large scale forest plantations enables trees to be harvested effectively and efficiently once they are ready for logging. Earthworks are required to construct and maintain roads, landings, tracks, stream crossings, and firebreaks.
Maintaining a buffer zone of riparian vegetation between a harvesting area and streams and rivers reduces the impacts of the logging operation by maintaining the stability of the stream bank, reducing sediments that are exposed and prone to run-off during harvesting, and minimises the amount of debris (logs and branches) entering a waterway during felling. (Although some wood is good for the stream, it should not smother it entirely.)
Roads, landings, and logging earthworks are potentially the greatest sources of sediment moving into waterways. Instream barriers, such as culverts, dams, and fords used to cross streams, can prevent the free movement of mahinga kai upstream of logging operations.
Best practice guidelines need to be maintained in order to minimise any adverse effects on water quality and mahinga kai.