Environmental Effects

Freshwater

Plantation forestry yields similar levels of nutrients as native forest and through most of the forestry cycle provides erosion and flood control so it is largely in association with felling that forestry has an impact on water quality and quantity. Although the effects of forestry on a catchment are generally short-term and localised, they can be significant. Soil disturbance in the form of road, track and landing construction can increase the potential for sediment loss.

Trees and associated riparian vegetation may provide soil stability through their roots holding onto soil together, by providing a physical 'umbrella' effect to direct/prevent rain splash on soil, and by providing a filter to soil and other contaminants entering waterways. Once trees are harvested, soil erosion may occur, resulting in the sediment getting into freshwater bodies, which reduce water clarity, chokes beds when it settles, and can be abrasive to the gills of fish in fast-moving water.  Tree roots progressively decay after harvest with complete loss within about seven years of harvest. The window of greatest risk for erosion occurs between 4-7 years after harvest. This is the time when old roots have rotted but new roots from replanted trees are not sufficiently established to provide full stability and the protective forest canopy has not closed over. Permanently deforested slopes are vulnerable to soil erosion and in some cases large-scale mass movement. 

The extent that these adverse effects will occur depends on the characteristics of each site, the care with which harvesting is planned and carried out, and subsequent site reinstatement executed, and the speed with which slopes are reforested.  Reforested areas are most vulnerable to instability during the period before new forest species establish their roots.  This will depend on the intensity and magnitude of rainfall during this period.

When harvesting is carried out close to streams, there may be little to stop rainwater washing the soil into the waterways, unless slash is left to form a barrier or ground cover. The extent of these effects, if any, depends on the topography and geology of the particular area, the type of soil and the amount of rain normally experienced. The worst effects are generally only felt for the first year after harvesting, but on some sites impacts may be detectable for up to three to five years.

The negative effects of forest harvesting may be mitigated by the establishment or retention of riparian planting on appropriate sites. This may protect the waterways from some of the sediment carried by any surface runoff from roads or bladed tracks created by harvesting machinery, or from other disturbed areas. ⁴⁸⁶³ Retention of riparian vegetation at harvest also maintains shade over waterways which lowers water temperatures and helps to stabilise stream banks. Leaving riparian planting in place also provides a buffer from forestry activities which reduces the risk of streambank erosion. 

The overall effects of forestry on the quality of New Zealand’s freshwater resources is generally positive when land is converted from pasture to plantation forest. Pasture causes much more sedimentation and nutrient runoff, so afforestation of an area previously used as pasture can improve water quality  within five to six years of planting. ⁴²⁰³  Conversion from pasture to forestry can also reduce the total water yield (run-off) in the catchment. A study undertaken at the Glendhu Forest in the upper Waipori Basin found that over a 10-year period there was a 30 per cent reduction in water yield after tall tussock grassland was converted to plantation forestry. ⁴²⁰⁴   Other studies have shown that this can be quite variable, but the effect is most pronounced in dry eastern areas where annual rainfall is less than 1000mm). ⁴²⁰⁵ Ths can be beneficial fro water regulation and flood control purposes but in areas where water is scarce forests may compete for water. 

Conversion in the reverse direction, from forestry to pastoral use, increases the supply of nutrients in runoff and to groundwater. Such conversion is now common in central and eastern North Island areas once thought to be unsuitable for pastoral use because of cobalt deficiency (bush sickness) which is now easily remedied.

Biodiversity

Historically, plantation forests were established on failed farm land, or indigenous vegetation was cleared to enable the establishment of plantation forests. This is now rare due to regulations and accords. However, at the interface between plantation forests and indigenous vegetation some disturbance may occur, although modern felling methods and approaches to caring for neighbouring indigenous vegetation can generally minimise this impact.  Harvesting activity including thinning will disturb the understorey of plantation forests. ⁴²⁰⁶  Although the biodiversity of plantation forests is not as rich as indigenous forests, they can provide excellent habitat for a range of indigenous fauna, including kiwi and falcon. ⁴²⁰⁷   These species and other rare threatened and endangered fauna and flora habitats are now commonly beneficially managed within the context of large-scale commercial plantation forestry.

Refer to the DoC approved guidance on the NZFOA web site.

Erosion

Around 10% of New Zealand is classed as severely erodible land. The clearance of this land and ongoing pastoral farming caused and continues to cause accelerated erosion. The conversion of land from pastoral farming to forestry generally mitigates soil erosion by reducing soil water levels and binding the soil together with roots. ⁴²⁰⁸ However, good practice is required to minimise soil erosion during site preparation, earthworks and harvesting which can activate or accelerate erosion by increasing soil exposure and instability and altering water flow runoff. ⁴²⁰⁹

Slash

Slash is woody debris which is produced during thinning and harvesting operations. Woody material in waterways is a natural process and provides instream benefits such as habitat for fish. However, too much slash can have negative effects on water flows, water quality, fish passage, fish habitat, property and infrastructure when it enters water bodies. Slash can have positive effects, returning nutrients to soil and reducing erosion, when retained on site. ⁴²¹⁰

Transport

The transportation of logs results in additional wear and tear on roads and increased traffic can impact on neighbourhood amenity and safety.  This can be mitigated by the prudent use of roading funds and appropriate allocation of revenue raised from rates.

Last updated at 2:00PM on August 30, 2024