Enviromental Effects

Effects of Plantation Forests
Greenplan’s forests are planted on land formerly used for farming of sheep and cattle. It is useful to contrast the environmental effects of plantation forests in comparison to agricultural activities. Such a comparison also helps to put the effects of pine forests in context.

The following is a summary of the main findings from the publication Environmental Effects of Planted Forests, J.P. Maclaren, New Zealand Forest Research Institute, 1996.

Water Yield
Forests reduce water yields relative to pasture by between 25 and 50% in some New Zealand sites.
In some situations forests reduce flooding, except in cases of poor forestry practices, such as poorly designed roading and skid sites, which increase sedimentation. On the other hand good practices can reduce erosion and flood effects.
Water Quality
Water quality in New Zealand’s remote areas is very good by international standards. However lowland areas where agriculture is the main activity have poor water quality. This is mainly due to run-off of inorganic fertilisers and faecal contamination.
Sedimentation is often increased by agricultural activities.
Forestry can have a negative effect on water quality mainly at the time of logging and roading if there is not good management. There are clear guidelines for ensuring water quality is not degraded.
The Resource Management Act, under which forestry is strictly regulated, but agricultural activities left alone, places strict controls on forestry activities in relation to water quality.
Soil Erosion
Some degree of erosion is the inevitable result of natural geological forces.
However in many parts of New Zealand erosion has been accelerated by large scale land clearance. An estimated 5.5 million ha currently under pasture is unsustainable in its present use.
If land is susceptible to erosion it is likely to be greater on farm land than within forests.
There is clear evidence that most forms of erosion are significantly reduced if forested with pine trees. Trees dry out the soil, and bind it with their roots to a much greater extent than grass species.
Soil erosion at harvest time can be largely eliminated by good management and immediate replanting.
Soil Deterioration
Soil can be said to deteriorate if the growth of any future crop is likely to be inferior to the growth of a similar, current crop.
There is no proof that agricultural or forestry practices cause deterioration of soils, provided appropriate site preparation and fertiliser treatments are used.
Studies have shown that forestry enhances the availability of soil nutrients, especially nitrogen and phosphorus, in the top soil layers.
Studies have also shown that there is a temporary increase in acidity levels under pine trees. This benefits trees as most prefer acidic soils. Acidity is neutralised over time as foliage and branches decompose.
Soil compaction is greater in pastoral sites than in well managed forests.
Greenhouse Effect
There is increasing agreement that global warming is occurring largely because of human activity.
Forestry is one option for offsetting the effects of global warming. Trees absorb carbon dioxide, one of the main Greenhouse gases. They convert it to wood, through photosynthesis.
A hectare of radiata absorbs between 20 and 25 tonne of carbon per hectare annually.
Although some carbon is released during the harvesting and processing of forests there is a positive net absorption of carbon.
Agricultural activities do not result in the storage of large volumes of carbon in the crop. Any uptake of carbon is more than offset by annual carbon losses.
New Zealand’s emissions of methane and nitrous oxide from livestock and high nitrogen pastures may contribute more to global warming than it’s total emissions of carbon dioxide from fossil fuel usage.
Aesthetics
There is wide ranging opinion as to the aesthetic value of plantations, and of pastoral land. Because of this little can be said in regard to the relative merits of the aesthetic values of each.
Indigenous forests are normally preferred to plantation forests, especially by New Zealanders.
Many foreign visitors seem to appreciate the aesthetic qualities of plantation forests to a greater extent, or do not notice a significant difference...
Forest Practices
Radiata pine is the predominant species planted in New Zealand because of its high productivity, relative high rates of return, length of growing cycle and established markets.
If pasture land is converted to forestry there is normally little site preparation required and only minimal clearance of existing woody vegetation.
Use of herbicides although unpopular with some environmental groups is likely to be less destructive than other forms of site preparation such as burning or ‘windrowing’.
Radiata forests are clear felled rather than selectively logged because it is not possible to establish a commercially viable mixed age forest of radiata. In some cases it may be preferable to progressively log some forests rather than fell a continuous area.
Riparian strips along waterways are a partial solution to problems of sedimentation, and flow of nutrients into streams.
Good site preparation, and management of road construction prior to harvest is of vital importance to minimise erosion and compaction.
Increased harvest volumes produce increased noise, road traffic, and road maintenance costs.
An offsetting factor is that forest owners contribute to roading and infrastructure costs through local body rates over the life of their forests, but only impact on local infrastructure for part of that time. The agricultural industry however contributes in the same way, but impacts on infrastructure continuously.
Biodiversity
Biodiversity describes the variety and complexity of life.
Plantation forestry in New Zealand has been shown to support a greater number of birds than any other type of mainland forest.
In comparison to pastoral land use forestry increases biodiversity to a great extent, especially in terms of understorey vegetation and birds, as well as aquatic life.
Monocultures, Pest and Disease Risk
Radiata forests are described by some as unnatural monocultures. This is debatable. Many monocultures, such as New Zealand’s beech forests, occur naturally.
Radiata forests contain greater diversity than many other types of forest, and certainly in comparison to biodiversity of agricultural systems.
There is no proof that New Zealand’s radiata forest would be more susceptible to outbreaks of disease or other pest than any other form of forest. This would largely depend on the type of pest or disease, the exact genetic composition of trees and local factors such as climate.
It is no more likely that New Zealand radiata forests will be significantly affected by introduced pests and diseases than agriculture would be affected by a significant disease such as Foot and Mouth.
Socio Economic Effects
The major socio economic concern regarding forestry is the loss of farmland, with the assumption that this will create a decline in employment and income in areas dependant on agriculture. This raises the question of whether afforestation is a cause or a symptom of the long term decline in pastoral farming.
In areas of small scale afforestation there is unlikely to be significant impact on agriculture.
In areas of larger scale afforestation there is likely to a short term lowering of employment levels and household income levels. However this will be offset by demand for forest workers during forest development and harvesting. There is also likely to be significant employment in associated businesses servicing forest operations.