The benefits of radiata pine plantations include providing shelter on farms, stabilising soil on erosion-prone hillsides, reducing the amount of sediment that ends up in rivers and streams, and absorbing water that could cause flooding.
Because trees can store carbon, radiata pine plantations are useful ‘carbon sinks’. About 50% of the dry matter in the wood is carbon – largely cellulose (about 65%) and lignin (about 30%). Depending on growth rate and wood density, a hectare of pine trees locks up 4–7 tonnes of elemental carbon per year, which is equivalent to 15–26 tonnes of carbon dioxide absorbed from the atmosphere. A 1,000-hectare forest can absorb 15,000–26,000 tonnes of carbon dioxide per year.
When burnt as fuel in a car engine, petrol releases 2.62 kilograms of carbon dioxide per litre. On this basis, a typical car produces 1 tonne of carbon dioxide for every 5,555 km driven, or 3 tonnes for an average year’s driving (16,666 km). This is equivalent to the amount of carbon dioxide that 33–60 pine trees absorb in one year.
Habitat for native species
For long periods while they grow, plantation forests are relatively undisturbed ecosystems, and provide habitats for many species of native plants and animals. For example, kiwi and falcons have found planted forests to be a suitable habitat and feeding ground.
Although the biodiversity of plantation forests is not as rich as indigenous forests, the number of native species that live there contribute to the biodiversity of the country.
New Zealand’s radiata pine forests are mainly planted on soils that were not considered suitable for intensive agricultural use. Some examples are the large volcanic ash fields of the central North Island, drifting coastal sand dunes and shallow, less-productive hill-country soils.
Growing trees on these soils often improves them because organic material in the form of leaf, bark and woody litter adds nutrients. The trees also extract mineral nutrients from deep in the soil. Because of this, the second crop of trees on such sites will often grow better than the first. Some areas that have produced one or two crops of trees have, with fertiliser treatment, been successfully converted to agricultural land.
Protecting the forest
Radiata pine can suffer from pests, diseases, windstorms and fire. Dothistroma needle blight, a pathogenic fungus that attacks the needles of young pine trees causing reduced growth, is the most widespread disease. It is most serious in places with higher rainfall or prone to prolonged misty conditions. However, it can be controlled by spraying with a fungicide.
Strong wind is a hazard to forests in New Zealand and has sometimes caused severe damage to pine plantations. Young trees are sometimes toppled by wind on sites with puggy soils, and older trees can be uprooted completely or snapped off.
Fire is an ever-present risk, particularly during prolonged droughts.