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Forestry

Harvest and manufacturing

The word “harvesting” is used in farming to imply the sustainable extraction of a crop. It is part of a continuing cycle of planting and growth and should not be confused with “deforestation” – which in some cases (e.g. in certain tropical countries) may be equivalent to pillaging of a natural resource.

Good foresters ensure that forests are not “overcut” – i.e. lose too much wood, particularly from the older, more valuable trees. On the other hand, very old trees do not grow as vigorously as middle-aged trees.

The typical harvest age of a crop of radiata pine is 28 years, whereas 45-60 years is more normal for Douglas-fir and most other species.

Radiata is sometimes called “fast growing” because of this relatively short rotation length, but the expression is also used to describe the huge amount of wood that is grown every year on an average hectare (typically about 20 m3/ha/yr for radiata pine, but it can be 30 or more).

Harvesting radiata pine involves clearfelling – i.e. cutting every mature tree in a large even-aged stand. So-called selective logging is not practical in a windy climate, on steep terrain, and with short-lived and light-demanding species.

Two clearfell logging systems are used: ground-based and cable. Ground-based systems (skidders, crawler tractors, and forwarders) are suitable for gentler slopes – perhaps 15⁰ or less – but steeper topography requires a more expensive cable-logging system.

This involves a complex structure of wire-ropes and winches to pull the logs, generally uphill, to a stationery hauler. Cable systems can minimise disturbance to the soil compared to the vehicle tracks necessary for ground-based systems, so are often preferred for environmental reasons.

People tend to become accustomed to a hillside of trees and often dislike the visual disruption caused by harvesting, but in warm and wet New Zealand this is fairly ephemeral – the hills “green up again” surprisingly quickly. Most debate over the ecological impact of harvest concerns the size of the felling coupes (i.e. clearfell areas), the disturbance of soil and the protection of water-courses. Some negative impacts are inevitable but with good planning and experienced operators (and logging is indeed a highly skilled occupation), these can be reduced to very low levels.

A “New Zealand Environmental Code of Practice” has been issued by the forest sector to ensure that loggers understand their obligations.

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