Gums
Gums have been grown in New Zealand since the mid-1880s. There are about 160 species in New Zealand, but only a few have shown real promise as timber trees suitable for farm woodlots.
Many early plantings of gums in New Zealand were on unsuitable soils or in districts that were too windy or cold for them – most of the gums planted in Canterbury by the first settlers died during the cold winters of 1886 and 1899. Some gums have proved particularly vulnerable to attack from both native and introduced insects, and farm foresters have become wary of investing too much effort in these trees.
Gums as tree crops
Sydney blue gum (Eucalyptus saligna) was widely planted in the upper North Island during the 1970s. It grows up to 50 metres tall. Mountain ash (E. regnans) – one of the tallest trees in the world – was planted in cooler areas around the central North Island in the 1970s and 1980s. Alpine ash (E. delegatensis) is also grown in cold districts. Its timber is suitable for joinery and construction work, but much of it is chipped and sold to Japan for making paper.
Since the late 1990s brown barrel (E. fastigiata) has been the preferred gum species, as it has been least attacked by pests and is tolerant of cold conditions.
Wattles
Various tree wattles (Acacia species) are grown in New Zealand. On farms, most were originally planted for firewood or quick-growing shelter.
Black wattle (A. mearnsii) was originally introduced for the tannin its bark produces. However, it was attacked by a fungus that causes brown galls to develop on its branches and it became uneconomic to grow for the tanning industry. Black wattle seeds survive in the soil for more than 50 years, and in parts of the northern North Island it has become a weed as it regularly germinates following soil disturbance.
Blackwood (A. melanoxylon) produces one of the finest woods for cabinet and furniture making, and is increasingly planted in farm woodlots. It needs shelter from strong winds and year-round moisture. It is suitable for planting in gullies and beside streams, and can be planted in light scrub.
Erosion control
Gums and wattles are also used for soil conservation on unstable east coast hills, where summer droughts make poplars or willows unsuitable.