One of the largest and longest-living trees in the world, New Zealand kauri (Agathis australis) belongs to the ancient conifer family, Araucariaceae. Agathis is one of three still-existing southern hemisphere genera of Araucariaceae, and is thought to have evolved in the Australia–New Zealand region. New Zealand kauri appeared about 20 million years ago. Agathis contains about 20 species found along the western Pacific and into New Guinea, the Philippines, Indonesia and Malaysia. The most southern-growing species, New Zealand kauri, is restricted to the sub-tropical forests in areas north of latitude 38° S (in Auckland, Northland and the Coromandel Peninsula), where it grows from sea level to 600 metres.
Bark, leaves and cones
The trunk of the full-grown kauri has greyish bark with a pattern like hammer marks, caused by the bark flaking off. Kauri gum (sometimes called New Zealand amber) oozes from the bark of mature trees. The tree’s oblong leaves are flat and leathery; bronze when young but turning bright green as they mature.
Kauri trees bear both male and female cones. Male cones are finger-shaped and fall once they have released their pollen in spring. Female cones are round and turn from green to brownish red as they mature. They ripen after three years of growth and release seeds that are dispersed by the wind.
Kauri gum or resin oozes from the bark, leaves and cones of all kauri trees, and large deposits form where the branches fork. It is how the kauri protects itself from injury and decay. The gum hardens to seal any damaged surfaces, slowing the onset of rot and warding off insect attack.
Like most conifers, young kauri have a narrow pyramid shape. As the trees reach the forest canopy, which takes 50 years or more, they shed their lower branches to become clean-stemmed ‘rickers’ – named after the ships’ spars for which they were once logged.
Mature kauri develop massive column-like trunks, with spreading crowns supported by whorls of large branches. These huge structures are balanced by spreading lateral roots anchored with peg roots that can extend 5 metres into the ground. The feeding roots form a fine surface mat within the litter mound that surrounds each tree.
Kauri’s final size depends on site and conditions, but heights average 30–40 metres and trunks can reach several metres in diameter. By 600–700 years of age, kauri reaches an average diameter of over 1 metre. Kauri can survive for 1,000 years or more (with an average diameter of 2 metres), but trees older than 1,700 years (average diameters over 3 metres) are now rare. The ages of the largest survivors like Tāne Mahuta (‘the god of the forest’, diameter over 4 metres) are not accurately known, but have been estimated at 1,500–2,000 years. Even bigger trees were known in the past, some with diameters of more than 7 metres.
Before people settled in New Zealand, forest containing kauri covered much of the Coromandel Peninsula and northern areas. Today, the remaining 7,455 hectares of mature kauri forest is scattered in remnant patches. Fossil evidence shows that it once grew as far south as Invercargill. Over the last few million years, kauri retreated to its present limits as a result of geological-scale disturbances such as sea-level changes, mountain-building, volcanic eruptions and glaciations, and the associated loss of suitable soils.