Agathis australis

Appearance

It is the largest but not tallest species of tree in New Zealand, standing up to 50 m tall in the emergent layer above the forest's main canopy. The tree has smooth bark and small narrow leaves. Other common names to distinguish ''A. australis'' from other members of the genus are southern kauri and New Zealand kauri.

Kauri forests are among the most ancient in the world. The antecedents of the kauri appeared during the Jurassic period . Although the kauri is among the most ancient trees in the world, it has developed a unique niche in the forest. With its novel soil interaction and regeneration pattern it can compete with the more recently evolved and faster growing angiosperms. Because it is such a conspicuous species, forest containing kauri is generally known as kauri forest, although kauri need not be the most abundant tree. In the warmer northern climate, kauri forests have a higher species richness than those found further south.
The young plant grows straight upwards and has the form of a narrow cone with branches going out along the length of the trunk. However, as it gains in height, the lowest branches are shed, preventing vines from climbing. By maturity, the top branches form an imposing crown that stands out over all other native trees, dominating the forest canopy.

The flaking bark of the kauri tree defends it from parasitic plants, and accumulates around the base of the trunk. On large trees it may pile up to a height of 2 m or more. The kauri has a habit of forming small clumps or patches scattered through mixed forests.

Kauri leaves are 3 to 7 cm long and 1 cm broad, tough and leathery in texture, with no midrib; they are arranged in opposite pairs or whorls of three on the stem. The seed cones are globose, 5 to 7 cm diameter, and mature 18 to 20 months after pollination; the seed cones disintegrate at maturity to release winged seeds, which are then dispersed by the wind. A single tree produces both male and female seed cones. Fertilisation of the seeds occurs by pollination, which may be driven by the same or another tree's pollen.

Distribution

In terms of local topography, kauri is far from randomly dispersed. As mentioned above, kauri relies on depriving its competitors of nutrition in order to survive. However, one important consideration not discussed thus far is the slope of the land. Water on hills flows downward by the action of gravity, taking with it the nutrients in the soil. This results in a gradient from nutrient poor soil at the top of slopes to nutrient rich soils below. As nutrients leached are replaced by aqueous nitrates and phosphates from above, the kauri tree is less able to inhibit the growth of strong competitors such as angiosperms. In contrast, the leaching process is only enhanced on higher elevation. In Waipoua Forest this is reflected in higher abundances of kauri on ridge crests, and greater concentrations of its main competitors, such as taraire are found at low elevations. This pattern is known as niche partitioning, and allows more than one species to occupy the same area. Those species which live alongside kauri include tawari, a montane broadleaf tree which is normally found in higher altitudes, where nutrient cycling is naturally slow.

Status

The small remaining pockets of kauri forest in New Zealand have survived in areas that were not subjected to burning by Māori and were too inaccessible for European loggers. The largest area of mature kauri forest is Waipoua Forest in Northland. Mature and regenerating kauri can also be found in other National and Regional Parks such as Puketi and Omahuta Forests in Northland, the Waitakere Ranges near Auckland, and Coromandel Forest Park on the Coromandel Peninsula.

The importance of Waipoua Forest in relation to the kauri was that it remained the only kauri forest retaining its former virgin condition, and that it was extensive enough to give reasonable promise of permanent survival. On 2 July 1952 an area of over 80 km² of Waipoua was proclaimed a forest sanctuary after a petition to the Government.
The zoologist William Roy McGregor was one of the driving forces in this movement, writing an 80-page illustrated pamphlet on the subject, which proved an effective manifesto for conservation.
Along with the Warawara to the North, Waipoua Forest contains three quarters of New Zealand's remaining kauri. Kauri Grove on the Coromandel Peninsula is another area with a remaining cluster of kauri, and includes the Siamese Kauri, two trees with a conjoined lower trunk.

In 1921 a philanthropic Cornishman named James Trounson sold to the Government for ₤40,000, a large area adjacent to a few acres of Crown land and said to contain at least 4,000 kauri trees. From time to time Trounson gifted additional land, until what is known as Trounson Park comprised a total of 4 km².

The most famous specimens are ''Tāne Mahuta'' and ''Te Matua Ngahere'' in Waipoua Forest. These two trees have become tourist attractions because of their size and accessibility. Tane Mahuta, named after the Māori forest god, is the biggest existing kauri with a girth of 13.77 metres , a trunk height of 17.68 metres , a total height of 51.2 metres ...hieroglyph snipped... and a total volume including the crown of 516.7 cubic metres . Te Matua Ngahere, which means 'Father of the Forest', is smaller but stouter than Tane Mahuta, with a girth of 16.41 m . Important note: all the measurements above were taken in 1971.

Kauri is common as a specimen tree in parks and gardens throughout New Zealand, prized for the distinctive look of young trees, its low maintenance once established .

Evolution

Just as the niche of kauri is differentiated through its interactions with the soil, it also has a separate regeneration 'strategy' compared to its broadleaf neighbours. The relationship is very similar to the podocarp-broadleaf forests further south; kauri is much more light demanding and requires larger gaps to regenerate, whereas broadleaf trees such as puriri and kohekohe show far more shade tolerance. These species can regenerate in areas where lower levels of light reach ground level, for example from a single branch falling off. Kauri trees must therefore remain alive long enough for a large disturbance to occur, allowing them sufficient light to regenerate. In areas where large amounts of forest are destroyed, such as by logging, kauri seedlings are able to regenerate much more easily due not only to increased sunlight, but their stronger resistance to wind and frosts. Kauri occupy the emergent layer of the forest, where they are exposed to the effects of the weather; however, the smaller trees that dominate the main canopy are sheltered both by the emergent trees above and by each other. Left in open areas without protection they are far less capable of regenerating.

When there is a disturbance severe enough to favour its regeneration, kauri trees regenerate en masse, producing a generation of trees of similar age after each disturbance. The distribution of kauri allows researchers to deduce when and where disturbances have occurred, and how large they may have been; the presence of abundant kauri may indicate that an area is prone to disturbance. Kauri seedlings still occur in areas with low light, of course, but mortality rates for such seedlings are much higher, and those that survive self-thinning and grow to sapling stage tend to be found in higher light environments.

During periods with less disturbance kauri tends to lose ground to broadleaf competitors which can better tolerate shaded environments. In the complete absence of disturbance, kauri tends to become rare as it is excluded by its competitors. Kauri biomass tends to decrease during such times, as more biomass becomes concentrated in angiosperm species like towai. Kauri trees also tend to become more randomly distributed in age, with each tree dying at a different point in time, and regeneration gaps becoming rare and sporadic. Over thousands of years these varying regeneration strategies produce a tug of war effect where kauri retreats uphill during periods of calm, then takes over lower areas briefly during mass disturbances. Although such trends cannot be observed in a human lifetime, research into current patterns of distribution, behavior of species in experimental conditions, and study of pollen sediments have helped shed light on the life history of kauri.

Kauri seeds may generally be taken from mature cones in late March. Each scale on a cone contains a single winged seed approximately 5 mm by 8 mm and attached to a thin wing perhaps half as large again. The cone is fully open and dispersed within only two to three days of starting.

Uses

Although today its use is far more restricted, in the past the size and strength of kauri timber made it a popular wood for construction and ship building, particularly for masts of sailing ships because of its parallel grain and the absence of branches for much of its height. Kauri is also a superb timber for building the hulls and decks of boats because of its resistance to rot. Kauri crown and stump wood was much appreciated for its beauty, and was sought after for ornamental wood panelling as well as high-end furniture. Although not as highly prized, the light colour of kauri trunk wood made it also well-suited for more utilitarian furniture construction, as well as for use in the fabrication of cisterns, barrels, bridges construction material, fences, moulds for metal forges, large rollers for the textile industry, railway sleepers and braces for mines and tunnels.

In the late 19th and early 20th centuries Kauri gum was a valuable commodity, particularly for varnish, spurring the development of a gum-digger industry.

Today, the kauri is being considered as a long-term carbon sink. This is because estimates of the total carbon content in living above ground biomass and dead biomass of mature kauri forest are the second highest of any forest type recorded anywhere in the world. The estimated total carbon capture is up to nearly 1000 tones per hectare. In this capacity they are bettered only by mature ''Eucalyptus regnans'' forest, and are far higher than any tropical or boreal forest type yet recorded. It is also conjectured that the process of carbon capture does not reach equilibrium, which along with no need of direct maintenance, makes kauri forests a potentially attractive alternative to short rotation ''Pinus radiata'' forests, for example.