Agathis australis

Agathis australis

Agathis australis, commonly known as the kauri, is a coniferous tree found north of 38°S in the northern districts of New Zealand's North Island. It is the largest (by volume) but not tallest species of tree in the country, standing up to 50m tall in the emergent layer above the forest's main canopy. The tree has smooth bark and small oval leaves. Other common names to distinguish A. australis from other members of the genus are southern kauri and New Zealand kauri.

Though kauri are among the most ancient trees in the world, they have developed a unique niche in the forest. With their novel soil interaction and regeneration pattern they are able to 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, though kauri need not be the most abundant tree. In the warmer northern climate, kauri forests have a higher species richness than others found further south.

Description

Young plants grow straight upwards and have the form of a narrow cone with branches going out along the length of the trunk. However, as they gain in height, the lowest branches are shed, preventing epiphytes from climbing. By maturity, the top branches form an imposing crown that stand out over all other native trees, dominating the heights of the forest.

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. While the reproduction of kauri seed cones takes place between male and female seed cones of the same tree, fertilisation of the seeds occurs by pollination, which may be driven by the same or another tree's pollen.

Kauri forests are among the most ancient in the world. The antecedents of the kauri appeared during the Jurassic period (between 190 and 135 million years).

Size

Agathis australis can attain heights of 40 to 50 metres and trunk diameters big enough to rival Californian Sequoias at over 5 meters. The largest kauris do not attain as much height or girth at ground level but contain more timber in their cylindrical trunks than a comparable Sequoia with its tapering stem.

The largest specimen of which there is any known record grew on the mountains at the head of the Tararu Creek that falls into the Hauraki Gulf just north of the mouth of the Waihou River (Thames). This tree was known as The Great Ghost. Local Thames Historian Alastair Isdale noted this tree was 8.54 metres in diameter, and 26.83 metres in girth. It was consumed by fire c.1890.

A kauri tree at Mill Creek, Mercury Bay was measured in the early 1840s to be 22 metres in circumference and 24 metres to the first branches. It is thought that this tree was felled around 1870.

Growth rate and age

In general over the lifetime of the tree the growth rate tends to increase, reach a maximum, then decline.

A 1987 study measured mean annual diameter increments ranging from 1.5 to 4.6 mm per year with an overall average of 2.3 mm per year. This is equivalent to 8.7 annual rings per centimetre of core, said to be half the commonly quoted figure for growth rate. The same study concluded only a weak relationship between age and diameter. Individuals in the same 10 cm diameter class may vary in age by 300 years, and the largest individual on any particular site is often not the oldest.

Experts agree that because of the variation in growth rate it is not possible to accurately assess the age of a standing tree from its diameter alone.

Trees can normally live longer than 600 years. Probably many individuals exceed 1000 years, but there is no conclusive evidence that trees can exceed 2000 years in age. (Ahmed & Ogden 1987)

Root structure and soil interaction

One of the defining aspects of this tree's unique niche is its relationship with the soil below. Much like podocarps, it feeds in the organic litter near the surface of the soil through fine root hairs. This layer of the soil is composed of organic matter derived from falling leaves and branches as well as dead trees, and is constantly undergoing decomposition. On the other hand, broadleaf trees such as Māhoe derive a good fraction of their nutrition in the deeper mineral layer of the soil. Although its root system is very shallow, it also has several downwardly directed peg roots which anchor it firmly in the soil. Such a solid foundation is necessary for a tree the size of a kauri to avoid blowing over, especially during storms and cyclones.

The litter left by kauri is much more acidic than most trees, and as it decays similarly acidic compounds are liberated. In a process known as leaching, these acidic molecules pass through the soil layers with the help of rainfall, and release other nutrients trapped in clay such as nitrogen and phosphorus. This leaves these important nutrients unavailable to other trees, as they are washed down into deeper layers. This process is known as podsolization, and changes the soil colour to a dull grey. For a single tree, this leaves an area of leached soil beneath known as a cup podsol. This leaching process is important for kauri's survival as it competes with other species for space. Leaf litter and other decaying parts of a kauri decompose much slower than most other species, however. Besides its acidity, the plant also bears substances such as waxes and phenols that are harmful to microorganisms. This results in a large buildup of litter around the base of a mature tree, in which its own roots feed. These feeding roots also house a symbiotic fungi known as mycorrhiza which increase the plant's efficiency in taking up nutrients. In this mutualistic relationship, the fungus derives its own nutrition from the roots. In its interactions with the soil kauri is thus able to starve its competitors of much needed nutrients and compete with much younger lineages.

Distribution

Local spatial 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 in 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, kauri trees are 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.

Changes over geological time

Kauri is presently found north of 38°S latitude, its southern limit stretching from Kawhia Harbour in the west to the eastern Kaimai Range. However, its distribution has changed greatly over geological time due to the phenomenon of climate change. This is exemplified in the recent Holocene epoch by migration southwards following the peak of the last Ice Age. During this time when frozen ice sheets covered much of the world's continents, kauri was able to survive only in isolated pockets, its main refuge being in the very far north. Radiocarbon dating is one technique used by scientists to uncover the history of this tree's distribution, with stump kauri from peat swamps being used for measurement. The coldest period in recent time occurred very roughly 15-20,000 years ago, and during this time kauri was apparently confined north of Kaitaia, which itself is not far from the northern most point of the North Island, North Cape. Much like kumaras grown in New Zealand, kauri requires a mean temperature of 17°C or more for the majority of the year. Kauri's retreat can in fact be used as a proxy for temperature changes during this period.

It remains unclear whether kauri recolonized the North Island from a single refuge in the far north or from scattered pockets of isolated stands that managed to survive despite climatic conditions. It spread south through Whangarei, past Dargaville and as far south as Waikato, attaining its peak distribution during the years 3000-2000 BP. There is some suggestion it has receded somewhat since then, which may indicate temperatures have declined slightly since this time. During the peak of its movement southwards, it was traveling as fast as 200 metres per year. Regardless of where it originated from, its spread southward seems relatively rapid for a tree that can take a millennium to reach complete maturity. This can be explained by its life history pattern.

Kauri relies on wind as its means of both pollination and seed dispersal, whereas many other natives may have their seeds carried large distances by frugivores (animals which eat fruit) such as the kererū, a native pigeon. However, kauri trees rapidly reach a stage at which they can produce seeds, taking only 50 years or so before giving rise to their own offspring. This trait makes them somewhat like a pioneer species, despite the fact that their long lifespan is characteristic of k-selected species.

Regeneration and life history

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 easier due not only to increased sunlight, but their stronger resistance to wind and frosts. Kauri resides in the emergent layer of the forest, where it is exposed to the effects of the weather, however 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.

Due to this special regeneration niche, kauri trees can live over a thousand years, whereas most other trees experience senescence long before this time. This extraordinary age is simply a reflection of how long this species must wait in order for there to be a disturbance large enough to favour its regeneration. The nature of this large disturbance also means that kauri trees regenerate en mass, resulting in a cohort or generation of trees of similar ages forming after each disturbance. Kauri in a given area are hence likely to be of similar age. Due to the nature of their regeneration, the distribution of kauri allows researchers to predict when and where disturbances have occurred, and how large they may have been; the presence of abundant kauri may be an indication that an area is prone to disturbances. 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 disturbances kauri tends to lose ground to broadleaf competitors which are more capable of establishing themselves in shaded environments. In the complete absence of disturbance, kauri tends to become more rare as it is excluded by its competitors. Biomass of kauri 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 terms of their age, with each tree dying at a different point in time, and regeneration gaps being 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 are impossible to observe in the lifetime of a human, research into current patterns of distribution, behavior of species in experimental conditions, and study of pollen sediments (see palynology) have helped shed light on the life history of kauri.

Ethnobotany

Deforestation

Heavy logging which began around 1820 and continued for a century has considerably decreased the number of kauri trees in New Zealand. It has been estimated that prior to European colonisation, the kauri forests of northern New Zealand occupied at least 12,000 square kilometres. By the 1950s this area had decreased to about 1,400 km², comprising some 47 forests which were depleted of their best kauri. By 1900, less than 10% of the original kauri had survived. It is estimated that today, there is 4% of uncut forest left in small pockets.

Estimates are that around half of the timber had been accidentally or wilfully burnt. More than half of the remainder had been exported to Australia, Britain, and other countries, while the balance was used locally for building houses and ships.

Much of the timber was sold for a return sufficient only to cover wages and expenses, plus reasonable interest on the capital employed in the industry. From 1871 to 1895 the receipts indicate a rate of about 8 shillings (around NZD$20 in 2003) per hundred superficial feet (34 shillings/m³).

The Government continued to sell large areas of kauri forests to sawmillers who, under no restrictions, took the most effective and economical steps to secure the timber, resulting in much waste and destruction. At one sale in 1908 more than five thousand standing kauris, totalling about twenty million superficial feet (47,000 m³), were sold for less than two pounds per tree (two pounds in 1908 equates to around NZD$100 in 2003) It is said that in 1890 the royalty on standing timber fell in some cases to as low as twopence (NZD$0.45 in 2003) per hundred superficial feet (8 pence/m³), though the expense of cutting and removing it to the mills was typically great due to the difficult terrain they were located in.

Uses

Although today their 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 due to its parallel grain and the absence of branches extending for much of its height. Kauri crown and stump (tree) wood was much appreciated for its beauty, and was sought after for ornamental wood panelling as well as high-end furniture. Though 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, railroad ties and braces for mines and tunnels, among many others.

In the late nineteenth and early twentieth centuries kauri gum (semi-fossilised kauri resin) was a valuable commodity, particularly for varnish, and was the focus of a considerable industry at the time.

Timber

Technical specifications

  • Moisture content of dried wood: 12%
  • Density of wood: 560 kg/m³
  • Tensile strength: 88 MPa
  • Modulus of elasticity: 9.1 GPa
  • After felled kauri wood dries to a 12% moisture content, the tangential contraction is 4.1% and the radial contraction is 2.3%

A considerable number of kauri have been found buried in what are today salt marshes, resulting from ancient natural changes such as volcanic eruptions, sea level changes and floods. Such trees have been radiocarbon dated to originating as far back as 50,000 years ago or older. The bark and the seed cones of the trees often survive together with the trunk, although when excavated and in contact with the air, these parts display rapid deterioration.

The quality of the disinterred wood varies, and some is in surprisingly good shape, comparable to that of newly-felled kauri, although often lighter in colour. This aspect can be improved by the use of natural dyes, which provide brown dark and greenish tones that heighten the details of the grain. After a drying process, such ancient kauri can still be made use of for furniture and other construction.

Conservation and kauri today

The small remaining pockets of kauri forest in New Zealand have survived in areas that were not subjected to burning by Māori settlers and were too inaccessible to 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. It contains three quarters of New Zealand's remaining kauri.

In 1921 a philanthropic Cornishman named James Trounson sold to the Government for 40 thousand pounds, a large area adjacent to a few acres of crown land and said to contain at least four thousand kauris. From time to time Trounson had added further areas by way of gift, 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 due to their size. Tane Mahuta, named after the Māori forest god, is the biggest existing kauri with a girth of 13.77 m (45.2 ft), a trunk height of 17.68 m (58.0 ft), a total height of 51.2 m (168 ft) and a total volume including the crown of 516.7 m³ (18,247 cu ft).

Te Matua Ngahere, which means 'Father of the Forest', is smaller but stouter than Tane Mahuta, with a girth (circumference) of 16.41 m (53.8 ft).

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 (though seedlings are frost tender), and small footprint.

References

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See also

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