| Agathis australis | |
|---|---|
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Scientific classification | |
| Kingdom: | Plantae |
| Clade: | Tracheophytes |
| Clade: | Gymnospermae |
| Division: | Pinophyta |
| Class: | Pinopsida |
| Order: | Araucariales |
| Family: | Araucariaceae |
| Genus: | Agathis |
| Species: | A. australis |
| Binomial name | |
| Agathis australis | |
 | |
| Natural range ofA. australis | |
| Synonyms[2] | |
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Agathis australis, orkauri, is aconiferous tree in the familyAraucariaceae, found north of38°S in the northern regions of New Zealand'sNorth Island.[3]
It is the largest (by volume) but not tallest species of tree in New Zealand, standing up to 50 metres (160 ft) tall in the emergent layer above the forest's main canopy. The tree has smooth bark and small narrow leaves. Other common names to distinguishA. australis from other members ofAgathis aresouthern kauri andNew Zealand kauri.
With itspodsolization capability and regeneration pattern it can compete with faster growingangiosperms. Because it is such a conspicuous species, forest containing kauri is generally known askauri forest, although kauri need not be the most abundant tree. In the warmer northern climate, kauri forests have a higherspecies richness than those found further south. Kauri even act as afoundation species that modify the soil under their canopy to create unique plant communities.[4]
Scottish botanistDavid Don described the species asDammara australis.Agathis is derived fromGreek and means 'ball of twine', a reference to the shape of the male cones, which are also known by the botanical termstrobili.[5]Australis translates in English to 'southern'.[5]
The Māori name is descended fromProto-Polynesian*kauquli, Samoan ebony orDiospyros samoensis.[6]
The young plant grows straight upwards and has the form of a narrow cone with branches going out along the length of thetrunk. However, as it gains in height, the lowest branches are shed, preventingvines from climbing. By maturity, the top branches form an imposing crown that stands out over all other native trees, dominating theforest 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 metres (6 ft 7 in) or more.[7] The kauri has a habit of forming small clumps or patches scattered through mixed forests.[8]
Kaurileaves are 3–7 centimetres (1.2–2.8 in) long and 1 centimetre (0.39 in) broad, tough and leathery in texture, with no midrib; they are arranged in opposite pairs or whorls of three on the stem. Theseed cones are globose, 5–7 centimetres (2.0–2.8 in) diameter, and mature 18 to 20 months after pollination; the seed cones disintegrate at maturity to release wingedseeds, which are then dispersed by the wind. A single tree produces both male and female seed cones.Fertilisation of the seeds occurs bypollination, which may be driven by the same or another tree's pollen.
Agathis australis can attain heights of 40–50 metres (130–160 ft) and trunk diameters big enough to rival Californiansequoias at over 5 metres (16 ft). The largest kauri trees did not attain as much height or girth at ground level but contain more timber in their cylindrical trunks than comparable Sequoias with their tapering stems.
The largest recorded specimen was known asThe Great Ghost and grew in the mountains at the head of theTararu Creek, which drains into theHauraki Gulf just north of the mouth of theWaihou River (Thames). Thames Historian Alastair Isdale says the tree was 8.54 metres (28.0 ft) in diameter, and 26.83 metres (88.0 ft) ingirth. It was consumed by fire inc. 1890.[9]
A kauri tree at Mill Creek,Mercury Bay, known asFather of the Forests was measured in the early 1840s as 22 metres (72 ft) in circumference and 24 metres (79 ft) to the first branches. It was recorded as being killed by lightning in that period.[10]
Another huge tree,Kairaru, had a girth of 20.1 metres (66 ft) and a columnar trunk free of branches for 30.5 metres (100 ft) as measured by aCrown Lands ranger, Henry Wilson, in 1860. It was on a spur of Mt Tutamoe about 30 kilometres (19 mi) south ofWaipoua Forest near Kaihau. It was destroyed in the 1880s or 1890s when a series of huge fires swept the area.[11][12]
Other trees far larger than living kauri have been noted in other areas. Rumors of stumps up to 6 metres (20 ft) are sometimes suggested in areas such as the Billygoat Track above the Kauaeranga Valley near Thames.[13] However, there is no good evidence for these (e.g., a documented measurement or a photograph with a person for scale).
Given that over 90 per cent of the area of kauri forest standing before 1000AD was destroyed by about 1900, it is not surprising that recent records are of smaller, but still very large trees. Two large kauri fell during tropical storms in the 1970s. One of these wasToronui, in Waipoua Forest. Its diameter was larger than that ofTāne Mahuta and its clean bole larger than that ofTe Matua Ngahere, and by forestry measurements was the largest standing. Another tree, Kopi, inOmahuta Forest near the standing Hokianga kauri, was the third largest with a height of 56.39 metres (185.0 ft) and a diameter of 4.19 metres (13.7 ft). It fell in 1973. Like many ancient kauri both trees were partly hollow.
In general over the lifetime of the tree the growth rate tends to increase, reach a maximum, then decline.[14] A 1987 study measured mean annual diameter increments ranging from 1.5–4.6 millimetres (0.059–0.181 in) per year with an overall average of 2.3 millimetres (0.091 in) 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 found only a weak relationship between age and diameter. The growth of kauri in planted and second-growth natural forests has been reviewed and compared during the development of growth and yield models for the species. Kauri in planted forests were found to have up to 12 times the volume productivity than those in natural stands at the same age.[15]
Individuals in the same 10 centimetres (3.9 in) diameter class may vary in age by 300 years, and the largest individual on any particular site is often not the oldest.[16][17] Trees can normally live longer than 600 years. Many individuals probably exceed 1000 years, but there is no conclusive evidence that trees can exceed 2000 years in age.[16] By combiningtree ring samples from living kauri, wooden buildings, and preserved swamp wood, adendrochronology has been created which reaches back 4,500 years, the longest tree ring record of pastclimate change in the southern hemisphere.[18] One 1700 year old swamp wood kauri that dates to approximately 42,000 years ago contains fine-scale carbon-14 fluctuations in its rings that may be reflective of the most recent magnetic field flip of the earth.[19]
Much likepodocarps, it feeds in theorganic litter near the surface of the soil through fineroot 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 undergoingdecomposition. On the other hand,broadleaf trees such asmāhoe derive a good fraction of their nutrition in the deeper mineral layer of the soil. Although its feeding root system is very shallow, it also has several downwardly directedpeg roots which anchor it firmly in the soil. Such a solid foundation is necessary to prevent a tree the size of a kauri from blowing over in storms and cyclones.
Thelitter left by kauri is much moreacidic than most trees, and as it decays similarly acidic compounds are liberated. In a process known asleaching, these acidic molecules pass through the soil layers with the help of rainfall, and release other nutrients trapped inclay such asnitrogen andphosphorus. This leaves these important nutrients unavailable to other trees, as they are washed down into deeper layers. This process is known aspodsolization, and changes the soil colour to a dull grey. For a single tree, this leaves an area of leached soil beneath known as acup podsol (de). This leaching process is important for kauri's survival as itcompetes with other species for space.[20]
Leaf litter and other decaying parts of a kauri decompose much more slowly than those of most other species. Besides its acidity, the plant also bears substances such aswaxes andphenols, most notablytannins,[21] that are harmful tomicroorganisms. This results in a large buildup of litter around the base of a mature tree in which its own roots feed. As with most perennials, these feeding roots also house asymbioticfungi known asmycorrhiza which increase the plant's efficiency in taking up nutrients. In thismutualistic 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 youngerlineages.
The fungi on kauri are a food source for the larvae of the New Zealand giraffe weevil,Lasiorhynchus barbicornis. The larvae ofL. barbicornis burrow into the wood of a tree for up to two years. ThenL. barbicornis exit the bark of the tree as a fully formed adult beetle. These adultL. barbicornis exit from trees in Spring and Summer and months. After emerging from the tree, these adultL. barbicornis only live for a few weeks.[22]
In terms of localtopography, 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 astarairi, at low elevations. This pattern is known asniche partitioning, and allows more than one species to occupy the same area. Those species which live alongside kauri includetawari, a montane broadleaf tree which is normally found in higher altitudes, wherenutrient cycling is naturally slow.
Kauri is found growing in its natural ecosystem north of 38°Slatitude. Its southern limit stretches from theKawhia Harbour in the west to the easternKaimai Range.[23]However, its distribution has changed greatly overgeological time because ofclimate change. This is shown in the recentHolocene epoch by its migration southwards after the peak of the lastice age. During this time when frozenice 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 the tree's distribution, with stump kauri frompeat swamps used for measurement. The coldest period in recent times occurred about 15,000 to 20,000 years ago, during which time kauri was apparently confined north of Kaitaia, near the northernmost point of the North Island,North Cape. Kauri requires a mean temperature of 17 °C (63 °F) or more for most of the year. The tree's retreat can be used as aproxy for temperature changes during this period. While not present in modern days, theAupōuri Peninsula in the far north was a refuge for kauri, as large quantities ofkauri gum were present in the soils.[24]
It remains unclear whether kauri recolonised 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 throughWhangārei, pastDargaville and as far south asWaikato, attaining its peak distribution during the years 3000BP to 2000 BP.[23] There is some suggestion that it has receded somewhat since then, which may indicate temperatures have declined slightly. During the peak of its movement southwards, it was travelling as fast as 200 metres (660 ft) per year.[23] Its southward spread 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 forpollination andseed dispersal, while many other native trees have their seeds carried large distances byfrugivores (animals which eat fruit) such as thekererū (native pigeon). However, kauri trees can produce seeds while relatively young, taking only 50 years or so before giving rise to their own offspring. This trait makes them somewhat like apioneer species, despite the fact that their long lifespan is characteristic ofK-selected species. In good conditions, where access to water and sunlight are above average, diameters in excess of 15 centimetres (5.9 in) and seed production can occur inside 15 years.
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 demand much more light and require larger gaps to regenerate than such broadleaf trees aspūriri andkohekohe that show far moreshade tolerance. Unlike kauri, these broadleaf 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 relatively strong 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, these smaller trees are far less capable of regenerating.
When there is a disturbance severe enough to favour their 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. Kauriseedlings can still occur in areas with low light but mortality rates increase for such seedlings, 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. Kauribiomass tends to decrease during such times, as more biomass becomes concentrated in angiosperm species liketōwai. 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, behaviour of species in experimental conditions, and study ofpollen sediments (seepalynology) have helped shed light on thelife 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 by 8 millimetres (0.20 in × 0.31 in) 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.
Studies show that kauri develop root grafts through which they share water and nutrients with neighbours of the same species.[25][26]
Heavylogging, which began around 1820 and continued for a century, has considerably decreased the number of kauri trees.[27] It has been estimated that before 1840, the kauri forests of northern New Zealand occupied at least 12,000 square kilometres (4,600 sq mi). The BritishRoyal Navy sent four vessels,HMSCoromandel (1821),HMSDromedary (1821),HMS Buffalo (1840), andHMSTortoise (1841) to gather kauri-wood spars.
By 1900, less than 10 per cent of the original kauri survived. By the 1950s this area had decreased to about 1,400 square kilometres (540 sq mi) in 47 forests depleted of their best kauri. It is estimated that today, there is 4 per cent of uncut forest left in small pockets.[28]
Estimates are that around half of the timber was accidentally or deliberately burnt. More than half of the remainder had been exported to Australia, Britain, and other countries, while the balance was used locally to build houses and ships. Much of the timber was sold for a return sufficient only to cover wages and expenses. From 1871 to 1895 the receipts indicate a rate of about 8 shillings (around NZ$20 in 2003)[29] per 100superficial feet (34 shillings/m3).[30]
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 a sale in 1908 more than 5,000 standing kauri trees, totalling about 20,000,000 superficial feet (47,000 m3), were sold for less than £2 per tree (£2 in 1908 equates to around NZ$100 in 2003).[29][31] It is said that in 1890 the royalty on standing timber fell in some cases to as low as twopence (NZ$0.45 in 2003)[29] per 100 superficial feet (8 pence/m3), though the expense of cutting and removing it to the mills was typically great due to the difficult terrain where they were located.[32]
Probably the most controversial kauri logging decision in the last century was that of the National Government to initiate clear fell logging of the Warawara state forest (North of the Hokianga) in the late 1960s. This created a national outcry as this forest contains the second largest volume of kauri after the Waipoua forest and was until that time, essentially unlogged (Adams, 1980). The plan also involved considerable cost, requiring a long road to be driven up a steep high plateau into the heart of the protected area. Because the stands of kauri were dense, the ecological destruction in the affected plateau area (approximately a fifth of the forest by area, and a quarter by volume of timber) was essentially complete (as of the early 1990s most of the affected area contained a thick covering of native grasses with little or no kauri regeneration). Logging was stopped in fulfillment of an election pledge by the Labour Government of 1972. When the National Party was reelected in 1975, the ban on kauri logging in the Warawara remained in place, but was soon replaced by policies encouraging the logging of gianttōtara and other podocarps in the central North Island. The outcry over the Warawara was an important stepping stone towards the legal protection of the small percentage of remaining virgin kauri-podocarp forest in New Zealand's Government-owned forests.
Although today its use is far more restricted, in the past the size and strength of kauritimber made it a popularwood for construction andship building, particularly formasts of sailing ships because of its parallel grain and the absence of branches for much of its height. Kauri crown andstump wood was much appreciated for its beauty, and was sought after for ornamentalwood 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, bridge construction material, fences, moulds for metal forges, largerollers for the textile industry,railway sleepers andcross bracing for mines and tunnels.
In the late 19th and early 20th centuriesKauri gum (semi-fossilised kauriresin) was a valuable commodity, particularly forvarnish, spurring the development of agum-digger industry.
Today, the kauri is being considered as a long-termcarbon 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 totalcarbon capture is up to nearly 1000 tonnes per hectare. In this capacity, kauri are bettered only by matureEucalyptus regnans forest, and are far higher than any tropical or boreal forest type yet recorded.[33] 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 toshort rotation forestry options such asPinus radiata.
Kauri is considered a first rate timber. The whiter sapwood is generally slightly lighter in weight. Kauri is not highly resistant to rot and when used in boatbuilding must be protected from the elements with paint, varnish or epoxy to avoid rot. Its popularity with boatbuilders is due to its very long, clear lengths, its relatively light weight and its beautiful sheen when oiled or varnished. Kauri wood planes and saws easily. Its wood holds screws and nails very well and does not readily split, crack, or warp. Kauri wood darkens with age to a richer golden brown colour. Very little New Zealand kauri is now sold, and the most commonly available kauri in New Zealand isFiji kauri, which is very similar in appearance but lighter in weight.
Prehistoric kauri forests have been preserved in waterlogged soils asswamp kauri.[34] A considerable number of kauri have been found buried insalt marshes, resulting from ancient natural changes such as volcanic eruptions, sea-level changes and floods. Such trees have beenradiocarbon dated to 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 exposed to the air, these parts undergo rapid deterioration. The quality of the disinterred wood varies. Some is in good shape, comparable to that of newly felled kauri, although often lighter in colour. The colour can be improved by the use of naturalwood stains to heighten the details of the grain. After adrying process, such ancient kauri can be used for furniture, but not for construction.
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 isWaipoua Forest inNorthland. Mature and regenerating kauri can also be found in other national and regional parks such as Puketi and Omahuta Forests in Northland, theWaitākere Ranges near Auckland, and Coromandel Forest Park[35] on theCoromandel Peninsula.
The importance of Waipoua Forest was that it is 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 square kilometres (31 sq mi) of Waipoua was proclaimed a forest sanctuary after a petition to the Government.[36]The zoologistWilliam 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.[37]Waipoua Forest, together with the Warawara to the north, contains three quarters of New Zealand's remaining kauri. Kauri Grove on theCoromandel Peninsula is another area with a remaining cluster of kauri, and includes theSiamese Kauri, two trees with a conjoined lower trunk.
In 1921 philanthropic Cornishman 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 square kilometres (1.5 sq mi).
The most famous specimens areTāne Mahuta andTe Matua Ngahere in Waipoua Forest. These two trees have become tourist attractions because of their size and accessibility. Tane Mahuta, named after theMāoriforest god, is the biggest existing kauri with a girth of 13.77 metres (45.2 feet), a trunk height of 17.68 metres (58.0 feet), a total height of 51.2 metres (168 feet)[38] and a total volume including the crown of 516.7 cubic metres (18,250 cubic feet).[39] 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). Important note: all the measurements above were taken in 1971.[40]
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 (although seedlings are frost tender).[citation needed]
Kauri dieback was observed in the Waitākere Ranges caused byPhytophthora cinnamomi in the 1950s,[41] again onGreat Barrier Island in 1972 linked to a different pathogen,Phytophthora agathidicida[42] and subsequently spread to kauri forest on the mainland. The disease, known as kauri dieback or kauri collar rot, is believed to be over 300 years old and causes yellowing leaves, thinning canopy, dead branches, lesions that bleed resin, and tree death.[43]
Phytophthora agathidicida was identified as a new species in April 2008. Its closest known relative isPhytophthora katsurae.[44][45] The pathogen is believed to be spread on people's shoes or by mammals, particularly feral pigs.[46] A collaborative response team has been formed to work on the disease. The team includesMAF Biosecurity, theConservation Department, Auckland and Northland regional councils,Waikato Regional Council, and Bay of Plenty Regional Council. The team is charged with assessing the risk, determining methods and their feasibility to limit the spread, collecting more information (e.g. how widespread), and ensuring a coordinated response. The Department of Conservation has issued guidelines to prevent the spread of the disease, including keeping to defined tracks, cleaning footwear before and after entering kauri forest areas, and staying away from kauri roots.[47]
