Agathis australis
Agathis australis, commonly known as kauri, is a species of coniferous tree in the family Araucariaceae, most commonly found north of 38°S in the northern regions of New Zealand's North Island.
It is the largest but not tallest species of tree in New Zealand, standing up to 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 Agathis are southern kauri and New Zealand kauri.
With its podsolization capability and regeneration pattern it can compete with 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. Kauri even act as a foundation species that modify the soil under their canopy to create unique plant communities.
Taxonomy
Scottish botanist David Don described the species as Dammara australis. The genus name Agathis is derived from Greek and means 'ball of twine', a reference to the shape of the male cones, which are also known by the botanical term strobili. The specific epithet australis translates in English to 'southern', and was used to distinguish the species from the other members of Agathis, which are typically found in more northern tropical areas.Etymology
The Māori name is descended from Proto-Polynesian *kauquli, a name used to describe various dark-coloured trees in Polynesian languages, including Samoan ebony, or Diospyros samoensis.Description
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 or more. Kauri often form small clumps or patches scattered through mixed forests.
Kauri leaves are long and 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, 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.
Size
Agathis australis can attain heights of and trunk diameters big enough to rival Californian sequoias at over. 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 as The Great Ghost and grew in the mountains at the head of the Tararu Creek, which drains into the Hauraki Gulf just north of the mouth of the Waihou River. Thames Historian Alastair Isdale says the tree was in diameter, and in girth. It was consumed by fire in.
A kauri tree at Mill Creek, Mercury Bay, known as Father of the Forests was measured in the early 1840s as in circumference and to the first branches. It was recorded as being killed by lightning in that period.
Another huge tree, Kairaru, had a girth of and a columnar trunk free of branches for as measured by a Crown Lands ranger, Henry Wilson, in 1860. It was on a spur of Mt Tutamoe about south of Waipoua Forest near Kaihau. It was destroyed in the 1880s or 1890s when a series of huge fires swept the area.
Other trees far larger than living kauri have been noted in other areas. Rumors of stumps up to are sometimes suggested in areas such as the Billygoat Track above the Kauaeranga Valley near Thames. However, there is no good evidence for these.
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 was Toronui, in Waipoua Forest. Its diameter was larger than that of Tāne Mahuta and its clean bole larger than that of Te Matua Ngahere, and by forestry measurements was the largest standing. Another tree, Kopi, in Omahuta Forest near the standing Hokianga kauri, was the third largest with a height of and a diameter of. It fell in 1973. Like many ancient kauri both trees were partly hollow.
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 per year with an overall average of 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.Individuals in the same diameter class may vary in age by 300 years, and the largest individual on any particular site is often not the oldest. 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. By combining tree ring samples from living kauri, wooden buildings, and preserved swamp wood, a dendrochronology has been created which reaches back 4,500 years, the longest tree ring record of past climate change in the southern hemisphere. 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.
Root structure and soil interaction
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 feeding 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 to prevent a tree the size of a kauri from blowing over in 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 more slowly than those of most other species. Besides its acidity, the plant also bears substances such as waxes and phenols, most notably tannins, 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. As with most perennials, 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.
The fungi on kauri are a food source for the larvae of the New Zealand giraffe weevil, Lasiorhynchus barbicornis. The larvae of L. barbicornis burrow into the wood of a tree for up to two years. Then L. barbicornis exit the bark of the tree as a fully formed adult beetle. These adult L. barbicornis exit from trees in Spring and Summer and months. After emerging from the tree, these adult L. barbicornis only live for a few weeks.