The Mighty Kauri Tree
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 New Zealand, 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.
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).
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], known as "Father of the Forests" 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.
Another huge tree, Kairaru, had a girth of 20.1 meters and a columnular trunk free of branches for 30.5 meters as measured by a Crown Lands Ranger, Henry Wilson, in 1860. It was on a spur of Mt Tutamoe about 30km south of Waipoua forest near Kaihau. It was destroyed some years later in the 1880's or 90's after a series of huge fires swept the area. Other trees far larger than living kauri have been noted in other areas also. Near the Billygoat Track above the Kauaeranga Valley near Thames, there are records of stumps up to 6 metres in diameter. Relocating these, if they still exist, would provide a tangible link to a lost past.
Given that over 90% 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 1970's. These were Toronui, in Waipoua forest. Its diameter was larger than that of Tane Mahuta and its clean bowl larger than that of Te Matua Ngahere, and by forestry measurements of the time was the largest standing. Another tree, Kopi, in Omahuta Forest near the standing Hokianga Kauri, fell in 1973. It was the 3rd largest in the country at that time with a height of 56.39 m (185') and a diameter of 4.19 m (13.75'). Like many very old kauri both trees were partly hollow, a characteristic shared by some very large old sequoias.
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 M?hoe 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. 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.
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 Ranges.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 BC. 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 kerer?, 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 R-selected species. In good conditions, where access to water and sunlight are above average, diameters in excess of 15 centimetres and seed production can occur inside 15 years.