Forest Vines to Snow Tussocks: The Story of New Zealand Plants
The kauri (Agathis australis) is one of the world's largest trees with an excellent timber that was extensively exploited following European settlement. Young trees are narrowly conical, but mature trees have widely branching crowns and huge cylindrical trunks with little or no reduction in diameter with height.
Although fossils of the kauri which date back to warmer geological times have been found in the far south of the country, at present it reaches its limit at about 38°S, so kauri forests are essentially a feature of the Northland and Coromandel peninsulas. Not all forests in these far northern areas, however, are dominated by kauri.
On more fertile soils, such as those derived from basalt, conifer broadleaf forest of the general type we have already considered prevails, while kauri forest is largely restricted to the less fertile soils derived page 112from consolidated sand dunes, clay stone and the sandstone known as greywacke. In the latter case kauris tend to be concentrated on the thinner soils of ridges and spurs with ordinary conifer broadleaf forest in the valleys.
The kauris with their great trunk columns are the overwhelming feature of these forests (Fig. 65). Their crowns form a high canopy at about 35-40 m and plant growth below this is often rather sparse. Most of the tree species of the non-kauri forests are present however, including rimu, taraire, tawa, towai and northern rata, but they tend to be rather small-crowned and spindly. Most of the shrubs and ground plants are found elsewhere, although some are particularly abundant in kauri forest: toru (Toronia toru), neinei (Dracophyllum latifolium) mairehau (Phebalium nudum) and a terrestrial form77 of the normally epiphytic Brachyglottis (Senecio) kirkii. Particularly conspicuous where they occur are the often huge tussocks of 'kauri grass' (Astelia trinervia) and the sedge Gahnia xanthocarpa.
The fallen leaves, bark flakes and twigs of the kauris form a litter poor in nutrients, which is slowly decayed by fungi rather than the more nutrient-demanding bacteria. In these circumstances the litter accumulates to considerable depths, sometimes up to 3 m near large trees. The litter becomes very acidic and this promotes heavy leaching from the upper layers of the soil and the formation of a thick concretelike iron pan at a lower level. The impervious nature of this can result in quite swampy conditions.
Kauris not only establish on less fertile soils; they also greatly impoverish them. It is not surprising, then, that there is little regeneration of kauris on the floor of a kauri forest. Cockayne63 thought that this was due to there being insufficient light on the forest floor, but more recent studies suggest that it results from the infertility of the litter and soil and perhaps also to competition from the roots of the mature trees.78 Whatever the cause, the lack of regeneration shows that kauris are unable to replace themselves in closed forests, and Cockayne postulated that the climax forest would be one without kauris in which the dominance would probably be assumed by taraire.
Figure 65 (opposite) Kauri (Agathis australis) forest interior. The large trunks are kauris. The narrow-leaved shrubs are neinei (Dracophyllum latifolium), and the narrowleaved ground herbs kauri grass (Astelia trinervia). Puketi forest near the Bay of Islands, northern North Island.
Photo: J. W. Dawson.
In some cases buried kauri logs are all aligned in the same direction, indicating destruction by a cyclone. Bieleski78 suggests that the upturned roots of the blown over kauris would break through the iron pan, improve the drainage of the soil and so allow a new sequence to commence.
At present there is extensive kauri regeneration in areas where forests have been logged or burnt in European times. The pioneer phase is dominated by the light-demanding manuka (Leptospermum scoparium) followed by kanuka (Kunzea ericoides). Numerous kauri seedlings and those of other species develop under the kanuka canopy and as the latter opens out the kauris grow through and overtop it.
Another type of shrubland widespread in the north is found on terrain known as 'gumland'.79 The kauri forests preceding the gumland vegetation left in the soil considerable quantities of resin, which was gathered intensively following European settlement for the making of polishes and varnishes. The soil is highly infertile and it is thought that repeated fires in both Maori and European times have led to the present degraded vegetation. Manuka (Leptospermum scoparium) is quite common, although stunted, and other characteristic shrubs are Dracophyllum lessonianum and the strikingly yellow-flowered kumarahou or 'Gumdigger's Soap'80 (Tomaderris kumeraho). Among herbs, sedges and the umbrella fern (Gleichenia tirtinata) are prominent. Undoubtedly the gumlands would have to be free of fire for a very long time before kauri forests could return.
Finally, the most puzzling sites containing kauri remains are swamps, as kauris do not grow in such sites except sometimes marginally. Not only is it common to find kauri gum in Northland swamps, but it is often located in several distinct layers indicating a succession of forests, presumably at times when drainage was good, alternating with treeless swamps.
Cheeseman,81 when considering the swamps of the low lying sandy peninsula leading to North Cape, was the first to suggest an explanation involving cyclical changes in height of the land above the sea. He was thinking in terms of raising and lowering of the land, but, although the effect was the same, we now know that it was a matter of raising and lowering of the sea level. At each glacial period during the recent (or page 115perhaps current) ice age, sea levels dropped by up to 100 m as large amounts of water were locked up in ice. Then, with each interglacial, ice would melt to varying extents and the sea level would rise again. With the lowered sea level, drainage of the swamps would improve and kauri forest would establish; with a raised sea level the forests would disappear as drainage deteriorated sometimes to the extent of inundation by the sea.
If this explanation for the former existence of kauri forests on currently swampy sites is correct it presents us with a curious contradiction: on such sites the kauri, now restricted to the warmest part of New Zealand, flourished during glaciations and disappeared during interglacials!