Mixed Broadleaf Podocarp and Kauri Forest

FORESTS, INDIGENOUS

by Alec Lindsay Poole, M.SC., B.FOR.SC., F.R.S.N.Z., Director-General of Forests, Wellington.

FORESTS, INDIGENOUS

In descriptions of New Zealand forests distinction is usually made between the broad classes: (a) Mixed broadleaf (almost entirely evergreen) types; (b) Mixed broadleaf forest (the same forest as above) containing coniferous trees, mainly podocarps (Podocarpus, Dacrydium and Phyllocladus) and kauri (Agathis australis); (c) Southern beech (Nothofagus) evergreen types; and (d) Transition forest between the southern beech and the other two.

In their primitive condition all these forests, though to a lesser extent the beech forests, are noted for the luxuriance of their growth; in particular, the forest floor is densely covered with ferns, mosses, liverworts, grasses, sedges, and many small flowering plants, and numerous tree seedlings. The trees and shrubs are in many tiers. Coniferous trees usually tower above broadleaf trees as scattered emergents, and only under certain conditions tend to form closed canopies themselves. The broadleaf trees below them form an irregular canopy, and below them again, growing under dense shade, are smaller trees, shrubs, and tree ferns. Epiphytes of Astelia, large grasslike plants, or seedlings of a number of families, grow in the crowns of the trees; and lianes of many species add to the tangle of vegetation. The canopy of the southern beech forests is more regular, consisting as it does of usually one or two species of beech only, and the trees, shrubs, and ferns below and the ground cover are all more sparse.

In their structure, the broadleaf forests are akin to wet tropical forest, and have been called by some ecologists “subtropical evergreen forest”. The beech forest, on the other hand, because it was thought that the origin of Nothofagus was in the antarctic (from fossil evidence), was called “subantarctic rain forest”. With the growing knowledge of Pacific vegetation and the reconstruction from pollen records of past vegetation movements associated with climatic changes, these concepts are, however, changing rapidly.

New Zealand lies between the latitudes 34°S and 47°S. Within the same latitudes in the Northern Hemisphere lie very large land masses. These are noted for once having carried very extensive forest of coniferous trees, pines, spruces, and firs, etc., and deciduous broadleaf trees, oaks, beeches, maples, birches, poplars, etc. The New Zealand forests bear almost no resemblance to these Northern Hemisphere forests, but they do bear a relationship to some Southern Hemisphere and high-altitude tropical forests. Although New Zealand is thought to have been isolated from other large land masses for a very long period, possibly since Triassic-Jurassic times, the same type of forest is to be found in other places throughout the Southern Hemisphere. New Zealand's isolation has, however, resulted in a high degree of endemism amongst species.

Pre-settlement Forests

The extent of forest before human settlement commenced is now impossible to determine, but it is thought that at about the time white settlement began, at least two-thirds of the country was forest clad. Both Maori occupation and other influences brought about constant changes in the forests. These will be described briefly. European settlement has, of course, brought about major changes, which will be described as the forest types are dealt with. Lastly, the introduction of mammals, of which the country was completely free, except for two bats, has everywhere changed the virgin condition of the forests. No forest is now free of their influence. Readily detectable effects are brought about by the ground-browsing animals, deer, goats, and escaped domestic animals, and the canopy browser, the opossum. These effects vary from complete destruction of certain types of forest by a combination of animals to slight effects on bird life and thus on the regeneration of the forest.

Before any human settlement took place, forests – as well as other vegetation – were undergoing slow but constant changes associated mainly with climatic changes. Beech forest invaded broadleaf, and vice versa. Botanists over the past decade have shown that changes of this nature now taking place indicate the development of a colder and wetter climate. These changes are occurring within the life spans of some of our major forest trees, rimu, totara, kauri, etc., which can be as much as several hundred or even a thousand or more years old.

The great central North Island volcanic eruptions of the past few millennia brought about cataclysmic changes in the vegetation throughout certain districts of the North Island. There is ample evidence, in the form of charred remains, of the widespread destruction of forest by fire and by burial under pumice. It seems that the first vegetation to cover raw pumice consisted mainly of grasses and some low pioneering shrubs. Because the region is, by and large, a forest one, forest vegetation gradually spread through the lower growing forms of vegetation from pockets of forest that escaped destruction and from the undestroyed forest around the edges of the showers. The result was a peculiar pattern of forest, related in part to nucleus forest areas and in part to the nature of the spread of the forest – the quick spread of podocarp-broadleaf forest, and the slow spread of beech.

Mixed Broadleaf Types

By and large, the coastal forests consisted of mixed broadleaf types, though in many places they contained some podocarps, or, in the north, kauri. Most of this forest has disappeared and the lower remnants are usually eaten out by stock; intact samples are rare. The coastal forest from the North Cape to the East Cape and Taranaki was dominated by karaka (Corynocarpus laevigatus), kohekohe (Dysoxylum spectabile), pohutukawa (Metrosideros excelsa), and puriri (Vitex lucens). In many places the nikau palm (Rhopalostylos sapida) occurred in large numbers. The coast to the south of this had forest containing much karaka, kohekohe, ngaio (Myoporum laetum), and nikau palm, extending as far as the tip of the South Island, and along the West Coast of the South Island as far as Westport. The coast of the Sounds-Nelson area, however, was principally covered by beech forest. On the West Coast the broadleaf forest contained a high proportion of podocarps, mainly rimu, growing right to the cliff edges. Much of this still remains as does southern rata (Metrosideros umbellata) forest on the rugged Fiordland coast. Although not strictly forming a forest in New Zealand, the mangroves (Avicennia resinifera) must be mentioned. They occupied – and still occupy – tidal mudflats from the north to about latitude 38°S.

The lowland and montane forests away from the coast, although they consisted mainly of broadleaf trees, contained a varying proportion of coniferous trees, usually podocarps. In some places there were broadleaf forests without the coniferous component, but these were limited in area. This, however, is the type of forest that remains now, because podocarps and kauri have been logged out leaving behind broadleaf trees only. There are a number of distinct types. One that is common on the Mamaku plateau and West Taupo region is dominated by hinau, Eleocarpus dentatus, with its olive-like fruit, and kamahi (Weinmannia racemosa). In other parts of the same area, and extending into the Urewera, are forests containing hinau, kamahi, much kohekohe, and some northern rata (Metrosideros robusta). Throughout most of these broadleaf types there is a content of kamahi. In the South Island southern rata becomes a dominant tree in many places, but kamahi remains very common.

High-altitude forests, if not of beech, are nearly all of mixed broadleaf types. Throughout the North Island kamahi is a very common tree at the upper limits of forests. Similarly, on the West Coast there are both kamahi and southern rata. Forests composed mainly of these two trees clothe the western flanks along the central part of the Southern Alps. These forests can be severely damaged by a combination of deer and opossums. In the Fiordland area much of high-altitude forest consists of southern rata, and in the Catlins area, on the south-east coast of the South Island, kamahi is again the principal tree. Upper timber-line belts are largely composed of broadleaf shrubs made up frequently of species of the two genera of tree daisies, Olearia and Senecio.

Mixed Broadleaf Podocarp and Kauri Forest

This was the most plentiful forest of the lowland and montane areas of New Zealand. It varied much in composition, according to latitude, altitude, slope, and aspect. It has provided, and continues to provide, a very large part of the milling timber cut in this country. The forest has been cleared from most of the more accessible areas as settlement progressed. The only substantial areas of it that remain are on the West Coast, where the soils are not suitable for agriculture, and on the central North Island, where agriculture, because of soil deficiencies, has only lately been developed. Pockets still exist in the valleys along all the mountain chains, and logged kauri forest containing kauri regeneration is present in small and large areas throughout the former extent of the kauri forest.

The forest in the north, particularly north of Auckland, and on the Coromandel Peninsula, frequently contains kauri as the dominant emergent tree. Trees sometimes reach very large dimensions and, with cylindrical boles and large spreading crowns, are a distinctive feature. Other conifers frequent in the same forest are rimu, tanekaha (Phyllocladus trichomanoides), and, in places, toatoa (P. glaucus). The lower tiers of broadleaf trees are various combinations of towai (Weinmannia sylvicola), kamahi, Hall's totara, taraire (Beilschmiedia taraire), and northern rata.

South of the kauri group of types, rimu becomes the dominant podocarp, but in the centre of the North Island on the pumice soils there is a mixture of podocarps, depending upon the age and the history of the forest that has arisen since the pumice showers. Matai (Podocarpus spicatus) and totara (P. totara) sometimes dominate, or matai, totara, rimu (Dacrydium cupressinum) and miro (P. ferrugineus) are all present. In parts of this area the two totaras, P. totara and P. hallii, meet and hybridise. In a few places only, the tree species of Phyllocladus also meet and hybridise. The most abundant hardwood in these types is kamahi, and tawa is frequent. In the centre of the North Island rewarewa (Knightia excelsa) and hinau are also plentiful. Throughout the remainder of the North Island rimu is the main podocarp tree up to about 2,000 ft elevation. Matai is present on the best soils and kahikatea on the swampy river silts. Totara is common on river soils, particularly shingly ones. Nearly all the matai, kahikatea, and totara forest has, however, been cleared. The hardwoods accompanying these podocarps are kamahi, Olea species, and the northern rata, which starts its life in the crowns of tall podocarps and gradually replaces its host.

The West Coast terraces are the home of rimu forest. In these, rimu grows frequently in groups with the crowns almost touching. A feature of this forest also is that rimus of all ages are found, ranging from seedlings to overmature trees. The hardwoods found in this forest are southern rata, Quintinia and kamahi. Off the terraces rimu is still plentiful, but it is usually a larger tree and there is an absence of juvenile forms. The hardwoods form a larger part of the forest.

At higher elevations in both islands the mountain toatoa (Phyllocladus alpinus) and kaikawaka (Libocedrus bidwillii) both form forests or belts of forest, sometimes of considerable extent.

Southern Beech Forest

In this forest four beech species occur, one with two major varieties. Whether they are found singly or in mixtures the forest itself contains mainly, if not entirely, beech as the dominant forest trees and the canopy is even compared with that of the mixed broadleaf coniferous forest. Such complete dominance of beech is made possible by the periodic occurrence of years of heavy flowering and seeding, normally followed by dense regeneration.

The brief account which follows of the habitat requirements of the individual species will help towards an understanding of the distribution of the species and forest throughout New Zealand. Black (Nothofagus solandri var. solandri) and hard (N. truncata) beeches are both species of a warm temperate climate, which in New Zealand seldom extends above 1,500 ft altitude. Hard beech withstands warmer conditions than do any of the other species, as indicated by its presence in the far north, while black beech is the most drought-resistant species. Although pure forests of both are to be found, these are of limited extent; they usually occur intimately associated in the same forest. Red beech (N. fusca) prefers somewhat cooler conditions and likes moist situations. Where it and hard beech occur together it takes the damper, more fertile sites, while hard beech is found on the drier sites and ridges. Mountain (N. solandri var. cliffortioides) and silver (N. menziesii) beeches are both species of a cold temperate climate and frequently form the timber line. They do descend to low altitudes in warmer climates, silver beech in quite extensive areas, often as a riparian tree, and mountain beech occupying very poor soils. At higher altitudes silver beech is present under moister conditions than is the case with mountain beech.

Hard beech is found in a few localities as far north as Kaitaia and around the Auckland district it becomes common. From the East Cape to the northern part of the South Island it is an important forest tree. Silver beech and red beech first occur on Mount Te Aroha and again on the Mamaku Plateau. From there southwards they become important. Black beech occurs from the Egmont-Wanganui district to about the centre of the South Island, and mountain beech from the mountains of the East Cape to the bottom of the South Island.

Beech forms the main forest all along the chain of mountains from the East Cape to Cape Palliser, with silver and mountain beeches at the higher altitudes depending upon site. Red beech occurs mainly at intermediate elevations, with hard and black beeches lower down the mountains and in the lowlands. In the north-west part of the South Island all species intermingle closely and form by far the greater part of the extensive forest cover. Mountain beech forms the forest at timber line all through the east of this region, and silver beech, together with mountain beech, throughout the west. West of the main divide beech forest virtually ceases at the Taramakau River and does not reappear for 100 miles further south at the Mahitahi River. By that time it is principally silver beech, and this tree dominates the greater part of the extensive forests throughout the south-western part of the South Island.

Hybridisation occurs between species of beech except silver beech. Between black and mountain beeches there are many transitional forms. Where the two appear on the same mountain range it is possible for mountain beech to occur at the timber line and black beech in the foothills, with a complete range of intermediates between the two. Mountain and red beeches do not associate frequently, but when they do they hybridise freely and adult hybrid trees are plentiful. Black and hard beeches, the species usually present in the same forest in intimate mixture, hybridise much less frequently and adult hybrid trees are not common.

The New Zealand southern beeches have interesting affinities with beeches occurring in other parts of the Southern Hemisphere. A great deal of knowledge has accumulated in recent years on this, much of it coming from studies of pollens. The New Zealand species have close affinities with those found in Australia and South America, and there is a peculiar association with a fungus, Cyttaria, occurring on silver beech and also on a similar species in South America, but on no other species. In the mountains of New Guinea and New Caledonia are a number of species of beech belonging to a section not now represented in New Zealand. Fossil pollens show, however, that it was present in past ages when conditions were warmer.

The Transition Forests

Where two or more types of forest meet, transition types occur, sometimes on an extensive scale. Podocarps in particular are frequent in certain localities as emergents in beech forest.

by Alec Lindsay Poole, M.SC., B.FOR.SC., F.R.S.N.Z., Director-General of Forests, Wellington.

  • Monograph on the New Zealand Beech Forests, Cockayne, L. (1926)
  • Vegetation of New Zealand, Cockayne, L. (1926)
  • Forests and Climate in the South Island of New Zealand, Holloway, J. T. (1954).

FORESTS, INDIGENOUS 23-Apr-09 Alec Lindsay Poole, M.SC., B.FOR.SC., F.R.S.N.Z., Director-General of Forests, Wellington.