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.