Forest Vines to Snow Tussocks: The Story of New Zealand Plants
More Restricted Forest Types
More Restricted Forest Types
This type is not greatly different from the general conifer broadleaf forest of better drained sites. It is best developed along the Northland coasts and adjacent islands and is dominated by three species that rarely occur very far from the sea. Pohutukawa (Metrosideros excelsd), which often grows alone on coastal cliffs, is restricted to the northern half of the North Island, but karaka (Corynocarpus laevigatus) with its large, dark green leathery leaves and ngaio (Myoporum laetum) extend to the northern and eastern South Island.
Coastal forest is of lower stature than inland forest and, as a result of the general absence of conifers, it lacks emergents.
In addition to the three trees already mentioned, of which pohutukawa is the largest (and when in flower the most spectacular with its bright red stamens) there may be a number of other trees, shrubs and herbs. Many of these also occur in inland forests, at least in the far north. Notable among these are the trees puriri (Vitex lucens), kohekohe (Dysoxylum spectabile) and the shrub kawakawa (Macropiper excelsum).
Of particular note are a number of large-to very large-leaved species in the northern North Island which range from moderately common to rare. Some of them have strong tropical affinities, and so can be regarded as relicts from warmer times.
On the mainland as well as on islands, and extending to East Cape and beyond, are the small trees parapara or the 'bird catching tree' (Pisonia brunoniana) and tawapou (Planchonella costata). Parapara has extremely sticky fruits to which small birds can become attached. It is also found in Australia and some Pacific Islands. The tawapou is closely related to species in Norfolk Island and Fiji.
The remaining species are found on the Three Kings Islands85,86 off the northern tip of New Zealand and are either restricted there to or also occur on the Hen and Chicken Islands or Poor Knights Islands further south.
|(a)||Genera with no other New Zealand species: Pukanui (Meryta sinclairii): a small tree with very large simple leaves which is also found on the Hen and Chicken Islands off the Northland east coast. Meryta is centred in New Caledonia with a few species elsewhere in the Pacific.
Tecomanthe speciosa: a liane with compound leaves and cream-coloured tubular flowers. Only one plant is known. Other species of the genus are found in Queensland and New Guinea. Elingamita johnsonii: an endemic genus related to Tapeinosperma of the tropics. Only one tree is known.
Davallia tasmanii: a fern belonging to a largely tropical genus.
|(b)||Genera with one or two more widespread species in New Zealand:
Pennantia baylisiana: one tree only is known.
Alectryon grandis: one tree only is known on the Three Kings. It is possibly also found on the Poor Knights Islands.87
Streblus (Paratrophis) smithii.
Cordjline kaspar: also on Poor Knights Islands.87
(Some of the rare species may have been more common before goats, which have since been exterminated, were released on the Three Kings Islands.)
It should not be thought that plants are restricted to coastal forest because they require a salty environment. They grow near the coasts because that is where the mildest climates are. If New Zealand extended further to the north, then many of them would occur at inland sites, and indeed the nearest relatives of pohutukawa in the tropical Pacific are not coastal at all, but are found in mountain forests. Conversely, in the far south of New Zealand some species, of inland forests further north, can be found growing close to the sea.
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!
Mangrove is the term applied to a number of small trees, mostly tropical, which grow between the tides in shallow seas bordering sheltered coasts. In New Zealand we have only one species, Avicennia resinifera. The derivation of the species' name is interesting; it was applied by the botanists on Cook's first voyage, in the mistaken belief that the bits of floating kauri gum they observed came from nearby mangroves.
Like the kauri, the mangrove is restricted to the warmest part of the country reaching the Bay of Plenty in the east and Kawhia in the west. Towards its southern limits it is no more than a shrub, but further north the mangrove can be a tree up to 10 m high forming low forests with a close canopy. For the most part these forests contain no other species of flowering plants.
Fresh Water Swamp Forest
In the lowlands throughout New Zealand, swamp forest often occurs in localised areas depressed below the general level of the landscape. Such depressions can be found in the irregular terrain of volcanic and formerly glaciated areas; in areas where land blocks have sunk along faults; where meandering rivers have changed their courses; and between consolidated sand dunes.
Figure 66 Swamp with cabbage trees (Cordyline australis) and New Zealand 'flax' (Phormium tenax). Near Kaeo, northern North Island.
Photo: J. W. Dawson.
Figure 67 (opposite) Exterior view of kahikatea (Dacycarpus dacrydioides) swamp forest. Harihari, west coast, South Island. Photo: J. H. Johns.
There is an often rather open undergrowth of small-leaved and largeleaved shrubs, many of which persist from earlier stages in the succession, and sedges are prominent among the forest floor plants. Herbaceous and woody epiphytes are often common and among the vines kiekie (Frejcinetia baueriana var. banksii) is particularly abundant, covering the forest floor in places and completely obscuring the trunks of trees up to a considerable height (Fig. 32).
Over time, when the soil has built up sufficiently above the water table, the swamp forest will give way to ordinary conifer broadleaf forest.
Bogs, although similar to swamps in that they are waterlogged, are much less fertile and much more acid, and the slow breakdown of plant remains which results from these conditions leads to the build up of peat, often with peat moss (Sphagnum) playing a prominent role. The greater fertility of swamps is due to the fact that most of their water content comes either from streams or from ponds or lakes fed by streams. The latter contain minerals dissolved from the rocks and soils through which they pass and in times of flood they may also carry quantities of fertile silt. Bogs on the other hand are found mostly in high rainfall areas and most of their water comes as rain, which is devoid of nutrients. Their other characteristics follow from this.
Raised bogs have convex surfaces and consequent outward drainage. They are best developed in the Waikato and parts of Southland and they may become established on former swamps.
In the North Island forested bogs are largely restricted to middle altitudes on the central volcanic plateau near to Mt. Ruapehu. In the South Island they are common on the lowlands of the wet western side and also in Southland and Stewart Island.
Figure 68 (opposite) Aerial view of the transition from pakihi bog through silver pine (Lagarostrobos colensoi) dominated forest to rimu (Dacrydium cupressinum) dominated forest on better drained terrain. South Westland.
Photo: J. H. Johns.
As with kahikatea swamp forest, it seems probable that as the soil level builds up and soil moisture declines sufficiently, the progression ultimately leads to the general conifer broadleaf forest of the region. Any trend towards increased regional rainfall would, of course, slow or even reverse such a progression.
Mark and Smith83 concluded that the pakihi in their south Westland study area had never previously supported forest, but Rigg,84 who studied pakihis in north Westland, suggested, on the evidence of buried logs, that forests (probably dominated by rimu) did formerly occupy such terrain and that there was possibly a succession of such forests. The soil of these pakihis is highly leached and infertile with a well developed iron pan, which also suggests a previous forest which provided an acid litter. Rigg proposes, as one possibility, that the podocarp forests would have died out as a result of the reduced soil fertility and, presumably, the impeded drainage they themselves induced. Between successive forests a regeneration sequence similar to that described for South Westland would have taken place. This hypothesised forest-bog-forest cycle is similar to the forest/swamp/forest cycle proposed for some kauri localities.
As with the gumlands of Northland, many of the pakihis of Westland have been repeatedly burnt in both Maori and European times.