A number of tropical trees exhibit structural features which appear strange to the eyes of those from most temperate regions. Many trees, particularly in swampy situations, develop thin flanges or plank buttresses from the bases of their trunks. It is suggested that these plank buttresses, which are more or less triangular in form and may extend for several metres up the trunks and out along the roots, confer greater stability on the often shallowly rooted trees. These plank buttresses are very inconvenient for tree fellers who often find it necessary to construct scaffolding so that the trunk can be sawn through above them.

Other trees increase support for their trunks by forming downwardarching prop roots. This can be seen, perhaps most conspicuously, in mangrove forests. In swampy situations many trees send up pneumatophores or breathing roots, through which air is taken in and transferred to the root system beneath the swamp. Pneumatophores are particularly conspicuous in saline coastal swamps inhabited by mangrove forests, but are also found in inland freshwater swamps. An unusual type of aerial root in tropical forests is the column root, which descends to the ground from horizontal branches, such as in the banyan figs. Individual banyan trees can sometimes spread out by this means over hectares of ground.

Plank Buttresses

In New Zealand pukatea (Laurelia novae-zelandiae) often has well defined plank buttresses (Fig. 9). Other trees may also have buttresses: e.g. kohekohe (Dysoxylum spectabile), kahikatea (Dacrycarpus dacrydioides), and some species of beech (Nothofagus), but these are usually not plank-like.

Pneumatophores

Pukatea, when growing in swamps, also forms large pneumatophores. These are basically shield-shaped and sometimes several times higher than they are wide (Figs. 10, 11). These pneumatophores originate when a root tip arches above the swamp surface and then grows back in again so forming a loop. Wood is then added mostly on the upper and lower sides of the loop which leads to the eventual shield-shape. The pneumatophores are covered with mosses and have pustules (lenticels) of loose corky cells, through which air enters the root system.

Figure 12 (above) Pneumatophores of swamp maire (Syzygium maire). The one on the right has been cut in half longitudinally. The arrow indicates the white air-filled tissue.
Photo: J. E. Casey.

Swamp maire (Syzygium maire) is often associated with pukatea in swamp forests and it too forms specialised pneumatophores. In this case, however, they are not woody loops, but smaller, finger-like and rather spongy root tips several centimetres long. They are orangey-brown in colour and often branch near the base to form coral-like clusters. A longitudinal section through such a 'peg root' reveals a white outer cylinder of air-filled tissue (Fig. 12).

In the same swamps, kiekie (Freycinetia baueriana var. banksii) often sprawls on the forest floor as well as climbing up tree trunks. In the former situation it forms what may be pneumatophores of an unusual type. They are slender and finger-like, and have a succession of tyre-like rings of white aeration tissue.

Finally the pneumatophores which catch the eye of many people in the Auckland region are those formed by the mangroves. These too are finger-like and project above the mud surface at low tide (Fig. 13).

Prop Roots

In the swamp maire the primary root in young plants often atrophies or becomes weak, and prop roots develop from the base of the trunk to provide the root system of the adult tree. In young trees the space between the base of the trunk and the ground is readily observable (Fig. 14).

The nikau palm may also produce a fringe of slender prop roots a little above ground level.

Column Roots

Pohutukawa (Metrosideros excelsa) forms aerial roots very readily, which run down the trunk to the ground. Sometimes it forms column roots from more-or-less horizontal branches. Except where the trees have a semi-sprawling habit on coastal cliffs, these roots often do not reach the ground, but branch profusely at the lower end to give a straw broom effect (Fig. 15). Similar roots can be observed in some tropical figs (Ficus).

The leaves of plants of moist tropical forests are evergreen, generally larger than those of temperate forests and are often leathery and smooth-margined. Average leaf size decreases with increasing height above the ground and some of the trees have distinct juvenile forms with leaves much larger and/or more compound than those of the adults. In some cases there are narrow prolongations from the ends of the leaves known as 'drip tips', thought by some to enable rapid drying of leaves after rain.

Pulvini or elastic swellings at one or both ends of the leaf stalk or petiole are a common feature of tropical forest plants. It has been suggested that bending movements at these pulvini enable the leaf to maintain the best orientation to the light.

Leaf size

The smaller leaf sizes in New Zealand probably correlate with lower temperatures.²³

Teeth, Pulvini and Drip Tips

The proportion of woody dicotyledon species in the conifer broadleaf page 37forest with smooth-margined leaves or leaflets is lower than that of tropical forests: about 56 per cent compared with 80 per cent recorded, in a Nigerian forest.²¹ Godley²⁴ has observed that some New Zealand trees have much more prominent teeth on the leaves of young plants than on those of adults; for example wineberry (Aristotelia serrata) and ngaio (Myoporum laetum). In other cases, some trees with smooth-margined adult leaves or leaflets have juvenile leaves with toothed or lobed margins; for example puriri (Vitex lucens), titoki (Alectryon excelsus), kohekohe (Dysoxylum spectabile) and several species of Hebe.

Pulvini also are not common. The species of maire (Nestegis) have dark-coloured pulvini at the petiole bases and kohekohe (Dysoxylum spectabile), titoki (Alectryon excelsus) and king fern (Marattia salicina) have them at the bases of the leaflet petioles. Hinau (Elaeocarpus dentatus) is an interesting case. The adult leaves do not have pulvini, but on juvenile leaves they can be observed at each end of the petioles. The larger-leaved tropical species of Elaeocarpus have prominent pulvini in the same position on adult leaves.

Drip tips are neither strongly developed nor very common in the New Zealand forest. In fact the most that can be said is that ten or so species tend to have slightly to moderately drawn out leaf tips especially when growing under sheltered, shady conditions. Perhaps both drip tips and pulvini were better developed in the ancestors of our present trees.

Juvenile Forms^24,25

Often very distinct juvenile and adult forms are a noticeable and, to those learning to identify native plants, a tiresome feature of the New Zealand flora. In the conifer broadleaf forest, some of the juveniles contrast with those of the tropics in that they are very freely branched with leaves much smaller than on the adults. These will be considered in Chapter 6 as part of the wider question of the small-leaved, freely branched or 'divaricating' shrubs, so prevalent in New Zealand.

The remaining rain forest species under this heading have juvenile leaves which are larger or longer or more compound or more dissected than those of the adults.

Raukawa (Pseudopanax edgerleyii) has palmately compound juvenile leaves with up to five deeply lobed leaflets. The adult leaves are simple and smooth-margined.

One variety of Pseudopanax simplex has juvenile leaves very similar to page 38those of P. edgerleyii and simple, toothed adult leaves (Fig. 18).

Pate (Schefflera digitata) is unusual in that juvenile leaves are only found on some plants in the northern half of the North Island. These leaves are palmately compound and similar in size to those of the adults, but the leaflets are deeply lobed rather than just toothed.

In three tree species, all belonging to the family Cunoniaceae, the leaves are basically pinnately compound. In Ackama rosifolia the juveniles have six to ten pairs of leaflets, the adults three to five. In towai (Weinmannia silvicola) the trend is from five to one pairs of leaflets; in kamahi (W. racemosa) from one or two pairs of leaflets to simple leaves (Fig. 19).

Figure 18 Juvenile and adult leaves of Pseudopanax simplex var simplex. Dissected compound juvenile leaves above; serrate, compound semi-juvenile leaves centre; simple, serrate adult leaves below. Photo: J. E. Casey.

Figure 19 Juvenile and adult leaves.
Lancewood (Pseudopanax crassifolius): la, juvenile leaf; lb, adult leaf; lc, d, brown, mottled leaves from a seedling.
Pseudopanax ferox: 2a, coarsely toothed juvenile leaf; 2b, adult leaf.
Hinau (Elaeocarpus dentatus): 3a, juvenile leaf; 3b, adult leaf (note domatia).
Rewarewa (Knightia excelsa):
4a, juvenile leaf; 4b, c, adult leaves.
Kamahi (Weinmannia racemosa): 5a, trifoliolate juvenile leaf; 5b, simple adult leaf.
Photo J. E. Casey.

In all cases where the juvenile leaves are compound and the adult leaves simple, the latter have a distinct joint at the end of the petiole as an indication of their compound derivation.

Only a few New Zealand species have juvenile leaves which are broad and much larger than the adults: wineberry (Aristotelia serrata), Nestegis

Figure 20 Adult lancewood (Pseudopanax crassifolius), with a basal shoot of completely juvenile form to the left. Nga Manu Reserve, Waikanae, southern North Island.
Photo: J. E. Casey.

page 41

Figure 21 (left) Juvenile (below) and adult foliage of rimu (Dacrydium cupressinum).
Photo: J. E. Casey.

Figure 22 (right) Juvenile (above) and adult foliage of kahikatea (Dacrycarpus dacrydioides).
Photo: J. E. Casey.

apetala and Coprosma chathamica. In several species of Dracophyllum, where all the leaves are long and narrow, the juvenile leaves are much larger than the adults. In species of other genera the juvenile leaves are themselves very small but they are still larger than the adult leaves, which are reduced to scales, as in several species of 'whipcord' Hebe, Helichrysum coralloides and a group of conifers formerly included in Dacrjdium; silver pine (Lagarostrobos colensoi), yellow silver pine (Lepidothamnus intermedius), bog pine (Halocarpus bidwillii)H. biformis and manoao (H. kirkii). In the latter cases there is an abrupt change from juvenile to adult leaves. Two other conifers with scale-leaved adults are our only Dacrydium — rimu (D. cupressinum) (Fig. 21) and kahikatea (Dacrycarpus dacrydioides) (Fig. 22). In both of these the juvenile leaves are small and needle-like. The juveniles of rimu are notable for their attractive weeping habit.

Figure 23 Domatia of the adult leaves of hinau (Elaeocarpus dentatus). The pouch-like domatia form at the junctions of the secondary veins and the midrib. Photo: M. D. King.

In most of the species that follow, juvenile leaves are as narrow as the adult or narrower, but may have a larger area by virtue of their greater length. This is not a pattern familiar in the tropics, although on Mauritius and Reunion Islands in the Indian Ocean there is a partly comparable pattern — one species each of 24 genera has juvenile leaves which are much narrower than the adult leaves and also much smaller in area.²⁶

Lancewood (Pseudopanax crassifolius) (Figs. 19, 20) is the best known of our trees with a juvenile form.²⁷ The juvenile leaves are several times longer than those of the adult and a little narrower. They hang down in a distinctive cluster from the tip of the slender stem, which does not branch until it attains a height of 4-5 m after 15 or more years. The stem is then able to branch into a dense and rounded crown with short, broad, upwardly directed adult leaves. The related but less common P ferox is very similar, but the juvenile leaves have very coarse irregularly-shaped teeth (Fig. 19). Lancewood also has a distinct seedling form in which the leaves are very variable in shape, size and degree of dissection. Rewarewa (Knightia excelsa) and hinau (Elaeocarpus dentatus) (Fig. 19) with their long narrow juvenile leaves have a juvenile-adult pattern similar to lancewood, but branching is initiated at an earlier stage. The juvenile leaves of hinau are soft, widest near the tip and have obscure teeth; those of rewarewa are stiff, of even width and have coarse teeth. As mentioned earlier, the juvenile leaves of hinau may have pulvini, which the adult does not, but the adult leaves have pouch-like cavities or domatia (Fig. 23),²⁸ where the secondary veins meet the midrib on the undersides.

Three of the four species of Nestegis or 'native olives' have narrow adult leaves. In the case of two species, black maire (N. cunninghamii) and white maire (N. lanceolata), the juveniles are much narrower than the adults, but as they are also often longer they may have about the same area as the adult leaves or somewhat less. The last species, N. montana, has adult and juvenile leaves which are almost equally narrow, but the juveniles, being longer, are greater in area than the adults.

Trees with distinct juveniles may exhibit a curious phenomenon whereby fully adult specimens give rise to shoots near the ground which bear leaves of juvenile form (Fig. 20). These are termed reversion shoots and they derive from dormant buds formed during the juvenile phase of the tree and so are 'programmed' to be juvenile. Pokaka (Elaeocarpus page 43hookerianus) may be an exception, as reversion shoots can occur 10 m or more above the ground; well above the height of the juvenile stage.

The Deciduous Habit

In north temperate forests many trees and shrubs are leafless during the winter, and in tropical forests some species, growing in areas where there is a distinct dry season, may be leafless during the unfavourable period. In the New Zealand rain forest, which grows in parts of the country where there is no well-marked dry season and winters are relatively mild, the deciduous habit is uncommon.²⁹'³⁰'³¹ The following New Zealand conifer broadleaf forest species are or may be leafless during winter: wineberry (Aristotelia serrata)Fuchsia excorticata, the vines Muehlenbeckia australis and M. complexa, and ribbonwood (Plagianthus regius). Some forms of kowhai (Sophora microphylla) lose their leaves in the spring before flowering. However, with at least Fuchsia and Aristotelia, leaf fall is more related to temperature than to declining day length, which is the triggering factor for most northern temperate deciduous trees. Both species may be evergreen in milder latitudes.⁶³