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.

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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).