Tree or Strangling Epiphytes

Northern rata (Metrosideros robusta)³⁷ is the most notable and common example here. It is found in lowland forest throughout the North Island and near the north-west coast of the South Island. It is much more frequent as an epiphyte than as a ground plant and it prefers the tall emergent conifers as supporting trees. The earlier stages of its life cycle are very similar to those of the puka. It usually establishes in asteliad nests, although young plants have been observed attached directly to rough bark. A distinctive feature of some small northern rata plants is the development of tuber-like swellings on the roots which, it has been suggested, may serve for water storage.⁴⁶ Eventually a root grows down the trunk to the ground giving off horizontal girdling roots at intervals (Fig. 50). Unlike puka this descending root does not remain relatively slender, but gradually enlarges to become a metre or more in diameter. It is often branched near the ground to form a tripod or tetrapod arrangement (Fig. 51). More complicated patterns develop where several branching roots descend from a northern rata crown to form complexes several metres in diameter. In some cases more than one rata may be involved, although this is not easy to determine.

With the development of such a massive root system, when the supporting tree eventually dies the northern rata is able to stand alone on its 'pseudo-trunk' (Figs. 52, 53). If the support was an emergent then the rata now replaces it in that role.

The northern rata and some tropical epiphytic trees of similar habit are often referred to as 'stranglers'. This implies that these epiphytes kill the supporting trees by compressing their trunks within a complete or partial network of roots. Popular writers on New Zealand plants have taken enthusiastically to this idea, describing the northern rata variously as a 'predatory gangster', 'forest bandit' or 'notorious strangler' which 'crushes', 'smothers', 'stifles', or 'squeezes' the supporting tree in an 'iron', 'deadly' or 'fatal' embrace.⁴⁷

Partly as a reaction to these verbal flights, some botanists in recent times have tended to take a contrary view.⁴⁸ They point out that the page 84

Figure 51 Mature northern rata (Metrosideros robusta) with a tripod based trunk-like root. The original supporting tree is no longer present.
Paraparaumu, southern North Island. Photo: M. D. King.

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Figure 52 (opposite) Mature northern rata (Metrosideros robusta) with a branched trunk-like root system. The original supporting tree is dead, but its trunk persists inside the northern rata roots. The broken top of the trunk is indicated with an arrow. Kaitoke, near Wellington, southern North Island.
Photo: M. D. King.

page 86light-demanding northern rata generally establishes in the well lit crowns of mature trees so that, by the time the rata is large enough to stand alone, the supporting tree might well have died of old age. It does seem, however, that the northern rata must have some deleterious effect on the supporting tree through partial overshading, root competition, and perhaps in cases where the supporting trunk becomes enlarged within a well developed rata root cage, through some restriction in the movement of water and nutrients.

Recently a distinctive new tree species of Metrosideros (M. bartlettii) has been described.⁴⁹ It is restricted to a few forest patches near North Cape and is similar in epiphytic habit to northern rata.

Southern rata (Metrosideros umbellata) is rare and localised in the North Island, but quite common in montane and higher latitude lowland forests in the west of the South Island. It is mostly terrestrial, but has been observed growing as a 'strangling' epiphyte in several places. Similar Metrosideros epiphytes are known in New Caledonia, Fiji and Hawaiʻi.