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Tuatara: Volume 1, Issue 3, September 1948

Rata the Killer

page 36

Rata the Killer

The peculiar “strangling” habit of the rata, Metrosideros robusta A. Cunn., presents one of the most spectacular botanical phenomena to be found in New Zealand rain forests. Hooker had some doubts as to the habits of this tree when he described it in his Handbook (1864). He said it was “tall, erect (never? climbing). …” The popular conception of the tree by the early settlers was recorded by T. Kirk in 1872 when he wrote an article on rata to dispel some of the erroneous ideas then current. It was believed “that these immense trees were originally weak climbing plants, the stems of which increased in bulk until they killed the fostering tree which had supported them, and ultimately united to form a solid trunk. …” Kirk pointed out that the weak climbing plants belonged in fact to a different species, but, he said, “there can be no question that M. robusta is often found destroying trees by which it is supported. … The seeds of M. robusta are conveyed by birds, or blown by wind, amongst the epiphytic masses of Asteliads, Lycopods and Ferns, so abundant in the trees of the northern forests. In this situation the plant takes root and forms a small bush, for a time obtaining sufficient nourishment from the decaying vegetation in which it is growing, until the limited supply proving insufficient for the increasing demand, its roots stretch boldly down the trunk of the supporting tree, in search of that full supply which can only be obtained from the earth. Sometimes only a single root is given off, at others one main root with one or two weaker roots are to be seen, and again several roots of about equal dimensions are to be found, but in nearly all cases the different roots or stems are bound together by smaller roots, which are given off at right angles to the trunk of the supporting tree, and become united with the adjacent main roots by inosculation; not unfrequently masses of fibrous roots are developed, which perish with the increase of the main root, after serving their purpose of deriving temporary nourishment from the atmosphere. In course of time the various stems become inosculated (coalesced), to a greater or lesser extent, along their course, and the supporting tree is literally strangled by their iron embrace.”

In his account (1938) of the vegetation of the Tararua Mountains the present author discussed briefly certain relationships of the rata to its environment. “It is a light demanding large bushy ‘shrub’ which cannot exist on the forest floor (at least not on the Tararuas). Instead it commences its life as an epiphyte high up in the forks of the tallest trees. It then sends to the ground one or more slender roots, which finally become established in the soil and function as ordinary stems. These stems often surround the foster tree at its base and in page 37 the early stages also clasp the trunk of the supporting tree more or less permanently by lateral roots. This fact, together with that of rata surviving long after the death of its foster tree led Kirk (1872) to believe that the former strangles the latter. On the Tararuas the foster tree is usually rimu, Dacrydium cupressinum, and the dead trunks of these are commonly to be seen. The author, however, was never able to detect any evidence of strangulation. The explanation of the death of the rimu is to be sought in its failure to compete successfully for light. Rimu, like rata, is also a strong light-demanding plant, as is evidenced by its habit of growth. Once, however, its crown is overtopped by the epiphyte it is doomed to die.”

Simpson and Thompson in a note (1942) on Metrosideros robusta took exception to the above observations. They said: “Other observers have suggested to us that root competition may be the destructive agent or a soil reaction set up by the closely matted fibrous roots. Kirk… states that puriri (Vitex lucens), to increase its girth, bursts apart the stems of the epiphyte and, if this be accepted, shade can have little effect on that species.” This latter supposition is probably quite true, since puriri is a fairly shade tolerant species throughout its life. They further observe: “No force, indeed, other than compression could cause the distortion that appeared in rimu trunks, cut while still growing, enclosed and over-topped, from forest near Kaituna River in North Nelson. These … had grown deep, full-length, irregular flanges into the spaces afforded by the rounded stems of the epiphyte, and the distorted timber was sound.” From the above observations it is impossible to predict from which cause these particular specimens of rimu would have died if they had not been cut. It might be added in passing that death from old age must have claimed many trees whose decaying trunks are still being supported by the “epiphyte”. It is to be remembered that by nature of circumstances the foster tree of rata is already a tall forest tree while the latter is but a mere seedling on it.

The story of rata strangling its foster tree has appealed to popular imagination and is often repeated. It is of interest to read the note of warning in Laing and Blackwell's “Plants of New Zealand”. “This, at least, is the generally accepted explanation. It must, however, be confessed that some of the details of the process have hitherto escaped observation.” Rather strangely the note continues: “Apparently the only tree which can resist this iron hug is the puriri—the strongest and toughest of all New Zealand trees. It may sometimes be seen bursting the encircling roots of the epiphyte.” In the last two sentences one has the mighty strength bursting the clutches of the giant. Such observations probably have their origin in Kirk's paper, already mentioned. “The only tree which the rata seems powerless to injure is the puriri …; a fine example, surrounded by three or four large stems, page 38 which it has forced outwards at the base, is to be seen … by the Hotea River, Kaipara; similar instances are rare.”

It must be observed that the strength of wood of the foster tree matters little, so long as it does not become crushed. To be able to break free, the foster tree must develop sufficient bursting power in the trunk through the osmotic pressure in the tissues laid by the cambium, to break the hug of the rata.

Literature Cited

HOOKER, J. D. 1864. “Handbook of the New Zealand Flora” p. 72.

KIRK, T. 1872. On the Habit of the Rata (Metrosideros robusta). Trans. N.Z. Inst. 4: 267-270.

LAING, R. M. and BLACKWELL, E. W. 1927. “Plants of New Zealand” 3rd Ed. p. 285.

SIMPSON, G., and THOMSON, J. S. 1942. Notes on Some New Zealand Plants and Descriptions of New Species (No. 2). Trans. Roy. Soc. N.Z.: 72: 21-40.

ZOTOV, V. D. et al. 1938. An Outline of the Vegetation and Flora of the Tararua Mountains. Trans. Roy. Soc. N.Z.: 68: 259-324 (esp. p. 275).