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Tuatara: Volume 11, Issue 2, June 1963

The Root Habit — of Some New Zealand Plants

The Root Habit
of Some New Zealand Plants

Little is known about the root habit of many New Zealand plants. One of the few published works is a paper by McIndoe (1932).

One feature of at least some species of forest tree is their ability to form root grafts. This phenomenon seems to be common in the Nothofagus species and is probably found in others. There are several interesting ecological implications arising from root grafting as it has been shown overseas (see Fraser and Gaertner, 1961), that materials are transmitted from individual to individual through the root systems. Weak plants may survive in a dense stand in this way and this raises the question of the nature of competition in such a situation.

A set of observations on a peculiarity shown by some New Zealand gymnosperms was made by Foweraker (1929) and bears repeating. Foweraker found that Podocarpus totara could survive burial by quite deep deposits of river silt by sending out adventitious roots near the new ground level. Photographs show trees with as many as three series of roots girdling the stem and each separated by several feet. This ability could be important to species inhabiting the flood plains of rivers. Christensen (1923) showed that a number of other New Zealand trees and shrubs survived burial by riverbed gravel in the same way, while others succumbed.

Some of the most aggressive colonisers of bare ground in the mountains are able to produce adventitious roots from stems buried by silt or gravel, or shoots from erosion-exposed parts which are apparently root tissue in nature. These include Muehlenbeckia axillaris, Raoulia spp., Gaultheria rupestris and Cyathodes fraseri. This capacity is shared with the aggressive adventive plant Rumex acetosella.

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It has been noted that some New Zealand trees and larger shrubs produce adventitious shoots readily from root systems which lie near the ground surface. Moar (1955) recorded this for Dacrydium colensoi and the author has seen it in species such as Hoheria glabrata. Griselinia littoralis, Dacrydium bidwillii and Olearia ilicifolia.

Amongst the alpine vegetation in New Zealand there are many shrubs, many woody small plants which could be called shrublets and numbers of semi-woody species. There are, in fact, very few soft-stemmed herbs proper. Most of the herb-like species are partially woody or fully woody perennials which creep along at ground level, sending down adventitious roots. This applies, of course, to dicotyledonous plants such as Celmisia discolor, C. lyallii, C. laricifolia, Forstera tenella, Gaultheria depressa, Drapetes dieffenbachii, Coprosma pumila, Ourisia sessilifolia and Geum uniflorum, but even the long-lived monocotyledons like the snowgrasses (Chionochloa spp.) and the Astelias have tough, fibrous, wood-like tissue in their main stems and have the usual monocotyledon capacity for sending out adventitious roots along their leaders. The carpet grass (C. australis) is an extreme example of this. Most of the subalpine scrub species also have the capacity for production of adventitious roots on branches lying along the ground. Gaultheria rupestris, Hebe macrantha, Coprosma serrulata and Podocarpus nivalis are some of these and Wardle (1960) recorded others in a description of downhill layering in subalpine scrub. Mr B. Fineran (pers. comm.) has similarly noted this characteristic in shrub species on the Snares Islands.

On a recent expedition to Western Fiordland it was noticed that under the thick growth of mosses and liverworts on the trunks of Nothofagus menziesii trees, as much a 8-10 feet above ground level, there were numerous adventitious roots. An investigation of other species in the same neighbourhood—Griselinia littoralis, Coprosma linariifolia, C. ciliata. C. astonii, Neopanax anomalum and Myrsine divaricata — showed that all the species examined exhibited the same characteristics. Presumably the plants extract moisture and nutrients from the humus which is present under the moss cushion. This is not so surprising when it is realised that most of the soils of Western Fiordland are peats so that the substrate in the moss cushion is similar to that on the ground. There was no evidence for this phenomenon in the same species in eastern Fiordland. It seems that very high rainfall is necessary for it. On the Snares Islands it was found that Olearia lyallii sometimes produced roots in the humus collected on branches. (B. Fineran, pers. comm,).

The ‘feeding roots’ of forest species both in beech forest and mixed broadleaved podocarp forest in New Zealand are found in the humus layer of the soil. Similarly in the alpine grasslands — the actively growing root systems proliferate in the humus layer page 80 in the uppermost part of the soil. In the snowgrasses — members on the genus Chionochloa, as in many other species, there are normally masses of old dead and rotting leaf bases which surround the living tillers. ‘Feeding roots’ are found in these rotting tissues which remain wet even in the driest weather. It is probable that both water and nutrient requirements are largely supplied in this way.

Some New Zealand forest species are obligate epiphytes but many other species are able to exist as facultative epiphytes if the atmosphere is moist enough. This phenomenon is at its best in places of very high rainfall such as Western Fiordland or the Upper Hokitika River. In the latter area the forest is dominated by Metrosideros umbellata and Weinmannia racemosa, with an admixture of other broadleaved and gymnospermous species. Almost every tree in the forest has started life as an epiphyte and there are huge ratas, as much as 20 feet in diameter, formed from the coalescence of several trees (natural shoot grafts) and covered with a great luxuriance of filmy ferns, mosses and liverworts. In such situations, at least in the early stages of their life, these plants must obtain nutrients entirely from humus formed in the crotch of their host tree. It is interesting that so many species of shrubs, trees and herbs, in addition to the obligate epiphytes, are adapted to take advantage of the epiphytic habitat if other environmental conditions permit.

The facility with which New Zealand plants produce adventitious roots and the ability to utilise humus or peaty material with little mineral matter in it for much of their nutrient supply is a striking feature of our vegetation. There is scope for much future research on the root systems of New Zealand plants.

Additional Note: Since this paper was written a paper has been published (P. Wardle, 1963: Growth habits of New Zealand subalpine shrubs and trees, N.Z.J. Bot. 1, 1, 18) in which are described further examples of downhill layering and development of adventitous root systems.

Literature Cited

Christensen, C. E. 1923. On the behaviour of certain New Zealand arboreal plants when gradually buried by river shingle. Trans. N.Z. Inst. 54: 546.

Foweraker, C., 1929. The rain forest of Westland. No. 2 — Kahikatea and Totara forests. Te Kura Ngahere 2: 2, 6.

Fraser, D. A., and Gaertner, E. E., 1961. Use of radioisotopes in forestry research. Recent Advances in Botany — The 9th. International Congress of Botany, 1959, 2, 1381.

McIndoe, 1932. An ecological study of the vegetation of the Cromwell district with special reference to root habit. Trans. N.Z. Inst. 62: 230.

Moar, N. T., 1955. Adventitious root-shoots of Dacrydium colensoi Hook. in Westland. South Island, New Zealand, N.Z. J. Sci, & Tech, A, 37: 207.

Wardle, P., 1960. The subalpine scrub of the Hokitika catchment, Westland, Trans. Roy. Soc. N.Z., 88: 47.