![Thumbnail: [Volume 68, 1938-39 page 275]](/journals/rsnz/images/rsnz_68/thumbnails/rsnz_68_02_0358_0275_ac_02.gif)
Interrelation of Plants.
So far only the general relation of plants to the climate has been discussed. Plants, however, modify greatly climatic conditions in their immediate neighbourhood, and this is especially pronounced within such formations as, e.g., tall forest.
Metrosideros robusta, the common rata, furnishes an outstanding example. It is a tall forest tree, but its massive trunk is not a true stem but a single or a number of more or less fused roots. 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 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.
Rata is found in abundance in the wet Western Area and is practically absent in the dry Eastern Area. This is in keeping with the profusion of epiphytes in the former and their paucity in the latter. In the forks of tall trees in the Eastern Area the conditions are generally far too dry for any large epiphytes, such as Astelia Solandri or M. robusta, to become established.
A few trees with true stems often behave, particularly in the Western Area, in a similar manner to rata. Among these Nothopanax aboreum, Melicytus ramiflorus, Weinmannia racemosa are conspicuous. They not infrequently begin their life 8 to 10 m. above ground but finally establish their roots in the ground. In this area large numbers of ferns, many shrubs, and a good few trees in their juvenile stage exist among true epiphytes well above ground. This is rendered possible through a dense growth of true epiphytes such as many filmy ferns, bryophytes and astelias. Further, many species are either driven out of existence in this area by the dense shade produced by more vigorous plants, or are driven up tall trees to assume an almost epiphytic habit. Senecio Kirkii is a noteworthy example of the latter group in the warm temperate belt. Under the entangle-
![Thumbnail: [Volume 68, 1938-39 page 276]](/journals/rsnz/images/rsnz_68/thumbnails/rsnz_68_02_0359_0276_ac_02.gif)
ments of Rhipogonum scandens, due, no doubt, to the dense shade produced by it, the ground is often practically bare of all other growth. Here Freycinetia Banksii is usually a lofty climber, but where plenty of light reaches the ground, as on some ridges and steep slopes, it frequently spreads over large areas of ground almost to the complete exclusion of all small plants and even shrubs.
Nothofagus Menziesii is usually a fairly deep-rooting tree, but in many places, especially in the upper cold temperate belt, its root system is to a large extent on the surface or even elevated up to some 50 cm. above ground. This is brought about by a dense unbroken mat usually of Hymenophyllum multifidum spreading over them and holding a permanent supply of moisture.
Interrelation of species is best observed in the succession and regeneration of the vegetation. Only brief notes can be made on these points at present. Regeneration of forests is more or less a continuous process depending on the amount of light falling on the forest floor. The soil is full of seed, and seedlings or what pass for such, 20 to 30 years old trees, begin to grow rapidly when opportunity arises. It is interesting to note that an examination of annual rings of Nothofagus Menziesii trunks of varying diameters up to 20 cm. at 1 m. from the ground proves them to be all of the same age in any one place.
In the warm temperate belt in the Southern Area in many places vegetation is unstable owing to continuous creeping down of screes which are very extensive, particularly in the Muku-muku basin. Nothofagus forest appears to be the climax association but this on the steeper slopes seldom develops. There are many places where screes have apparently resumed active movement comparatively recently. Their surfaces are strewn with dead logs.
When the movement slows down Raoulia tenuicaulis, Danthonia semiannularis, Poa anceps, Acaena sanguisorbae, Paesia scaberula, Blechnum procerum, B. fluviatile, Epilobium spp. and some other herbaceous plants arrive among the first colonisers. These are followed by Leptospermum ericoides generally in association with Cassinia leptophylla, Cyathodes acerosa, Leucopogon fasciculatus, Helichrysum glomeratum, Hebe salicifolia, Rubus cissoides, Brachyglottis repanda, Cyathea Smithii, and others. In the succeeding stage Melicytus ramiflorus, Alectryon excelsum, Weinmannia racemosa, Beilschmiedia tawa, Hedycarya arborea, Metrosideros spp., Freycinetia Banksii, and Rhipogonum scandens become established and often form a fairly stable association. Various transitional stages are quite commonly met with. Occasionally old trees of Leptospermum ericoides with trunks up to 35 cm. in diameter are encountered in mature associations subsequent to that of Leptospermum.
Somewhat similar successions take place in other parts of the warm temperate belt on the Tararuas, but Pteridium aquilinum often takes a prominent part in the initial colonisation and is usually succeeded with difficulty by other plants. The screes, however, in the Eastern Area are not very frequent, while in the Northern and Western they are almost unknown. The rock in these screes weathers almost directly into clay and the place of the screes is taken by
![Thumbnail: [Volume 68, 1938-39 page 277]](/journals/rsnz/images/rsnz_68/thumbnails/rsnz_68_02_0360_0277_ac_02.gif)
frequent landslips. Usually landslips are quickly occupied by Blechnum procerum, Microlaena avenacea, Brachyglottis repanda. These are rapidly followed by most common species of undergrowth.
In the cold temperate belt throughout, regeneration on the landslips is very similar to that in the warm temperate in the Western Area, but Hebe salicifolia is generally the first to establish quickly a dense association.
Regeneration after fire, which is not infrequent in the warm temperate belt, particularly on the eastern side, whether accidental or purposeful, even after sowing down of pasture grasses, provided man does not interfere further energetically with natural processes, quickly brings the land under forest back to its original state. Here Pteridium aquilinum, Pteris incisa, Blechnum fluviatile, Acaena sanguisorbae, Erechtites prenanthoides, Brachyglottis repanda, with perhaps some Leptospermum scoparium and Cassinia leptophylla rapidly become established. These are followed by Blechnum discolor, Hebe salicifolia, Geniostoma ligustrifolium, Rubus cissoides, Schefflera digitata, Coprosma australis, and Weinmannia racemosa. On the western side the whole is often dominated by Cyathea Smithii with some C. dealbata. Successions then follow in the usual manner. In the drier regions Nothofagus always forms the climax association. In the wetter, the climax association is dominated by Metrosideros robusta, which supplants more or less completely Dacrydium cupressinum. Beilschmiedia tawa often forms a distinct association, which, however, in most cases appears to be an intermediate stage in succession, preceding D. cupressinum.
In the cold temperate belt fires occur almost exclusively, and then only rarely, in the Eastern Area. A conspicuous feature of regeneration here is the almost immediate growth of Nothofagus and Weinmannia, or only the former, resulting in practically pure stands which for a long time afterwards are almost free from undergrowth.
In the warm subpolar belt fires are also restricted to the eastern side. The burns, as they are to be seen on Mount Mitre and in Park Valley, are quickly occupied by Poa imbecilla and Agrostis Dyeri. Leucogenes leontopodium and Ranunculus geraniifolius appear in quantity. Most of the perennial plants revive, as also do the tussocks of Danthonia antarctica, although more slowly.