FORESTS, EXOTIC – PLANTATIONS AND FARM SHELTER
Introduced or, as they are usually termed, exotic trees now help to form the typical New Zealand landscape of farm and open land where once there was native forest, shrub land or grassland. Today there are rye-grass-dominant swards on the easy lowland and montane country, and the paddocks, as often as not, are surrounded by shelter belts of exotic trees and shrubs. Exotic trees are also planted around the homesteads and there may be a scattering of farm woodlots. The steeper country usually has browntop-dominant pastures and it is often quite destitute of trees, or there may be a few shelter belts near the bottom of the hills and a few scattered exotic trees about. Sometimes the steeper and darker valleys may have remnants of native forest in them, and the mountains behind, except on the east coast of the South Island, are still covered with native forests. Exotic forests, ranging in size from a few thousand acres to the large Kaingaroa forest of some 300,000 acres, occur occasionally throughout lowland and montane areas.
A great variety of trees has been introduced into New Zealand. Many of them can be grown successfully, and some have become acclimatised in so far that they will regenerate naturally under certain conditions. In fact, there would be few other parts of the world of comparable size where such a selection of introduced trees can be grown. Yet we still have not learned to grow them to produce the best effects, and examples of trees and shelter belts planted for the wrong purpose and rows of unkempt trees are by no means uncommon.
Radiata pine must take pride of place for the frequency with which it is planted in shelter belts and woodlots. The rapidity of its growth and the ease with which it is handled make it an attractive tree for these purposes; but it is not a particularly gainly tree and can soon look out of place unless thought has been given to the size to which it finally grows. The next most important, though not nearly so widely planted, is Cupressus macrocarpa. Like radiata pine it, too, was introduced from California. In New Zealand it grows into a very large, spreading tree which yields a high-quality durable heartwood timber of general utility on farms. It can become very unkempt and must be planted with an eye to its future size. A relative, Lawson's cypress, a graceful tree, was at one time planted widely for ornament and shelter, but has been killed in many places by a canker. Its place has been taken partly by another cypress, C. lusitanica, and the closely related C. benthamii. Radiata pine is also the commonest tree grown in exotic forests. The next most common, Corsican pine (P. nigra), and ponderosa pine (P. ponderosa), are sometimes planted on farms and open country on sites somewhat too difficult for radiata pine, particularly in the south. Southern North American pines are grown in plantations in the north. The European maritime pine (P. pinaster) is sometimes seen in waste shrub land because fire encourages its regeneration. Fire does the same thing to radiata pine, and after a succession of fires a few original trees will form a dense stand. The cones, which remain closed for many years, open with the heat and release the seed. Douglas fir, or Oregon pine, has been planted extensively in State plantations south of the centre of the North Island, and is sometimes seen in woodlots. The oldest and largest trees, some of them approaching 200 ft in height, are, in fact, found in these. In the early days of settlement spruces were planted widely by Europeans. While some of these thrived for a time, they mostly later disappeared because of the attacks of spruce aphis. Bishop's pine, P. muricata, which looks somewhat like radiata pine, has also been planted widely but is not wind firm.
Amongst the broadleaf trees, the most successfully established are some species of willows and Australian eucalypts. Of the 600 species of Eucalyptus, almost all Australian, many have from time to time been introduced into New Zealand. By and large they have not been successful as plantation trees because of the difficulties the utilisation of their timbers presents in this country. The timber of only one species, the Tasmanian bluegum, E. globulus, is marketed in any quantity. Unfortunately, this tree, which was once planted widely, is now severely attacked by a scale and a weevil. This is a fate that befalls other eucalypts as pests find their way across the Tasman. Nevertheless, eucalypts are still planted widely on farms and it is an important genus in this country. Species of another large Australian genus, Acacia, or wattles, have been introduced here. The golden flowers of the black and silver wattles are a common sight in the spring. A. melanoxylon, the blackwood, is sometimes seen and is likely to be planted more frequently because of the fine cabinet wood it produces.
A common sight is to see rivers and streams lined, or even sometimes almost filled, with the European crack willow (Salix fragilis). The male and female flowers of Salix are borne on separate trees and are greenish. They are propagated by cuttings for river-control work, and they also propagate themselves as twigs and branches fall into river beds. Finally, some waterways become so impeded that clearing operations have to be resorted to. Other willows that have become more or less naturalised along rivers are the weeping willow (S. babylonica) and the golden willow (S. vitellina). The first species hybridises with the crack willow and many of the hybrids propagate themselves vegetatively. The grey willow (S. cinerea), one of the pussy willows, is widely grown as an ornamental tree and has become naturalised in very wet swamps, especially in the Thames. Other pussy willows grown are S. caprea and S. discolor. Willows and their cousins the poplars – in particular, the tall, fastigate, Lombardy poplar – are frequently planted as poles on country that is subject to slipping. The weeping willow is occasionally planted along water courses to provide a supply of browse in pasture country that gets very dry in the summer. Poplars are planted throughout New Zealand, but they require deep fertile silt to thrive. The species planted are mostly hybrid black poplars and are usually difficult to identify. During recent years many modern hybrids have been planted out.
Broadleaf trees, such as European oaks, elms, and walnuts do not, in general, do well in New Zealand, though good specimens and stands are to be seen where the sites are particularly favourable. Usually wind and disease affect the growth of these trees. In some places the European sycamore has become naturalised.
by Alec Lindsay Poole, M.SC., B.FOR.SC., F.R.S.N.Z., Director-General of Forests, Wellington.