Tree biology and how it influences woodland ecology

1.3.1 Fitness of various species for particular uses

All tree species possess a unique combination of morphological, physiological and reproductive traits which fit them for particular niches in the ecosystems they occupy. In the case of exotic plantations the silviculturally and economically most suitable tree species may have originated in a far distant country. Two quite different examples illustrate this.

The Monterey or radiata pine Pinus radiata is an impoverished and stunted tree in its natural range on a handful of sites on the coast of California, mostly notably, the Monterey Peninsula. It has been left marooned in less than favourable growing conditions as its range has been reduced by climate changes since the last ice age. Yet elsewhere it is capable of magnificent growth and has been extensively planted in many parts of the world including New Zealand and South America. In New Zealand it now accounts for some 90% of the exotic trees grown and develops so rapidly that it can be harvested at an age of 25 years, whereas even Douglas fir (whose timber may sell for roughly double the price) would normally have to grow for 45-60 years under the same conditions. It also has excellent form, wounds incurred when lower side branches are pruned heal rapidly, and it does not coppice so any unwanted trees die when felled. Its seeds are easy to collect and store and have a high germination rate, while bare-root seedlings and cuttings can be grown rapidly without shading and withstand weeding with herbicides. Planting stocks have a survival rate > 95% on a wide range of sites; the tree grows well and predictably even on infertile soils, its vigorous early growth often outstripping gorse and other weeds while in some situations its roots penetrate to a depth of 5 metres. The tree also has a degree of tolerance to frost, snow, salt winds and severe drought. Its genetics have been widely studied, clones being developed to suit particular conditions. This shade-intolerant species is most easily grown under clear-felling regimes and tends to shade out other species when planted in mixtures: the multi-species commercial forests that thrive in Europe are not found in New Zealand.

The genus Eucalyptus, which consists of around 500 species of trees and shrubs, has a native distribution largely confined to Australia, but extending into New Guinea, eastern Indonesia and Mindanao (Hora, 1981). Eucalypts show a most remarkable range of size and habitat and various species of this vigorous and adaptable tree, which evolved in isolation even from New Zealand, are now widely planted in many parts of the world, especially California which has the largest range of eucalypt species in the USA. The smallest is less than a metre in height, whereas mountain ash E. regnans can live for 300 years and is the tallest hardwood tree in the world, growing to more than 100 m on deeper well-watered soils in the foothills of Victoria, South Australia. The river red gum E. camaldulensis, a robust tree up to 35 m high, is found in most of Australia and can live for 500 years; older trees shelter parrots in their cavities. Tasmanian blue gum E. globulus, which reaches 35-45 m in height, is the species most widely planted in the Mediterranean area and California. Conditions in tropical North Australia vary from the normally extremely hot and dry to the suddenly deeply flooded when tropical rainstorms cause the rivers to overflow. This is an area well suited to extremely territorial frilled lizards which feed mainly on insects and make rapid two-legged dashes from tree to tree to avoid attack by predatory birds. Ghost gums E. papuana grow in New Guinea and arid parts of northern Australia, while in the southern state of Victoria snow gums E. pauciflora grow high on the Australian Alps, tolerating winter temperatures as low as —20 °C and providing food and shelter to parrot populations which feed on their fruits. It is on the lower slopes of these hills that the mountain ash flourishes on good soils with adequate water.

Almost all eucalypts are evergreen, having leaves that are hard and rich in nutrients. Apart from the koala (see Section 5.7.2), few animals can digest them. The bark of ghost gums is shed to the ground leaving a strikingly white surface, which reflects sunlight, and the leaves tend to hang down, thus staying cooler. Fire affects almost the whole of Australia, whose trees are well adapted to it, many of them having developed the ability to coppice or sucker in response to millennia of natural fires. Eucalypt fires develop rapidly and burn intensely. Many trees can survive all but the most severe fires and some species need fire to release their seeds. Buds buried beneath the bark produce new leaves and branches and life often resumes within a few weeks. Jarrah E. marginata is one of many species with a lignotuber, like a huge wooden radish, which enables it to coppice.

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