Biodiversity is, as we have seen, a measure concerned with the variety and abundance of living things, of which the number of species present is an important aspect. High biodiversity is seen as a desirable attribute, hence communities possessing it are more likely to be a higher priority for conservation than those which do not. The rationale of conservation, however, is far from simple. Communities with rare species facing possible extinction are often given more emphasis than more diverse communities composed of common species. Conservation issues often also involve questions of aesthetics, while many communities which much of the public regard as natural actually depend for their continued existence on continued human influence. The beautiful open countryside of the Lindis Pass, New Zealand, where little taller than tussock grasses occurs, for example, is dependent for its continued existence on regular burning, fertilization and grazing. In their absence woody vegetation would invade and presumably dominate.
Maintenance of the grassland and heathland on the Long Mynd hills, Shropshire, in the English Midlands is similarly dependent on human intervention in preventing the development of woodland. In time past, senescent heather Calluna vulgaris was periodically burnt off and the development of woodland prevented by intensive sheep grazing. In recent decades there has been a considerable spread of bracken Pteridium aquilinum and various trees and shrubs are also beginning to move in. Rowan Sorbus aucuparia is scattered here and there; a few of its trees are very large while many young trees are developing from seeds deposited in bird droppings. Young hawthorn Crataegus monogyna are developing in much the same way. Intriguingly some hill sides tend to have many young rowan and others hawthorn, rather than the admixture that might have been expected. The occurrence of wood-sorrel Oxalis acetosella, a shade species common in ancient woodlands, on the Long Mynd is also of great interest (Packham, 1979). The plant occurs on screes and grasslands on open hills and mountains where protection from exposure is much less than in forests. The slope and aspect of a variety of wood-sorrel sites on the Mynd were plotted on a slope-aspect polargraph in which increasing slope is represented by greater distance from the centre of a circle. Grasslands on north-facing slopes are relatively cool and moist, receiving little or no direct sunlight when steeply sloping, and often provide conditions suitable for the growth of woodland plants such as wood-sorrel, dog's mercury Mercurialisperennis, and wood anemone Anemone nemoralis as well as plants found in marshes such as glaucous sedge Carex flacca and marsh thistle Cirsium palustre.
New Zealand is unique in possessing large areas of natural forest in which there is a great deal of public interest. The indigenous biodiversity of the country is protected, and invasion of the original forests by exotic species is prevented as far as possible. A major problem here is that of wilding trees which arise from seeds carried from exotic plantations. Though many pioneer tree species are intolerant of shade and so cause little problem in southern beech forests, Douglas fir Pseudotsuga menziesii has considerable shade tolerance and its wildings invade canopy gaps in indigenous forest, while sycamore Acer pseudoplatanus is the most shade-tolerant tree in the country, is multi-leadered, coppices, and is very difficult to remove as European foresters know only too well. Fortunately, herbivores find it highly palatable and it is not commonly planted. Wilding trees, often of exotic species introduced by foresters in earlier times, also cause problems by invading open countryside. Lodgepole pine Pinus contorta causes the most difficulty, as it has enough shade tolerance to grow close to the base of grass tussocks and sets seed at 5-8 years of age when it may still be hidden by the grass. This tree can spread quite rapidly across the landscape. It also forms dense stands which suppress any understorey.
Some southern beech forests grow under such high harsh environments at higher altitudes that the trees themselves are the only higher plants (i.e. seed plants) present for hundreds of metres. Biodiversity could hardly be lower yet few would deny the value of these unique areas. In other cases southern beech forests with abundant rain, adequate nutrients, higher temperatures and sufficient light beneath the tree canopy have a dense and varied understorey.
A high diversity of indigenous understorey plants can also occur under monocultures of radiata pine Pinus radiata when conditions are good; this may be influenced by soil improvement due to mycorrhizal fungi. An extreme example of high biodiversity in an exotic stand was the occurrence of 30 different indigenous orchids beneath black pines at Itwatahi, while two old radiata pines near Fox Glacier bore 8 species of fern and 11 angiosperms as epiphytes.
The biota of conifer plantations and native broadleaf woodlands sometimes differ markedly. In Britain Tickell (1994) is reported as having shown that the latter contained 18 times as much insect life per unit area, yet the current conclusion is that conifer forests are not the 'deserts' they seem to some observers. Maclaren (1996) compares the biodiversity of pasture with that of natural and exotic forests in New Zealand, pointing out that the most important contrast is between pasture and forest plantation since these two land uses are interchangeable, whereas almost all indigenous forest is now protected. In practically every case planting exotic trees on pasture would increase indigenous biodiversity for understorey vegetation and birds, and probably aquatic species (including fish) also. Hulks, or snags as they are called in North America (large, standing dead trees - see Section 7.7.1), are uncommon in modern plantations and are a potential source of danger and disease. They do encourage a variety ofother organisms, however, and a cavity in such a radiata hulk near Tokoroa once contained 20 long-tailed bats. Others have large and interesting populations of insects, mites and fungi; cores from them may furnish climatic information also.
Although bird populations of young conifer plantations are low, those of older exotic stands are usually as abundant, though different from, those of indigenous forests. In New Zealand, plantations miss out primarily on fruit-feeding and nectar-feeding guilds, from an avifauna which is relatively poor anyway: parakeets Cyanoramphus spp., kaka Nestor merldionalis, yellowhead Mohoua ochrocephala, native pigeon Hemiphaga novaeseelandiae and kingfisher Halcyon sancta are amongst the birds which tend to be absent. The endangered kokako Callaeas cinerea, however, feeds on insects, which are abundant in the pine plantations where it is frequently encountered. Exotic plantations can thus be useful storehouses of biodiversity (and, as in New Zealand, sometimes more diverse than native forests). Plantations also offer protection to the native forest by preventing their unsustainable exploitation by providing the necessary wood products; this allows natural assemblages of species to be more easily conserved.
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