In countries with huge areas of plantation forest such as New Zealand, there is a choice between planting a number of tree species or just one. With several tree species significant losses are likely to occur annually because there will be a greater variety of damage-causing pests and pathogens; with a single species at present without many pest and pathogens the annual risk is low but there is the possibility of a catastrophe should they arrive. As Maclaren (1996) puts it, the forest owner has to balance the risks of a high-probability but low-impact damage against a low-probability but high-impact damage. In choosing the second option and planting radiata pine in New Zealand, foresters have lowered the costs of research and protection; the real concern is that these forests will no longer be so productive when, as is almost inevitable, pathogens and pests from other parts of the world finally invade. Similar arguments apply to other species and other places.
The greatest difficulties arise when genetic variability in a crop is low (as in clonal forestry) since a disease or pest overcoming host resistance can easily sweep through the entire population. The opposite problem is equally damaging where a pest or pathogen has evolved the ability to overcome a wide range of host resistances and can invade populations even with high genetic variability. This is the case with honey fungus Armillaria spp. and the gypsy moth Lymantria dispar. Honey fungus is one of the world's most important and voracious root diseases, made up of some 40 species worldwide (not all of which are pathogens) to which very few woody plants are immune (see Section 5.4.2). The gypsy moth is one of North America's most devastating forest pests, introduced from Europe in 1868-9, and able to feed on the foliage of literally hundreds of species of plant. If this moth becomes established in New Zealand it will attack a wide range of hosts including hardwoods, just as Armillaria mellea does already.
The ultimate example of an introduced tree lacking genetic variability is that of the English elm Ulmus procera, all of whose individuals are now believed to have arisen clonally from a single individual brought to Britain by the Romans. Its origin has been traced to the Atinian elm, a variety from central Italy that reproduces asexually. It was widely employed to support and train vines in wine production, having been recommended for this purpose by Columella in his AD 50 classic De Re Rustica (Of Rural Affairs). Gil et al. (2004) compared DNA from the English elm to four lineages drawn from across Europe in efforts to trace its origin. The closest match was with lineage C, that also occurs in the Iberian peninsula, and appears to originate in the region around Rome. It seems, therefore that the English elm reached Britain via Spain - which was also under Roman control in the first century AD. The rapid and devastating spread of Dutch elm disease in the 1970s (Section 5.4.5), when more than 25 million trees were killed, was promoted by the fact that this susceptible variety had been preserved genetically unaltered as the core of the English elm population for 2000 years.
On a world basis, land is increasingly in short supply so there is every reason to reclaim so-called brown-field sites which have been disturbed by mineral workings or other industrial processes, and are now unused. This is particularly the case when attempts are being made to improve the countryside near population centres. Besides being unproductive, derelict land is often dangerous with old buildings liable to collapse, crumbling quarry faces or exposure to chemical contamination. During a 12-year period centred on the 1980s over 157 000 ha of English land was reclaimed for beneficial use, including forestry, at a cost of almost £750 million. The British Forestry Commission devoted considerable resources to assessing and solving the many problems involved when the ultimate use is forestry (Moffat and McNeill, 1994).
On relatively infertile sites, including many former colliery spoil tips, conifers prove to be both appropriate and economically productive. In other places broadleaves (such as willows Salix spp., alders Alnus spp. and birches Betula spp.) can yield valuable timber and are also desirable for wildlife and aesthetic reasons. In both cases sewage sludge greatly improves growth, being cheaper and promoting a greater increase in growth than mineral fertilizers. Foliar analysis provides a good indication of nutrient absorption by young trees. Different substrates pose different problems, for example fresh pulverized fuel ash (pfa) produced by the burning of coal in thermal power stations commonly has a pH of 11 or 12. Though this drops to pH 8 or 9 after being stored in a lagoon (lagooning), it is still too alkaline for sensitive species such as beech, ash or sycamore. Sitka spruce and alder can be planted directly in it, though admixture with a surface layer of soil or colliery shale improves growth. Drainage is a key factor and on many sites deep discing or ripping of the substrate is essential. Ridge and furrow methods with young trees being planted and firmed up in notches on the ridges help soil aeration.
Restoration plans and planning applications need particular care and it is essential to devise and execute a long-term programme of aftercare to ensure that drainage, soil nutrient levels, weed control and tree density are properly maintained. In the case of new mineral extractions careful soil stripping and storage will make subsequent restoration of the site very much easier. It is vital that those who may be responsible for the site in later years be made aware of problems that may arise if deeply buried contaminants such as iron pyrites are disturbed.
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