Since the early 1980s, foresters and others interested in forests have been debating and defining sustainable forest management. Many definitions have been produced with a similar theme, exemplified by the forest principles set out by the UNCED conference (United Nations Conference on Environment and Development, also called the Earth Summit or Rio Summit) held in Rio de Janeiro, Brazil in 1992:
Forest resources and forest lands should be sustainably managed to meet the social, economic, ecological, cultural, and spiritual needs of present and future generations.
It is easy to see the value of sustainability as encapsulated by such a definition but it is notoriously difficult to achieve. This is partly because different groups have different priorities within this definition; some are concerned mostly with water quality, others the conservation of biodiversity, yet others with maintaining wood production, and the list could go on. In theory these are all compatible but in practice different objectives often require different management. Moreover there are problems in defining and predicting the needs of future generations. For example, if forests are managed to produce timber with the cost that two species of insect become extinct, how do we judge whether this is important to future generations? If we decide that all species must be preserved just in case and this reduces timber output in poor tropical countries to the detriment of the present population, is this acceptable? Using the above definition, the answer is clearly that the needs of the present human population are paramount, so where does the compromise lie? Even when objectives have been set it can be equally difficult to define exactly what these mean in practical terms that can be measured. This outline of the problems is in no way meant to undermine the need for sustainability but to highlight the difficulty in making sustainable forest management a reality. Discussion is still going on within the forestry community (e.g. Floyd et al, 2001). Regardless of the ongoing debate, a number of bodies are putting sustainable forestry into practice. The Forest Stewardship Council (FSC) based in Germany (see www.fsc.org/en/) has established a worldwide certification scheme based on the UNCED forest principles. Now, many major retailers in Europe, North America, South America and Asia ask for FSC certification when buying timber or forest products.
Before sustainable forest management as described above became an issue, foresters were primarily concerned with sustainable yield, the amount of wood that can be continuously produced at a given intensity of management. This is comparatively straightforward to measure since it involves the healthy production of at least as much timber per successive rotation (of the same duration) over a prolonged period. This aim has undoubtedly been achieved in many European forests that have been carefully tended for several centuries. There is concern about the rate at which infection by honey fungus Armillaria mellea has built up in radiata pine stands in North Island, New Zealand; frequently 20% of second rotation trees are infected whereas the figure on ex-pasture sites is only 2%. If more severe problems occur, the possibility of a crop rotation system involving clear felling, the use of the land for agricultural crops or pasture, and the complete decay of woody material before more tree seedlings are planted should be considered.
Problems of tree health frequently involve pests as well as pathogens and here effective control involves a careful consideration of the biology of both the pest and its host. This has led to the concept of integrated pest management (IPM), a very effective approach considered in relation to tropical trees by Speight and Wylie (2001). A classic example is that of grasshopper attack on eucalypts in Paraguay. This can cause losses of 70-80% due to ring-barking followed by tree fall during the first 3 months after establishment. The tougher grey bark of older trees seems relatively immune to attack so the solution to the problem is the use of older transplants (planted seedlings) in less risk-prone sites. Attempts to employ IPM with Hypsipyla shoot borers have been less successful; these Lepidopteran pests virtually prevent the plantation cultivation of mahoganies (Swietania, Khaya), cedars (Cedrella, Toona) and other valuable members of the Meliaceae in South-East Asia and Australia. Attempts to achieve classical biocontrol by introducing parasitoids and predators of these shoot borers have not eliminated the problem in their native country or in introduced populations elsewhere. Cases where susceptible species of Meliaceae have been grown with minimal shoot-borer damage often involve rearing the desirable tree in mixtures with other tree species; there are many opinions as to exactly why this procedure is sometimes successful. As with pathogens, some genetic strains of vulnerable tree species are less at risk than others, and in both cases attempts are being made to improve the situation by the breeding and selection of more resistant genotypes.
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