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4. Conclusions

The basic ecological concept of diversity leading toward stability in ecosystems would lead one to conclude that mixed species natural forests are less susceptible to damaging forest pests and diseases than single species plantations. However, both natural forests and plantations are known to be susceptible to a variety of damaging pests and diseases. Some northern temperate natural forests tend to be relatively simple ecosystems. These forests have a history of periodic outbreaks of bark beetles, defoliators and other insect pests. Moreover both natural forests and plantations are damaged by a variety of fungi, bacteria, parasitic plants and other disease causing agents. Consequently, effective monitoring and protection of forests against damaging pests and diseases must be an integral part of forest management regardless of whether the forests are natural or plantations, composed of exotic or indigenous species, single species or mixed.

Many records of devastating pest losses are reported in forest plantations. Plantations of exotic tree species can be attacked by indigenous pests, which have adapted to the new host, or introduced pests, which, in the absence of natural enemies, can build up into large numbers. Inadequate plantation management of either exotic or indigenous species will increase the susceptibility of plantations to pest and disease damage. These including poor species-site matching, planting stock with low genetic variability, failure to maintain tree vigour through intermediate cuttings, and reliance on one or two species in a national plantation programme.

While logic might dictate that mixed species plantations are less susceptible to damaging pests and diseases, and cases exist that support this notion, it is by no means certain this is a good tactic. Mixed species plantations may not necessarily meet management objectives. There are also cases where pest species have evolved that can survive on several closely related hosts or have alternative life stages on two unrelated hosts. These agents are best adapted to mixed-species situations.

Both natural and plantation forests should be managed with the objective of keeping them in a healthy, productive condition, one in which pests and disease are kept at low levels and do not interfere with management objectives. Integrated pest management (IPM) is a framework of decision-making and action tools designed to maintain and improve forest health. Pest and disease monitoring ensures early detection of potential problems. Combined with analysis of the economic, social and ecological impacts of pests, a sound basis on which to decide for or against control is derived. Two basic strategies, prevention or direct suppression, each with a range of tactics, can be applied. Prevention consists of actions taken to make trees and forests less hospitable to the build-up of pests and diseases and/or preventing new introductions. Direct suppression consists of biological, chemical or mechanical tactics designed to reduce pest and disease populations and subsequent losses. IPM systems consist of a combination of monitoring and action tools designed to reduce pest-induced losses. These systems are continuously evolving and are capable of accepting new technologies, as they become available.

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