Diseases of Forest Trees: Consequences of Exotic Ecosystems?
نویسنده
چکیده
MUCh attention is now given to risks and impacts of exotic pest introductions in forest ecosystems. This concern is for good reason because, once introduced, an exotic pathogen or insect encounters little resistance in the native plant population and can produce catastrophic losses in relatively short periods of time. Most native fungal pathogens of forest trees have co-evolved for eons with their hosts and have reached a sort of balance between them and populations of susceptible tree species. Recent studies on various forest types have indicated a higher incidence of certain fungal Pathogens than were previously thought to occur. These pathogens are either the type not normally thought of as highly virulent or are those that have not been previously reported as a serious problem on a particular host. For example, Pathogenic fungi belonging to both the Leptographium complex and Heferobasidion annosum, are associated with mortality afier prescribed burning in Certain longleaf pine stands. Yet, this tree species has traditionally been ranked as highly tolerant to these fungi. Could these observations reflect some manifestation of “exotic ecosystems,” whereby the conditions under which particular tree Sp3CieS evolved are no longer present or are altered in some way that increases their susceptibility to these fungi? With the CUrrent emphasis on ecosystem restoration and alternative silviculturel regimes, it is critical to address such questions in order to avert losses in forest productivity. Forest tree species and all other living organisms have ,,volved under var ious env i ronmenta l cond i t ions th rough c,ons of time. Nearly all species that have ever lived are ,,OW extinct (Raup 1988). Adaptations to climatic factors, so,ls, pests, diseases, and a host of disturbance events, operating at a variety of scales, have forged the ,;~laracteristics of each tree species we now observe, lrlcluding their functions in forest ecosystems. f’rom the perspective of forest pathological processes, irldlvidual tree species and ecosystems are in some form of rluasi-equilibrium with various pathogens. This is in c:ontrast to situations involving introduced pests or exotic organisms, which generally cause rapid and catastrophic mortality on native tree species. On the other hand, many root disease pathogens that have co-evolved with their hosts often cause excessive mortality and disruption of long-term stand management goals. Why then, in a theoretically stable system from the host-pathogen Perspective, are there significant problems with various diseases in coniferous forest stands over a wide range of forest types and ecological conditions? Have presettlement forest conditions changed, through past land uses, to the extent that unstable or “exotic ecosystems” are created by VariOUS management activities which have led to undesirable losses due to various forest tree diseases? For many decades, forest pathologists have studied the effects of various management regimes and their relationships to forest tree disease. As a result, an empirical understanding of relationships between site factors, disturbance, past and present management Practices, and silvicultural procedures relative to many forest diseases has been attained. In the light of these discerned relationships, the purpose of this paper is to introduce the concept of exotic ecosystems, defined as unstable ecosystems arising from rapid edaphic and environmental changes brought about by past land use or current management practices. These will be presented in the context of how various silvicultural regimes, disturbances, and past land use practices have interacted to create disease problems.
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تاریخ انتشار 2000