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Forests are indispensable as sources of harvested products and a variety of services. Unfortunately forests throughout the world, and especially in the tropics, are threatened by natural and human-induced disturbances. Regeneration of harvested or otherwise severely disturbed forest stands typically occurs in four stages: (1) stand initiation and regeneration stage, (2) thinning or stem exclusion stage, (3) transition or understory regeneration stage, and (4) steady-state or old-growth stage. Blocks to stand regeneration may occur in each of these stages. Revegetation of severely-disturbed forests often is very slow and unpredictable because of complex interactions among propagules as well as site and climatic conditions. Stand regeneration depends on an abundant and viable seed supply, a suitable medium for seed germination, favorable environmental conditions, and capacity for sprouting or layering of some species. Massive losses of seeds in seed banks occur because of seed aging, failure of seed germination, predation, diseases, and death of seeds. Mortality of seedlings also is high, especially in the cotyledon stage of development, because of low reserves of carbohydrates and mineral nutrients. Growth of older seedlings, saplings, and mature trees is inhibited by air and soil pollution, drought, flooding, soil compaction, insect attacks, and diseases. After canopy closure occurs in a developing forest stand changes in species composition are traceable to competition among plants for light, water, and mineral nutrients. Following severe disturbance of a multi-aged, mixed forest, a mixed-species stand of fast growing, generally short-lived trees typically occupies a site and is succeeded by species that dominated the stand before the disturbance occurred. After a minor disturbance gaps form in the canopy and are recolonized. If plant succession in a developing stand runs its full course the resulting old-growth forest is impacted by frequent minor disturbances and is maintained in an oscillating steady-state in which stand establishment, thinning, gap formation, and colonization recur. In some regions frequent fires maintain forest stands in a subclimax stage of development. Natural regeneration of harvested or otherwise severely disturbed stands likely will be too slow and unpredictable to provide all the forest products and services required by increasing population growth. Hence, greater emphasis on several concurrent strategies will be needed, including heavy emphasis on artificial regeneration of disturbed forests; conservation of the remaining tropical forests; expansion of plantations, agroforestry systems, and forest reserves; expansion of tree improvement programs; more intensive and improved forest management; and expanded research with particular emphasis on seed biology, responses of tree species and genotypes to environmental stresses, tradeoffs between beneficial and harmful effects of environmental stresses on tree species and genotypes, genetic engineering, potential effects of global warming on forests, agroforestry systems, models of stand productivity; and appraisal by remote sensing of injury to forest ecosystems. |
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