The ?rst incidences of air pollution impacts on the genetic constitution of forest tree populations were those documented near point sources of sulfur dioxide (SO2), particulates, and heavy metals. Localized extinction of forests around these point sources was documented by ecologists in the past two centuries. In North America, the most spectacular of these areas were those surrounding ore smelters in Trail, British Columbia, Sudbury, Ontario, and Copper Basin, Tennessee. In Europe, the most dramatic areas included the Black Triangle area (of eastern Germany, Poland, and the then Czechoslovakia) which was largely due to soft coal burning in power plants and numerous situations where industrial facilities were located in valleys such that toxic emissions destroyed vegetation on the surrounding hillsides. With these early pollution problems, large areas of forests have simply been replaced by grasses or other tolerant vegetation.
[...] Recent trends in the focus of agroforestry research and extension are informative. We can chart its entry to the international stage, by the emergence of ICRAF, then the International Council for Research in Agroforestry, now the World Agroforestry Center, in 1978. This was 5 years after the Center for Tropical Agricultural Research and Higher Education TIE), which pioneered research, education, and extension on agroforestry with perennial tree crops in Latin America, had come into existence. For the ﬁrst decade or so, the focus was on describing various traditional agroforestry practices around the world and developing methods for analyzing land use systems involving trees and their potential improvement paths, by drawing on contributing disciplines. [...]
[...] Knowledge Gaps and Research Needs Restoration of forests destroyed by severe air pollution (as in the region of the Black Triangle in Europe) remains a great challenge today, and there are currently no methods available to guide these restoration projects to recreate previous genetic diversity and genetic structure. Furthermore, reforestation under today's climate (light, temperatures, phenology, and moisture) and soil conditions (many former severely degraded areas by air pollution still have acidic soils or soils contaminated with heavy metals) may preclude simple replanting with single species. [...]
[...] In this article, we ﬁrst discuss genetic aspects of air pollution effects on forests and then examine how the changing climate may impact the genetics of forest trees. Genetic Aspects of Air Pollution For air pollution to induce natural selection, there must be variation in air pollution sensitivity, the variation must be heritable, and the pollution must be a strong enough selection force to disadvantage sensitive trees severely. According to Anthony Bradshaw, who has studied natural selection in grasses growing in the presence of heavy metals, evolutionary population change takes place in three stages: * Stage elimination of the most sensitive genotypes * Stage elimination of all genotypes except the most tolerant (note: elimination of all forest tree genotypes, as has occurred in many point source pollutants, results in extinction, not evolution) * Stage interbreeding of the survivors to give even more resistant genotypes which are then further selected The rate of selection is dependent on the severity of the pollutant stress, the type of reproduction (sexual or asexual reproduction), and the level of competition between genotypes (the more intense the competition between sensitive and tolerant trees, the faster the effects occur). [...]
[...] Understanding how climate change and related changes in greenhouse gases, such as CO2 and O3, will impact intra- and interspeciﬁc competition will help us better predict impacts of climate change on forest tree populations. Since these changes are rapid (from a historical viewpoint) but still long-term (from a research project duration viewpoint), long- term studies of population dynamics and competition will need to be done under realistic future climate scenarios. The extensive sets of provenance trials established in the twentieth century around the world should also prove useful in addressing questions related to adaptation to warming temperatures. [...]
[...] Some well-known trees fall into this category, such as the shea butter in West Africa (Vitellaria paradoxa), from which comes a range of local foods, as well as expensive cosmetic products and a cocoa substitute sold in industrialized countries (in 2003 pure shea butter was selling in the UK for over d140 per liter as a skin cream). The baobab (Adansonia digitata) is a distinctive landscape feature in farmers' ﬁelds across much of Africa, from which ﬁbers, fodder, and many other locally important products and cultural values are derived. [...]
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