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Hyrcanian forest granted the areas with unique richness of biological diversity, its endemic and endangered species, its natural beauty and its masterpieces of nature creative genius in the form of this ancient forest. North of Iran as along band has diverse natural, economic and social conditions. It characterized by various ecological conditions from 550 to 2200mm precipitation, zero to 5671 m elevation and various vegetation landscape from conifers to broadleaved to Mediterranean plants. These conditions caused great diversity in species. It due to its diverse ecological condition is rich in relict species that some of them referred to the Tertiary period.
Hyrcanian forest contain the most important and significant natural habitats for in-situ conservation of biological diversity, including those containing threatened species of outstanding universal value from the point of view of science or conservation. It also contains superlative natural phenomena or areas of exceptional natural beauty and aesthetic importance. It is outstanding examples in the record of significant on-going geological processes in the development of landforms and significant geomorphic or physiographic features. It is also outstanding example representing significant ongoing ecological and biological processes in the evolution and development of terrestrial, ecosystems and communities of plants.
Hyrcanian forest has a high degree of authenticity and integrity. This ancient forest contains its full original complement of wild plants and animals, much of which has been lost from other adjoining areas. This natural area with its geological diversity and altitudinal range, different weathers, plant communities with high numbers of endemic species, in its most area is substantially unmodified by people and their effects. Having a total area of 1847886 ha is large enough to survive as a natural area, and to maintain genetic diversity even though there are outside influences. Management of the forest, particularly after 2003, demonstrates that the natural environmental quality of the area has been maintained and improved.
Along with climate, geology, and topography, which are primary factors that determine plant distribution, similarities between the floras of Hyrcanian Forests, Middle European deciduous broadleaved forests and western North America deciduous broadleaved forests are increased by historical biogeography. Hyrcanian Forest contains remnants (refugia) of the broad leaf forests that once covered most of the North Temperate Zone 25 - 50 million years ago, in the early Cenozoic Era. As the climate and land forms changed, this forest disappeared from most of its former range, being replaced by shrublands, grasslands, and woodlands. During the Pleistocene ("Ice Ages"), glaciations had minimal impact on Hyrcanian Forest. Instead of the massive ice sheets that covered much of northeastern America, glaciers were largely restricted to high elevations in western North America.
When the Europe covered by ice during the Pleistocene, Hyrcanian Forest were alive and at the end of frozen, plant species of Hyrcanian forests immigrated to Europe and these forests is mother of European forests. There are little similarities in the physiognomic and taxonomic aspects between Hyrcanian forests and European and American deciduous forests. Fertile soil, suitable precipitation and temperate climate causes that some species such as "Fagus orientalis", "Quercus castaneifolia", "Alnus glotinosa" and "Aecr velotinum" can reach 50 meter height and 3 meter diameter at breast height but they have maximum about 25 meter height and 1 meter diameter at breast height in Europe. The Caspian forest areas are among the most unique and splendid biomes of the world. The Caspian forests have high moisture content, so forest trees are host to many epiphytes such as mosses, ferns, lichens, mistletoes and some flowering plants. Soils are productive and rich in minerals and organic matter. There are many well-known and unique communities in these forests, the most important ones being: