On the Ural range the elevation of upper timberline changes at grade 71 m per degree of latitude in linear regression. Much lengthy cross-section -for the semi-arid regions of middle Siberia and adjacent Kazakhstan, and for the regions of eastern Siberia dominated by larch forests -exhibit parabolic regression of timberline levels upon geographic latitude. The longitudinal gradient of timberlines presumably depends on radiation balance related with the amount of precipitation. The arctic boundary of taiga in eastern Europe and Siberia lies mostly on average latitude 69 ° 36' E. It correlates with mean July temperature 11.2 °C, or with duration of the growing season 128 days with stable temperature of air exceeding O°C which amounts to 876 °. Daily temperatures exceeding 5 ° and especially 10 °C are seemingly less influential there. The value of 11.2 °C deviates by about 1 °C from the value of 'above 10 °C' for three summer month reported by Langlet 1935, which shows the close environmental control regulating the northern and upper boundary of the northern, mostly coniferous forest on the northern hemisphere.
A minimum variance cluster analysis of 87 species of vascular plants occurring in south Siberian birch woodlands, based on their total distribution ranges, grouped them into 9 phytogeographic elements. The main distribution patterns of each element are illustrated by means of maps obtained by automatic mapping programs, showing the joint occurrence of the species of each element on a world scale. The results indicate a predominance of Eurasiatic-temperate taxa. The south Siberian birch woodlands can be considered as the easternmost extensions of the European deciduous forest belt, which in Siberia is compressed between the taiga biome in the north and the steppe biome in the south. The marginal position of Siberian Betula stands is reflected by the persistence of plants which are 'ecologically marginal' with respect to true forest vegetation; most of the Eurasiatic species are typical, in Europe, of forest-meadows and forest-margins
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