The factors are revealed that limit the spread of white spruce (Picea glauca) over North America, and ecological amplitudes of the species with relation to those factors are determined. Based on these data, the eco-geographical model of the spruce dispersion is designed. It is shown that the northbound dispersion of white spruce over North America is limited by heat supply. The species range at the south-western border (from the central part to the Rocky Mountains) is shaped by lack of humidification, at the south-eastern one, presumably by competitive relations with foliar edificators. For P. glauca, the northern limit of heat supply turns out to be about 1200 degrees C. At the southern border of the range, white spruce become uncompetitive with deciduous trees starting from the growing degree days of 3000 degrees C approximately. The limits of the species dispersion over arid zones are determined by the hydro-thermal coefficient, the value of which should not exceed 1.2. Validation of the dispersion model for P. glauca demonstrates discrepancy between actual and model range as of +/- 50-100 km. At that, within the discrepancy zone between potential range and that ascertained by Little (1971), the not yet detected populations of P. glauca may turn up, as is affirmed by a new finding of white spruce in the Noatak National Park.

The comparison is conducted of ecological boundaries of P. glauca spread with those ones of the Picea order in Eurasia. It is found out that the eco-geographical model designed for P. glauca describes the dispersion of Eurasian spruce species with high accuracy. This testifies not so strong ecological divergence between North American and Eurasian spruce species. Deviation of the modeling results for white spruce from actual range of Eurasian spruces is observed in the regions with permafrost and some others. Therefore, additional environmental factors are required for the eco-geographical model when reallocating it to another continent.