High species richness in hemiboreal forests of the northern Russian Altai, southern Siberia

Questions We discovered forests with very high local numbers of vascular plant species in southern Siberia. Are these the most species-rich forests of the Eurasian boreal and temperate zones, and which factors cause such high species richness? Location Altai Mountains in southern Siberia, Altai Republic, Russia. Methods We sampled vegetation and measured environmental variables across the Altai Mountains in plots of 100 m(2) and established series of nested plots of the size from 1 to 1000 m(2) at sites with the highest species richness. We compared the richest forests with other forests of the Altai and with literature records of species-rich vegetation types from elsewhere. Results The richest forests contained 45 vascular plant species per 1 m(2), 82 per 10 m(2), 114 per 100 m(2) and 149 per 1000 m(2). They are open hemiboreal forests dominated by Pinus sylvestris, with an admixture of Betula pendula, and sparse shrub and moss layers. About 95% of their vascular plant species are in the herb layer. All species are native. These forests occur in a forest-steppe landscape of the northern Altai, where the mean January temperature is between 12.5 and 10 degrees C, mean July temperature is 16-18 degrees C, and total annual precipitation is 560-610 mm. Soils are deep and mesic, with a pH of 6.26.4. Conclusions Hemiboreal forests of the northern Russian Altai are probably the most species-rich forests in boreal and temperate Eurasia. Such a high concentration of many species in small plots probably results from the incidental combination of several factors conducive to high species richness. These factors include the rich regional flora of the Altai Mountains, stability of the environment of this region since the Pleistocene, heterogeneity of the mountainous forest-steppe landscape with a combination of forested areas on north-facing slopes and steppes on south-facing ones, an open canopy that ameliorates microclimate conditions but does not exclude light-demanding species through strong shading, a macroclimate that is not too harsh, mesic soils with near-neutral pH, and limited competition within the herb layer.