Climate-induced mountain tree-line evolution in southern Siberia

The elevational tree-line change within the transitional zone between boreal forest and Mongolian steppes was quantified for the last millennium. The basic approach included studies along transects and measurements of tree-line positions to identify current, historical, refugee and regeneration tree lines. Tree mortality and natality were determined based on dendrochronology analysis. Tree mortality in the sixteenth to eighteenth centuries coincided with the Little Ice Age, while tree establishment was stimulated by warming at the end of nineteenth century. Downward shifts in tree line varied by an order of magnitude. The current tree-line position reoccupied the historical tree line in some transects, and was below or above the historical line in others. The regeneration line surpassed the historical tree line by 91 +/- 46 m (mean +/- SD). Such a heterogeneous response was attributed to local topoclimatic conditions and sapling recruitment efficiency. A mean annual 1 degrees C increase in temperature was associated with an upward shift of the tree line by about 70 m. The upward migration rate of the current tree line was about 0.8 m year-1 during the last century. The regeneration migration rate was about 2.3 m year-1 over the past three decades. Finally, the transformation of krummholz forms of larch and Siberian pine into arborescent form was documented.