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The Potential Impact of CO2 and Air Temperature Increases on Krummholz Transformation into Arborescent Form in the Southern Siberian Mountains

Abstract Trees in the southern Siberian Mountains forest-tundra ecotone have considerably increased their radial and apical growth increments during the last few decades. This leads to the widespread vertical transformation of mat and prostrate krummholz forms of larch (Larix sibirica Ledeb) and Siberian pine (Pinus sibirica Du Tour). An analysis of the radial growth increments showed that these transformations began in the mid-1980s. Larch showed a greater resistance to the harsh alpine environment and attained a vertical growth form in areas where Siberian pine is still krummholz. Upper larch treeline is ≥10 m higher than Siberian pine treeline. Observed apical and radial growth increment increases were correlated with CO2 concentration (r  =  0.83–0.87), summer temperatures (r  =  0.55–0.64), and “cold period” (i.e. September–May) air temperatures (r  =  0.36–0.37). Positive correlation between growth increments and winter precipitation was attributed to snow cover protection for trees during wintertime.