Effects of Climate, Site Conditions, and Seed Quality on Recent Treeline Dynamics in NW Russia: Permafrost and Lack of Reproductive Success Hamper Treeline Advance?
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Treeline advance alters albedo and carbon storage and is an important feedback mechanism to the global climate system. Establishment of trees north of the treeline requires favorable climate, suitable microsites, and viable seeds. Here we studied the influence of climate and microsite conditions on tree and seedling growth at four transects from forest through woodland to tundra in NW Russia, and tested the viability of seeds from the region. General growth patterns and establishment periods of the treeline species Picea obovata are similar across the study sites suggesting a regional driver (for example, climate). Individuals established as early as the 1640s, but mainly between 1850 and 1880, and during a major and continental scale establishment wave in the 1950s and 1960s. No establishment occurred after 1982. Older trees mainly showed significant and stable positive relationships to growing year summer temperatures and significant stable negative correlations to previous year summer temperatures in nearly all plots. Trees from the last establishment wave showed more mixed responses, but current year summer temperature positively affected growth. Active layer depth was similar in all plots with trees but decreased sharply in treeless tundra. A major role for the lack of recent establishment seems to be very low seed viability, possibly combined with early strong fall frosts, which might have severely limited successful recruitment in the last decades of the twentieth century. For a successful establishment of P. obovata in tundra areas of NW Russia, permafrost degradation and (generally) warmer winters might be a prerequisite.
Keywordsdendroecology Picea obovata recruitment tree rings seed viability climate warming
This study was supported by a Sofja Kovalevskaja Award from the Alexander von Humboldt Foundation (M. Wilmking), the German National Scholarship Foundation (S. Kenter) and the EU-Project CARBO-North (6th FP, Contract No. 036993).
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