| Summary: | Following a period of land degradation lasting more than one thousand years, Iceland has undertaken ambitious restoration and afforestation efforts for one century now. Afforestation has also been a central venture of the Icelandic government in order to meet the commitments assigned by the Kyoto Protocol, because vegetation represents an important carbon sink. The aims of the study are how above- and belowground woody biomass and organic carbon is accumulated along a chronosequence in South Iceland and how afforested and remnant mountain birch areas react as carbon sinks. In summer 2009, 31 monocormic trees (0.1-5.5 m height) were measured and excavated. The excavated trees formed the dataset to establish allometric biomass functions of Icelandic mountain birch. The functions were statistically fitted by using numerical nonlinear regression in Matlab. Tree inventories (n=519) were made at four study sites, whereof three were planted stands and one represented a natural grown woodland. Subsequently, forest biomass and carbon stock and sequestration rates of the four different old sites were estimated by the allometric relationships. The total biomass and carbon stock estimations were in the 10-year old birch stand 3.9 Mg DM ha-1 and 2.0 Mg C ha-1, in the 15-year old 21.1 Mg DM ha-1 and 11.1 Mg C ha-1, in the 60-year old 166.5 Mg DM ha-1 and 87.4 Mg C ha-1 and in the old-growth woodland 73.6 Mg DM ha-1 and 38.7 Mg C ha-1, respectively. Between 2004 and 2009 the average annual sequestration rates were 0.8 Mg DM ha-1 and 0.4 Mg C ha-1 (10-years old stand), 3.3 Mg DM ha-1 and 1.8 Mg C ha-1 (15-years old stand), 1.9 Mg DM ha-1 and 1.0 Mg C ha-1 (60-years old stand) and 2.0 Mg DM ha-1 and 1.1 Mg C ha-1 (old-growth), respectively. The ratio between long-time aboveground and belowground organic carbon stock increased along the chronosequence from 2:1 to 3:1. The root systems of mountain birch were stored in a thinner soil layer than expected. The results were well comparable with published studies on Icelandic mountain ...
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