Biomass and composition of understory vegetation and the forest floor carbon stock across Siberian larch and mountain birch chronosequences in Iceland

In English Changes in understory biomass, forest floor carbon (C) stock and vegetation composition were studied in six age-classes of Siberian larch (Larix sibirica) and two age-classes of native birch (Betula pubescens) in Iceland. The ground vegetation was less in the larch during the thicket stage and in the old-growth birch compared to a treeless pasture. Understory biomass was strongly related to canopy gap fraction across forest stands (P < 0.001), but not to soil pH or soil C/N ratio. Increased mass of dead wood and alterations in vegetation composition increased the forest floor C-stock of older forests. The forest floor had reached as high C-stock as the pasture's ground vegetation in ca. 50 years in the managed larch plantations and in ca. hundred years in the unmanaged birch forest. This study clearly shows the importance of which time-step is used when changes in forest floor C-stocks are computed for afforestation areas.