Fire severity is increasing in boreal forests in conjunction with global climate warming. Whether increased fire activity creates a positive, negative, or neutral feedback to climate warming depends on the balance between CO2 emissions during a fire and changes in forest successional dynamics after fire, which influence a forest’s ability to uptake and store carbon (C). One way that increased fire severity could alter C dynamics is through consumption of the soil organic layer (SOL), which alters the seedbed, and thus, forest stand composition and density. The primary objective of this study was to assess how fire-driven changes in stand density influence understory vegetation cover and associated C pools within larch (Larix cajanderi) forests of far northeastern Siberia. We hypothesized that a fire-driven increase in larch stand density decreases shrub, moss, and grass cover and their C pools due to decreased light and soil temperatures and increased leaf litter inputs. We quantified understory vegetation composition and C pools, canopy cover, and soil conditions across a stand density gradient (0.04 to 3.7 trees m-2) within a 75-yr old fire scar near the Northeast Science Station in Cherskii, Russia.
Moss cover decreased with increasing larch stand density, averaging 48% in low density stands compared to 2% in high density stands. Grass, small shrub (e.g., Labrador tea, blueberry, cranberry etc.) and lichen cover also decreased with increasing stand density, peaking at 28%, 47% and 56%, respectively, in low density stands compared to 2.5%, 20%, and 42% in high density stands. Aboveground C pools in larch trees increased as stand density increased; tall shrub (dwarf birch and willow) C pools decreased along the same gradient. However, the magnitude of this change varied substantially between these two pools; high density larch tree stands stored ~1,742 g C m-2 and 961 g C m-2 in low density stands while shrubs stored 75 g C m-2 and 92 g C m-2 respectively. These decreases in understory cover and C pools with increasing stand density were coincident with increased canopy cover and decreased soil temperature. Thus, understory vegetation contribution to forest floor cover and C pools will be greater in low to medium density stands, while high density stands will store most of their C in larch trees. This may cause a shift in C sequestration patterns across stands with fire-driven changes in stand density.