Stand age and tree density effects on carbon accumulation patterns in post-fire Cajander larch (Larix cajanderi) forests of Far Northeastern Siberia

Background/Question/Methods

Global change models predict that climate warming will increase fire frequency, severity, and extent in boreal forests. Because boreal forests contain a large proportion of global terrestrial carbon (C) stocks, an altered fire regime could have substantial impacts on global C cycling. In this study, we investigated post-fire C accumulation patterns in Cajander larch (Larix cajanderi) forests of northeastern Siberia. Increased fire frequency could affect C pools within larch forests by altering stand age distribution, and severity increases could increase larch stand density by creating more favorable microsites for establishment. In summer 2010, we surveyed 17 forest stands near Cherskii, Russia, representing a low-density successional gradient (5-205 yr old) and a mid-successional density gradient (0-3.7 tree m^-2). Within each stand, we quantified tree size and density to estimate aboveground tree C pools and net primary productivity (ANPP) using allometric equations from previous harvests. We also quantified C pools within snags, woody debris, and soil organic and upper mineral (0-10 cm) horizons and assessed changes in understory light and forest floor cover.

Results/Conclusions

Aboveground tree C and ANPP increased with stand age and density; values in high density stands (2255 g C m^-2 and 147 g C m^-2 yr^-1, respectively) were 3-4x higher than those in low-density late-successional stands (891 g C m^-2 and 43 g C m^-2 yr^-1, respectively). Understory light decreased 15 and 53% across successional and density gradients, respectively, and was strongly correlated (R² = 0.84) with increased aboveground tree biomass. Soil C pools did not vary with tree density, but were 30% higher in late- (6781 g C m^-2) compared to early-successional (5324 g C m^-2) stands. Woody debris and snag C pools were greatest in early-successional stands (405 and 1227 g C m^-2, respectively) and were unaffected by tree density. Total understory cover increased with successional stage and decreased with stand density. Findings highlight the influence of stand age and density on C accumulation patterns within Siberian larch forests and suggest that if increased fire threat leads to younger, denser Siberian larch forests, an increase in aboveground tree C pools may be sufficient to offset differences due to a shorter successional cycle. Further studies addressing the role of successional stage and stand density on C pools as a function of ground-layer development and permafrost stability would greatly improve our understanding of climate-driven alterations to the fire regime and resultant effects on long-term C balance.