From flames to forests: Carbon dynamics during post-fire succession in Alaskan boreal forests

Background/Question/Methods

Global change models predict high-latitude boreal forests will become increasingly susceptible to an intensifying fire regime as climate warms and dries. This altered disturbance regime could affect global terrestrial carbon (C) stocks and biophysical processes by decreasing stand age or altering plant demographic processes, with potential feedbacks to global and regional climate. Increased fire activity may also consume nitrogen (N) stocks, affecting nitrogen availability and plant productivity over the successional cycle. Here, we assess the interactions between fire, stand age and composition, and C and N dynamics within boreal forests of interior Alaska, where increased fire severity is predicted to shift forest composition from predominantly black spruce (Picea mariana) to greater deciduous dominance. We measured patterns of aboveground C and N accumulation and storage within 63 forest stands varying in age (20-119 years since fire) and deciduous importance value (IV), determined by relative density, basal area, and frequency of deciduous trees and large shrubs within each stand.

Results/Conclusions

C and N pools stored within aboveground biomass of tree and large shrubs increased with both stand age and deciduous IV, which explained 56 and 52% of the variability in these pools, respectively. Aboveground productivity (ANPP) also increased with stand age and deciduous IV (R² = 0.21). Forest type affected C and N accumulation and storage. Accumulation rates of C and N pools and ANPP were 3-4 times higher in deciduous stands dominated by (Populus tremuloides) or Alaskan paper birch (Betula neoalaskana) compared to black spruce stands. This trend was driven by the larger size and higher foliar N content of deciduous trees. At the stand level, ANPP of black spruce increased with stand age, but ANPP of aspen and Alaska birch stands did not. This was like likely due to decreased density over time, as ANPP at the tree level increased with stand age in all but the oldest aspen stand. Feedbacks between climate, fire, and vegetation that lead to shifts in species dominance are likely to impact landscape-level C and N cycling, with implications for sustainability of forest productivity and global and regional climate.