Effects of alternative successional trajectories on carbon pools within boreal forests of Interior Alaska

Background/Question/Methods

Climate warming in high latitude boreal forests has increased fire severity and frequency. High severity fires tend to expose mineral soil and favor establishment of light-seeded deciduous species. Consequently, forest successional trajectories are predicted to shift from black spruce self-replacement to those which favor deciduous dominance. Because these vegetation types differ in their flammability, longevity, standing biomass, and productivity, they are expected to have different impacts on carbon (C) pools. To assess how different trajectories affect ecosystem C dynamics during the intermediate successional stage, we surveyed 12 fire scars (22 to 52 years after fire) within Interior Alaska during summers 2008 and 2009. Using a combination of satellite imagery, fire scar boundaries, and ground-truthing, we located forest stands undergoing different successional trajectories (spruce, mixed, or deciduous) within each fire scar. Within each stand, we utilized 100-m line transects to quantify the composition, size, and density of trees and large shrubs and used these estimates to calculate the relative contribution of coniferous and deciduous species to total aboveground biomass. We also measured the mass of snags and woody debris on the forest floor.

Results/Conclusions

Within each successional trajectory, aboveground biomass significantly increased with stand age, but the rate of increase was significantly lower in black spruce stands compared to mixed and deciduous stands. By ~ age 50, aboveground biomass of black spruce stands (~ 20 Mg ha^-1) was 3x and 5x less than that in mixed and deciduous stands, respectively. The mass of standing dead trees varied little with stand age, but overall values were 4x and 6x lower within spruce stands (1.4 Mg ha^-1) compared to mixed (4.1 Mg ha^-1) and deciduous stands (6.4 Mg ha^-1), respectively. Woody debris decreased linearly with age in mixed and deciduous stands, likely due to decreased inputs following the initial fire disturbance. Woody debris in black spruce stands (~ 25 Mg ha^-1) peaked ~ 30 yr post-fire, and then declined to values similar to the other trajectories (< 10 Mg ha^-1). Our findings highlight the importance of successional trajectories in controlling biomass accumulation during intermediates stages of succession and suggest that if increased fire severity leads to increased deciduous tree cover, then landscape-scale C storage in biomass and standing dead will increase. However, because deciduous trees and shrubs have shorter life-spans than black spruce, the long-term cumulative effect on these C pools may be less in mature stands undergoing mixed and deciduous trajectories.