Plant functional traits reinforce alternate successional trajectories in Alaskan boreal forest
Climate-sensitive disturbances, such as wildfire, can feed back positively to climate warming via the carbon (C) cycle if C released by combustion is not replaced over post-fire succession. In boreal forests, burning of deep organic soils is not only an important determinate of ecosystem element balance over the disturbance cycle, but also sets the conditions that control tree seedling recruitment, species dominance and successional trajectory. Species dominance, in turn, has the potential to exert strong control over the plant-soil-microbial feedbacks that determine C and nutrient coupling, C storage, and ultimately, replacement of combusted C. We examined the consequences of increasing fire severity for C and N coupling and C storage in Interior Alaska boreal forests. Our approach was three-fold. First, we estimated combustion losses of C and N and reconstructed pre-fire C and N pools in 90 black spruce (conifer) stands that burned in 2004. Over the next seven years, we followed natural tree seedling establishment in these stands and used seedling species and functional type dominance to hind-cast C and N loss and initial post-fire element pools for conifer versus deciduous successional trajectories. Second, we assembled data from 90 stands that varied in time after fire and successional trajectory, and estimated C and N dynamics across 150 years of post-fire succession for each trajectory. Finally, we are using litter, soil, and moss transplant experiments to examine the plant-soil-microbial feedbacks that reinforce differing C and N dynamics across these two trajectories.
Stands that transitioned to deciduous dominance after the 2004 fires had smaller ecosystem pools of C and N before fire, lost a larger proportion of these elements pools the fire, and began succession with smaller residual pools than stands that returned to conifer dominance after fire. Over secondary succession, these deciduous stands accumulated about 10 times more carbon in aboveground biomass than conifer stands. Belowground biomass and soil carbon accumulation, by contrast, was about three times higher in the black spruce stands than in deciduous stands. Nitrogen accumulation did not differ between the trajectories; high C:N ratio biomass accumulation in deciduous stands balanced low C:N ratio soil organic matter accumulation in conifer stands. We hypothesize that the high carbon storage capacity of deciduous stands over succession is reinforced by plant-soil-microbial feedbacks whereby deciduous litter decomposes rapidly and suppresses the growth of recalcitrant mosses, maintaining high rates of nitrogen turnover.