The boreal forests of interior Alaska have recently been experiencing an increase in wildfire frequency, severity, and spatial extent. Conditions caused by severe burning have been linked to significant variations in post-fire vegetation recruitment patterns, and in particular, a shift from black spruce to aspen seedling dominance in severely burned sites. This abrupt shift in vegetation dominance could have important implications for climate-albedo feedbacks, future fire regime, wildlife habitat quality and subsistence resources.Studies conducted in burned black spruce stands in interior Alaska during the first 7-8 years after fire showed that levels of aspen recruitment responded positively, while black spruce recruitment responded negatively to increasing burn severity. Our objective was to understand whether these initial recruitment responses to variations in burn severity would be reinforced or dampened by growth responses of surviving vegetation in subsequent years after fire. In this study we examined patterns of post-fire vegetation growth in response to variations in burn severity 10-20 years after fire. We revisited previously sampled stands, and measured vegetation composition and growth parameters, residual organic layer depth, and soil moisture in 65 plots across four stands that burned between 10 and 20 years ago.
Results/Conclusions
Aspen displayed a strong positive growth response to decreasing residual organic layer depth, with respect to stem density/hectare as well as average basal area/hectare. Moreover, this response was significantly stronger than that observed during the initial recruitment stage. Black spruce growth responded negatively to decreasing residual organic layer depth. However, the relationship between black spruce growth and residual organic layer depth was significantly weaker than that observed in the case of aspen. The negative effect of a deep residual organic layer on growth was ameliorated in sites which had high soil moisture, in the case of black spruce, but the same was not true for aspen. Our results indicate that 1) the initial effects of burn severity on aspen seedling recruitment are reinforced and amplified by plant growth responses 10-20 years after fire, and are likely to determine eventual stand dominance, 2) residual organic layer depth plays a major role in controlling aspen growth, but is not as important in the case of black spruce growth, and 3) soil moisture is likely to be a key factor in determining the vulnerability or resilience of boreal forests to shifts in vegetation dominance in response to variations in burn severity.
