Variation in plant community composition and vegetation carbon pools a decade following a severe fire season in interior Alaska


Questions How does fire severity, measured as depth of burn of ground layer fuels, control the regeneration of understorey species across black spruce-dominated stands varying in pre-fire organic layer depths? Are successional shifts from evergreen to deciduous understorey vegetation more likely to occur with greater depth of burn? Does a shift in understorey vegetation community towards more deciduous species influence carbon accumulation in vegetation biomass? Location Northern boreal forest, interior Alaska. Methods We sampled 32 stands in interior Alaska that burned in 2003 and 2004 in which depth of burn had been recorded soon after fire. In 2014 we characterized tree density, understorey vegetation composition, above- and below-ground vegetation carbon pools, and a suite of environmental variables. We used ANOVA and multivariate redundancy analysis models to analyse the dominant controls on vegetation composition and carbon pools. Results Fire severity was a strong control on post-fire tree and non-vascular species composition. Ten years post-fire, sites that experienced deeper burning of organic layers had a higher abundance of deciduous tree species, fire-adapted mosses, forbs and graminoids, and lower abundances of evergreen shrubs and Sphagnum mosses. Environmental variables (elevation, soil bulk density and mineral soil pH, moisture and temperature) served as important controls on tree and vascular understorey species composition. We found no evidence that carbon pools associated with recovering vegetation biomass were influenced by landscape position, fire severity or environmental variables. Conclusions Both fire frequency and depth of burn are projected to increase with a warmer climate in interior Alaska. These shifts in fire regime are expected to favour the regeneration of deciduous species over conifers, which have the potential to regulate successional pathways and large-scale trends in albedo, which both ultimately could influence ecosystem–climate feedbacks and dampen future fire cycles. Our results show that understorey species composition is controlled by fire severity, suggesting that more severe burning causes a decline in the self-replacement of conifer communities across a range of hydrologic settings. Overall, our results suggest a loss of resilience in black spruce forests in the face of a changing fire regime. However, we found no evidence that fire severity influences vegetation carbon pools 10 yrs post-fire, suggesting that there is less landscape variation in biomass carbon pools than in vegetation composition and succession during early post-fire succession.

Journal of Vegetation Science