The influence of post-fire successional trajectory on understory composition in Alaskan boreal forests

Background/Question/Methods  Wildfires are a major natural disturbance in boreal forests of interior Alaska and play an important role in determining forest plant composition and productivity by influencing parameters such as nutrient availability, light transmission, and forest floor heterogeneity. Increased temperatures in boreal forests as a consequence of climate change have been correlated with increased fire severity. This altered disturbance regime has the potential to alter forest successional trajectories, as high-severity fires consumes the organic soil layer, exposing mineral soil, which is easily colonized by light-seeded deciduous tree species. Because canopy type is an important determinant of understory vegetation composition, a potential shift towards a deciduous successional trajectory has implications for future understory composition in boreal forests. To assess how successional trajectory influences understory vegetation composition and cover, we surveyed fire scars (22-62 yr since fire) within interior Alaska during the summers 2008 and 2009. Within each scar, data were collected within stands encompassing a range of successional trajectories (black spruce, mixed, deciduous). Within each stand, understory vegetation was sampled at 20-m intervals along a 100-m transect using a 0.5-m2 point frame grid subdivided into 25 5-cm2 sections. Vegetation hitting the corner of each section was identified to the lowest possible taxonomic level in the field and then later re-classified by functional type (lichens, moss, herbaceous plants, and shrubs and expressed as percent cover.

Results/Conclusions  Over the time period examined, understory composition and cover remained relatively consistent within the spruce successional trajectory. In contrast, stands undergoing a deciduous trajectory exhibited a large shift in functional type cover with stand age, with herbaceous plants and shrubs decreasing 16-fold and 2-fold respectively with time, and mosses increasing nearly 30-fold. The mixed-wood trajectory displayed a similar decrease in herbaceous plants and shrubs but moss cover decreased as well. These results indicate that understory vegetation composition covaries with tree canopy composition, and suggest that climate-induced shifts in fire regime and successional trajectories could also lead to changes in understory composition, with implications for ecosystem services such as carbon storage and habitat.