Fire severity affects community assemly through filters on regeneration traits in Alaska boreal forests

Background/Question/Methods

Community assembly is a critical process that shapes the composition of biotic communities following disturbance, but the mechanisms that determine divergent patterns of assembly are often poorly understood. Variation in fire regime is tightly linked to vegetation patterns and this has led to fire-adapted plant communities across biomes with similar functional traits. In the boreal forest, climate change is predicted to change boreal fire regime by increasing frequency and severity of fires. In 2004, wildfires burned 2.7 million ha of forest in interior Alaska, offering a unique opportunity to study the effects of variation in fire severity across a regional landscape.  Here we measured the composition of plant communities early after wildfire to assess how variations in disturbance characteristics influenced post-fire community assembly via effects on the relative success of different plant regeneration strategies. We compared patterns of post-fire community composition and regeneration trait abundance across a range of fire severities observed within a single pre-fire forest type, in black spruce forests of interior Alaska. 

Results/Conclusions

Patterns of community composition, as captured by multivariate ordination with non-metric multi-dimensional scaling, were primarily related to gradients in fire severity (biomass combustion and loss) and secondarily to gradients in soil pH, and regional climate. This pattern was apparent in both the full dataset (n=87 sites) and for a reduced subset of mesic sites (n=49). Changes in community composition across the fire severity gradient were highly correlated to variations in plant traits such as regeneration strategy and rooting depth. The dominant alignment between fire severity, regeneration traits, and community composition provides strong empirical support the hypothesis that disturbance characteristics influence patterns of community assembly by affecting the relative success of different regeneration strategies. Furthermore, our observations suggest that variations in disturbance characteristics may dominate over environmental constraints in determining early patterns of community assembly. The results have important biogeographical and functional implications for Alaska’s boreal forest and suggest that trends favoring moderate/high severity fires will alter community assembly rules, thus significantly altering the future landscape of boreal Alaska.