Differential Post-Fire Vegetation Recovery of Boreal Plains Bogs and Margins

Abstract

Peatland margins are a distinct ecotone especially vulnerable to deep smouldering in the Boreal Plains because they can experience greater water table drawdown during dry periods compared to peatland middles. Margin recovery trajectories have potentially important implications for wildfire behaviour as both the rate of vegetation recovery and community composition control fuel load and flammability. We compared peatland margin and middle vegetation trajectories using a chronosequence of time-since-fire in boreal Alberta, Canada. Margins had unique post-fire indicator species, with a higher broadleaf cover and limited Sphagnum moss colonization. Middles and margins became less distinct with greater time-since-fire, where both were dominated by feathermoss as canopy closure increased. High burn severity in margins can expose the seedbank in the underlying mineral soil to favourable conditions, causing rapid accumulation of broadleaf aboveground biomass and limiting Sphagnum establishment. The rapid accumulation of aboveground biomass increases potential fuel load, while exclusion of Sphagnum increases future smouldering potential given the dense peat in the margin ecotone. However, the dominance of deciduous vegetation for several decades post fire would serve to limit wildfire compared to a conifer-dominated system, particularly post leaf-out. Thus, peatland margins could represent a positive feedback to peat carbon loss for early season fires and a negative feedback for post leaf-out fires due to the interplay between fuel load, fire seasonality, and species flammability. Characterization of margins as distinct ecotones with a separate vegetation structure and species composition from peatland middles provides critical insight about wildfire vulnerability and carbon storage in the Boreal Plains.