Seedbed filter controls post-fire succession

Abstract Using black spruce ( Picea mariana )- Kalmia angustifolia dominated communities of eastern Canada we tested the hypothesis that habitat filter pre-empts biotic filter as a community structuring force in early post-fire succession leading to Kalmia dominated heath. We considered post-fire seedbed as an abiotic (habitat) filter and post-fire aboveground biomass of dominant plants as a biotic filter. First we surveyed and categorized post-fire seedbed types and tested the seedbed filter by a black spruce seeding experiment in 155 small plots (30×30 cm 2 ) on nine sites burned between 1 and 38 years previously. We then quantified black spruce seedling establishment in a competition removal and nitrogen addition experiment in twenty four 3×3 m 2 plots on four sites burned 7, 24, 23 and 29 years previously. We also conducted a vegetation survey in 200 plots (1×1 m 2 ) on 20 sites along a 1- to 76-year post-fire chronosequence. Only about 10% post-fire seedbed with little or no residual organic matter was suitable for black spruce regeneration. On the rest of the seedbeds made up of charred humus and lichen substrates Kalmia grew profusely by sprouting from sub-surface fire survived components. Removal of aboveground competition and addition of nitrogen did not help black spruce seedling establishment. Kalmia dominated the chronosequence with 50–90% cover until 60 years after fire. Black spruce cover was insignificant during the first 20 years followed by a slow increase surpassing Kalmia cover around 60 years. Herbaceous cover was initially high followed by a decline coinciding with high Kalmia cover and then increased again with decreased Kalmia cover. Lichen cover followed a similar trend. We interpret this post-fire vegetation assembly as a seedbed controlled phenomenon where seedbed limitation filters out the pre-fire dominant, black spruce, in favour of vegetatively regenerating Kalmia turning coniferous forest into Kalmia heath during the first four decades of secondary succession.

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