Fire severity as a key determinant of aboveground and belowground biological community recovery in managed even-aged boreal forests

Changes in fire regime of boreal forests in response to climate warming are expected to impact post-fire recovery. However, quantitative data on how managed forests sustain and recover from recent fire disturbance are limited.  

Two years after a large wildfire in managed even-aged boreal forests in Sweden, we investigated how recovery of aboveground and belowground communities, i.e., understory vegetation and soil microbial and faunal communities, responded to variation in the severity of soil (i.e., consumption of soil organic matter) and canopy fires (i.e., tree mortality).

While fire overall enhanced diversity of understory vegetation through colonization of fire adapted plant species, it reduced the abundance and diversity of soil biota. We observed contrasting effects of tree- and soil-related fire severity on survival and recovery of understory vegetation and soil biological communities. Severe fires that killed overstory Pinus sylvestris promoted a successional stage dominated by the mosses Ceratodon purpureus and Polytrichum juniperum, but reduced regeneration of tree seedlings and disfavoured the ericaceous dwarf-shrub Vaccinium vitis-idaea and the grass Deschampsia flexuosa. Moreover, high tree mortality from fire reduced fungal biomass and changed fungal community composition, in particular that of ectomycorrhizal fungi, and reduced the fungivorous soil Oribatida. In contrast, soil-related fire severity had little impact on vegetation composition, fungal communities and soil animals. Bacterial communities responded to both tree- and soil-related fire severity.

Synthesis: Our results two years post-fire suggest that a change in fire regime from a historically low-severity ground-fire regime, with fires that mainly burns into the soil organic layer, to a stand-replacing fire regime with a high degree of tree mortality, as may be expected with climate change, is likely to impact the short-term recovery of stand structure and above- and belowground species composition of even-aged P. sylvestris boreal forests.