Identifying functional impacts of heat-resistant fungi on boreal forest recovery after wildfire

Fungi play key roles in carbon (C) dynamics of ecosystems: saprotrophs
decompose organic material and return C in the nutrient cycle, and
mycorrhizal species support plants that accumulate C through
photosynthesis. The identities and functions of extremophile fungi present
after fire can influence C dynamics, particularly because plant-fungal
relationships are often species-specific. However, little is known about
the function and distribution of fungi that survive fires. We aim to
assess the distribution of heat-resistant soil fungi across burned stands
of boreal forest in the Northwest Territories, Canada, and understand
their functions in relation to decomposition and tree seedling growth. We
cultured and identified fungi from heat-treated soils and linked sequences
from known taxa with high throughput sequencing fungal data (Illumina
MiSeq, ITS1) from soils collected in 47 plots. We assessed functions under
controlled conditions by inoculating litter and seedlings with
heat-resistant fungi to assess decomposition and effects on seedling
growth, respectively, for black spruce (Picea mariana), birch (Betula
papyrifera), and jack pine (Pinus banksiana). We also measured litter
decomposition rates and seedling densities in the field without
inoculation. We isolated seven taxa of heat-resistant fungi and found
their relative abundances were not associated with environmental or fire
characteristics. Under controlled conditions, Fayodia gracilipes and
Penicillium arenicola decomposed birch, but no taxa decomposed black
spruce litter significantly more than the control treatment. Seedlings
showed reduced biomass and/or mortality when inoculated with at least one
of the fungal taxa. Penicillium turbatum reduced growth and/or caused
mortality of all three species of seedlings. In the field, birch litter
decomposed faster in stands with greater pre-fire proportion of black
spruce, while black spruce litter decomposed faster in stands experiencing
longer fire-free intervals. Densities of seedlings that had germinated
since fire were positively associated with ectomycorrhizal richness while
there were fewer conifer seedlings with greater heat-resistant fungal
abundance. Overall, our study suggests that extremophile fungi present
after fires have multiple functions and may have unexpected negative
effects on forest functioning and regeneration. In particular,
heat-resistant fungi after fires may promote shifts away from conifer
dominance that are observed in these boreal forests.