Shifts in the composition of soil fungal communities following a mountain pine beetle outbreak

Background/Question/Methods

Large-scale outbreaks of mountain pine beetle (Dendroctonus ponderosae) have killed millions of hectares of lodgepole pine (Pinus contorta) forests throughout North America. The rapid and widespread death of lodgepole pine will likely have cascading effects on biodiversity. The composition of soil fungal communities in particular, is predicted to be sensitive to the loss of mature pines from the landscape. In the first few years following beetle-induced tree mortality, ectomycorrhizal fungi ought to decrease in abundance with the loss of photosynthetically derived carbon inputs to the soil, and saprotrophic fungi increase with the addition of carbon inputs through root mortality. To understand the potentially cascading effects of mountain pine beetle on soil fungi, we identified eleven pine stands in northwestern Alberta, Canada that exhibited a gradient of beetle-induced tree mortality (0 -  80% basal area attacked since 2009). In each of these sites we collected 80 randomly distributed soil cores (4 cm wide x 20 cm deep). We assessed fungal diversity and composition by sequencing barcoded samples of the ITS1 region of rDNA using the Ion Torrent™ and used the QIIME bioinformatics platform for subsequent sequence analysis. 

Results/Conclusions

Sequencing yielded 31.5 million reads across sites. Quality filtering reduced this value by approximately 50%. Across a subset of sites representing the extreme ends of the tree mortality gradient, over 900 operational taxonomic units (OTUs) matched reference sequences belonging to the UNITE database.  Richness of fungal communities across the subset of sites was unaffected by the extent of pine loss. The composition of fungal communities, however, changed along the tree mortality gradient. Species belonging to the genus Saccharomycetes (yeasts) tended to increase under conditions of high pine mortality. Soils from sites with high tree mortality contained more total organic carbon than soils from sites without beetle-induced tree mortality, which may in part explain the increase in the abundance of soil yeasts. Of the ectomycorrhizal fungi, Cenococcum geophilum was the most abundant across sites; however, contrary to our prediction, it increased with high pine mortality. This fungal species may benefit from the release of non-pine ectomycorrhizal trees following pine mortality. Here we show for the first time that complex shifts in soil fungal communities occur following insect outbreaks likely due to differences in the dissipation of carbon following widespread tree mortality.