Lack of ectomycorrhizal fungi may hinder forest recovery following mountain pine beetle

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. In general, conifer seedling establishment in these forests is enhanced by ectomycorrhizal fungi and pine establishment in particular, is highly sensitive to the abundance of compatible ectomycorrhizal fungi. Consequently, mountain pine beetle may influence forest composition not only directly through mortality of canopy trees, but also indirectly through impacts on the ectomycorrhizal community. To understand the potentially cascading effects of mountain pine beetle on ectomycorrhizal fungi, and subsequent implications for seedling regeneration we identified eleven pine stands in northwestern Alberta, Canada that exhibited a gradient of mountain pine beetle attack intensity (0 -  50 m²/ha basal area attacked since 2009). In these sites we measured abundance of ectomycorrhizal fungi in two ways: 1) monthly sporocarp surveys, and 2) hyphal abundance in soils through the use of char-filled buried pouches. We also monitored germination of sowed seeds of lodgepole pine and white spruce (Picea glauca), a common species regenerating in the understory of these sites, into created microsites along the gradient of attack severity. 

Results/Conclusions

Sporocarp surveys yielded an average of 30 species of fungi per site; richness was insensitive to the severity of mountain pine beetle attack. However, the proportion of ectomycorrhizal sporocarps (relative to saprophytic fungi) decreased with attack severity. Specifically, approximately 60% of sporocarps in stands free of mountain pine beetle attack were ectomycorrhizal compared with less than 10% in sites experiencing high attack.  Aboveground patterns in sporocarps were mirrored in hyphal abundances in the soil. Further, lodgepole pine seedling germination decreased along the gradient of attack, while germination of white spruce was unaffected by patterns of mountain pine beetle attack. Here we show for the first time that sexual reproduction of ectomycorrhizal fungi and their abundance in soils declines following severe attack by mountain pine beetle.  Further, reduced germination of lodgepole pine, despite increases in canopy openness, suggest mountain pine beetle infestation will have a legacy effect altering the trajectory of forest recovery. Our results suggest the differential response of lodgepole pine and white spruce to changing site conditions with attack severity may govern future overstory composition of these stands.