Arbuscular mycorrhizal host plant diversity increases with mountain pine beetle attack severity

Background/Question/Methods

In boreal forests, understory communities are the reservoir of the majority of plant species diversity, influence nutrient cycling, represent habitat and forage for wildlife, and have strong impacts on canopy succession. Consequently, understanding the factors that influence changes in understory composition is critical to understanding general functioning of the forest system.  Historically, light and soil moisture have been identified as strong determinants of understory community composition; both of which can change in response to forest pest activity. Understory species can further be influenced by the specific mycorrhizal association formed, which can be affected by forest composition and structure. Currently, mountain pine beetle is rapidly spreading through lodgepole pine (Pinus contorta) forests of northwestern Alberta, resulting in a mosaic of stands at different stages of attack. The objective of this study was to examine the effects of mountain pine beetle caused tree mortality on initial changes in understory community composition and diversity. We established a total of 110 1-m² plots across eleven mature lodgepole pine stands ranging from 0 – 82% lodgepole pine basal area killed by mountain pine beetle since 2009. Changes in understory diversity and productivity, as well as light and soil moisture, were measured throughout the 2012-growing season. 

Results/Conclusions

Understory biomass increased three-fold along the MPB attack intensity gradient, though herbaceous species richness and evenness were invariant. Understory species diversity increased with higher soil moisture levels and increased severity of beetle-killed stands. However, hidden beneath the apparent short-term stability of understory diversity in response to mountain pine beetle outbreak is variation in responses as a function of the form of mycorrhizae different host species form.  In general, understory species composition shifted from ericoid to arbuscular mycorrhizal (AM) dominated plant communities with increased severity of beetle-killed stands. The diversity and evenness of arbuscular mycorrhizal associated species increased by twenty-five percent as soil moisture levels rose and tree mortality increased across stands. In contrast, non-AM host plant species decreased in richness and diversity by fifty-percent from unaffected to severely beetle-killed stands. Results are consistent with previous observations of increased understory diversity and productivity in severely beetle-killed stands. However, our results also suggest that the type of plant-mycorrhizal association may have a strong effect on initial understory community response to insect outbreak.