Moth Outbreaks Alter Root-Associated Fungal Communities in Subarctic Mountain Birch Forests

Abstract

Climate change has important implications on the abundance and range of insect pests in forest ecosystems. We studied responses of root-associated fungal communities to defoliation of mountain birch hosts by a massive geometrid moth outbreak through 454 pyrosequencing of tagged amplicons of the ITS2 rDNA region. We compared fungal diversity and community composition at three levels of moth defoliation (intact control, full defoliation in one season, full defoliation in two or more seasons), replicated in three localities. Defoliation caused dramatic shifts in functional and taxonomic community composition of root-associated fungi. Differentially defoliated mountain birch roots harbored distinct fungal communities, which correlated with increasing soil nutrients and decreasing amount of host trees with green foliar mass. Ectomycorrhizal fungi (EMF) abundance and richness declined by 70-80 % with increasing defoliation intensity, while saprotrophic and endophytic fungi seemed to benefit from defoliation. Moth herbivory also reduced dominance of Basidiomycota in the roots due to loss of basidiomycete EMF and increases in functionally unknown Ascomycota. Our results demonstrate the top-down control of belowground fungal communities by aboveground herbivory and suggest a marked reduction in the carbon flow from plants to soil fungi following defoliation. These results are among the first to provide evidence on cascading effects of natural herbivory on tree root-associated fungi at an ecosystem scale.

The biogeochemical consequences of litter transformation by insect herbivory in the Subarctic: a microcosm simulation experiment

“…It was, however, in disagreement with the field study by Kaukonen et al (2013) finding decreasing soil fungi:bacteria after repeated defoliations, suggesting that in the field, plant ? symbiont feedbacks (Saravesi et al 2015;Parker et al 2016) may be more important than the direct effects of litter degradation studied here. Similarly, our findings contradict the general trends for field studies in temperate and boreal forests (Frey et al 2004;Treseder 2008), where chronic inorganic N fertilisation had a negative impact on fungal biomass, while bacteria showed no response.…”

“…More work on the relative importance of seasonal timing and indirect effects, such as altered plant allocation and changes in plant community composition, will be required to more accurately resolve the effects of the outbreaks on total soil C cycling (e.g. Saravesi et al 2015;Arnold et al 2016). Nonetheless, we found enhanced decomposition of added substrate-C in frass treatments compared to the pure litter treatments (Table 2), which may suggest that addition of substrates with high C:N (litter) induced severe N-limitation (Kamble and Bååth 2014) due to rapid immobilisation of available N; this is also supported by the consistently low ammonium concentrations in the litter treatments ( Figure S2).…”

“…Thus, while it is clear that the outbreaks exert a range of important impacts on vegetation and soil, a detailed picture is still missing of the underlying mechanisms regulating observed changes in soil processes. This is partly due to the difficulty of disentangling the effects of the increased surface input of herbivory transformed labile litter, and the decreased belowground labile C-allocation, due to reduced photosynthesis, which occur simultaneously (Saravesi et al 2015).…”

“…For example, studies have reported enhanced resource turnover and decreased fungal:bacterial ratios (Kaukonen et al 2013), slowed biogeochemical cycling linked to changes in ectomycorrhizal (ECM) associations (Parker et al 2016), and decreased ECM-abundances and richness benefitting saprotrophic decomposers (Saravesi et al 2015). Moreover, the substantial short-term N-enrichment following outbreaks in the Subarctic mountain birch system (e.g.…”

Complex effects of mammalian grazing on extramatrical mycelial biomass in the Scandes forest‐tundra ecotone

“…The large abundance of /cortinarius is in line with other recent studies from the Scandes forest‐tundra ecotone (Parker et al., 2017; Saravesi et al., 2015), which nuances the view that Cortinarius spp. are unwilling to colonize mineral substrates (Wallander et al., 2013).…”