Ecosystem Resilience and Limitations Revealed by Soil Bacterial Community Dynamics in a Bark Beetle-Impacted Forest

Abstract

Forested ecosystems throughout the world are experiencing increases in the incidence and magnitude of insect-induced tree mortality with large ecologic ramifications. Interestingly, correlations between water quality and the extent of tree mortality in Colorado montane ecosystems suggest compensatory effects from adjacent live vegetation that mute responses in less severely impacted forests. To this end, we investigated whether the composition of the soil bacterial community and associated functionality beneath beetle-killed lodgepole pine was influenced by the extent of surrounding tree mortality. The most pronounced changes were observed in the potentially active bacterial community, where alpha diversity increased in concert with surrounding tree mortality until mortality exceeded a tipping point of ~30 to 40%, after which diversity stabilized and decreased. Community structure also clustered in association with the extent of surrounding tree mortality with compositional trends best explained by differences in NH4+ concentrations and C/N ratios. C/N ratios, which were lower in soils under beetle-killed trees, further correlated with the relative abundance of putative nitrifiers and exoenzyme activity. Collectively, the response of soil microorganisms that drive heterotrophic respiration and decay supports observations of broader macroscale threshold effects on water quality in heavily infested forests and could be utilized as a predictive mechanism during analogous ecosystem disruptions.

The Abundance of Fungi, Bacteria and Denitrification Genes during Insect Outbreaks in Scots Pine Forests

“…[34,39]. Therein, the biogeochemical response to the outbreak was most prominent in the mineral soil and dependent on the percentage of dead trees [34,39]. Even though the trees at our study site were not killed by the outbreaks, the higher intensity of the nun moth outbreak compared to the pine-tree lappet outbreak can be one explanation for the observed differences between the sites.…”

“…For example, the bacterial community structure shifts in a bark beetle (Scolytidae spp.) infested forest site have been shown to correlate with soil the NH 4 + concentrations and C/N ratio [34]. At our study sites, high amounts of dissolved organic C and N in throughfall, needle litter, larval cadavers, and particularly feces were introduced to the soil (cf.…”

“…forest infested with bark beetles (Scolytidae spp.) [34,39]. Therein, the biogeochemical response to the outbreak was most prominent in the mineral soil and dependent on the percentage of dead trees [34,39].…”

“…Conventional DNA-based methods allow the assessment of the potential activity of genes by quantification, but are unable to differentiate between DNA derived from active, dormant or dead microorganisms [34]. Abundance and activity patterns of denitrifying microorganisms often do not respond congruently to environmental changes [50].…”

Factors Affecting the Leaching of Dissolved Organic Carbon after Tree Dieback in an Unmanaged European Mountain Forest

“…Štursová et al 50 have shown that soil microbial community significantly changed in the Plešné catchment after the tree dieback, which has also been observed elsewhere. 51 At Plešné, fungal community biomass decreased, despite a relative increase in saprotrophic taxa, due mostly to the disappearance of mycorrhizal fungi following tree death. In contrast, bacterial biomass increased or remained unaffected after the disturbance, which resulted in a substantial decrease in the soil fungi-to-bacteria ratio.…”