Abstract
ABSTRACTForested 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.
Highlights
Forested ecosystems throughout the world are experiencing increases in the incidence and magnitude of insect-induced tree mortality with large ecologic ramifications
Our results suggest that forests with lower overall tree mortality levels are able to maintain “normal” ecosystem function, as the bacterial community appears resistant to tree death
Microorganisms contribute to changes in greenhouse gas flux through heterotrophic respiration [12] and regulate biogeochemical cycling [13], but communities have the ability to resist changes resulting from these perturbations [14], potentially muting ecosystem responses after a large-scale disturbance
Summary
Forested ecosystems throughout the world are experiencing increases in the incidence and magnitude of insect-induced tree mortality with large ecologic ramifications. 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. It has been determined that a certain threshold of localized tree mortality must be attained before observable changes in soil carbon and nitrogen pools and respiration rates beneath beetle-killed lodgepole pine trees occur [21], presumably because adjacent live trees exert a compensatory effect following a forest disturbance. Observed impacts on ecosystem processes and resident soil microbial communities during forest harvesting have occurred only after compaction exceeded a certain level [22] and can be witnessed decades after the disturbance [23]
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