Biogeochemistry in Subarctic birch forests : Perspectives on insect herbivory

• Main Author: Ågård Kristensen, Jeppe
• Format: Doctoral or Postdoctoral Thesis
• Language: English
• Published: Lund University, Faculty of Science 2019
• Subjects: Ecology; Biogeochemistry; Aboveground-belowground interactions; Herbivory; Insects; Subarctic; Birch; soil microbes; Fennoscandia; Northern Sweden
• Online Access: https://lup.lub.lu.se/record/f5b40e04-bc40-4962-830d-127549ceb8bd; https://portal.research.lu.se/files/68895904/Jeppe_g_rd_web_1_.pdf

Herbivory can influence ecosystem processes, partly through long-term changes of the plant community compositions, but also more rapidly through the herbivores’ digestive alteration of the organic matter that is cycled through the soil and back to the primary producers. In the Subarctic mountain birch (Betula pubescens ssp. czerepanovii) forest (SMBF) in Northern Fennoscandia, outbreaks by the geometrid moths (Epirrita autumnata and Operophtera brumata) are well-described, widespread, and increasing with global warming. In contrast, the ecosystem effects of background insect herbivory (BIH) in this ecosystem lacks quantification, although belowground responses to aboveground perturbations in high-latitude systems may accelerate global warming due to their storage of large terrestrial organic carbon (C) pools. We quantified the ecosystem impact of BIH in the SMBF of Northern Sweden. An initial literature review showed that the clear increase in organic matter turnover rates under insect infestations was primarily driven by outbreak conditions. In line with this, our conversion of an average BIH-rate of ~1.6% of the leaf area to annual canopy-tosoil fluxes of nitrogen (~3.5% N) and phosphorus (~2.0% P) showed that the background rates were relatively small compared to internal recycling through litter, and inputs from external sources, such as atmospheric deposition, biological fixation and weathering.In addition, we showed that the insects themselves efficiently conserve N, as 70-80 % of the ingested N was converted to insect biomass, while respiring 30-50% of the ingested C. When insect excreta (frass) was added to the soil, we showed that another ~30 % of the C was respired by soil organisms. Hence, a total of ~60 % of the C ingested by insect herbivores would be respired during the first growing season, compared to ~10 % of the C added as senesced litter, suggesting a decreased litter C-sink in soils during outbreaks. In microcosm incubations, frass addition stimulated fungal growth more than bacterial growth ...