Conclusion
While the biogeochemical consequences of geometrid moth outbreaks in
subarctic birch forests have attracted a lot of attention, less
consideration has been given to background insect herbivory and its
contribution to elemental cycling in this system. We showed that the
leaf area loss due to background insect herbivory
(~1.6%) was comparable to what was previously found in
dwarf shrub tundra, yet lower than ecosystems at lower latitudes, and an
increase with climate warming should therefore be anticipated. The
nutrient fluxes from canopy to the soil associated with current
background herbivore intensities were 1-2 orders of magnitude lower than
the fluxes through senesced litter and soil input from external sources.
The variation in background insect herbivory rates was substantial,
however, and tended to increase with elevation at the local scale, and
annual solar radiation, but showed no link to overall elevation,
temperature, nor humidity variation. This supports previous reviews
suggesting that most commonly recorded abiotic driver variables are not
always sufficient to predict variation in insect herbivory and other
ecological processes in mountainous landscapes. We speculate that
accounting for context dependencies by introducing moderator variables
in ecosystem models, e.g. latitude or biome, may be a fast way forward
to allow better integration of data from elevational gradients, until we
have sufficient understanding of the discrepancies between
biotic-abiotic relationships in space-for-time substitution studies at
different scales.
Acknowledgements: We thank Edward Diggory, Anders Voss, Jens
Arne Antoft, Mads Engelstoft, Simon Risum, Tue Mariager, Marco Lund,
Thomas Heister, Jens-Jakob Kratmann Nissen and Rasmus Svingel for
valuable assistance in the field, and Femke Pijcke and Esben Nielsen for
help in the laboratories in Lund and Copenhagen.
Data Accessibility Statement: All data will be made available
at a public repository once the manuscript has been accepted. Until
then, it is available upon request from the corresponding author
(jeppe.aa.kristensen@gmail.com).
Competing interests: The authors declare no conflicts of
interest.
Author contributions: JAK and DBM conceived the ideas and
designed the research. JAK performed the fieldwork, while JAK and AM
conducted the laboratory analyses. JAK analysed the data and wrote the
first draft of the manuscript. DBM and AM made substantial contributions
during revisions and approved the final version for publication.
Funding: This work was supported by the Swedish Research
Council (VR 2014-05177, received by DBM), the European Research Council
(ECOHERB – 682707, received by DBM), received by DBM, the Royal
Physiographic Society of Lund (received by JAK) and Helge Ax:son
Johnsons stiftelse (received by JAK).