Background insect herbivory increases with local elevation but makes
minor contribution to element cycling along natural gradients in the
Subarctic
Abstract
Herbivores can exert major controls over biogeochemical cycling. As
invertebrates are highly sensitive to their environment (ectothermal),
the abundances of insects in high-latitude systems, where climate
warming is rapid, is expected to increase. In subarctic mountain birch
forests research has focussed on geometrid moth outbreaks, while the
contribution of background insect herbivory (BIH) to elemental cycling
is poorly constrained. In northern Sweden, we estimated BIH along 9
elevational gradients distributed across a gradient in regional
elevation, temperature and precipitation to allow evaluation of
consistency in local vs. regional variation. We converted foliar loss
via BIH to fluxes of C, nitrogen (N), and phosphorus (P) from the birch
canopy to the soil to compare with other relevant soil inputs of the
same elements, and assessed different abiotic and biotic drivers of the
observed variability. We found that leaf area loss due to BIH was
~1.6% on average. This is comparable to estimates from
tundra, but considerably lower than ecosystems at lower latitudes. The
C, N and P fluxes from canopy to soil associated with BIH were 1-2
orders of magnitude lower than the soil input from senesced litter and
external nutrient sources such as biological N fixation, atmospheric
deposition of N and P weathering estimated from the literature. Hence,
despite the minor contribution to overall elemental cycling in subarctic
birch forests, the higher quality and earlier timing of the input of
herbivore deposits to soils compared to senesced litter may make this
contribution disproportionally important for various ecosystem
functions. BIH increased significantly with leaf N-content as well as
local elevation along each transect, yet showed no significant
relationship with temperature or humidity, nor the commonly used
temperature proxy, absolute elevation. The lacking consistency between
the local and regional elevational trends calls for caution when using
elevation gradients as climate proxies.