Abstract
Herbivores can exert major controls over biogeochemical cycling. As invertebrates are highly sensitive to temperature shifts (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 versus 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. 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 lack of consistency between the local and regional elevational trends calls for caution when using elevation gradients as climate proxies.
Highlights
Herbivores alter element cycling in terrestrial ecosystems (Bardgett & Wardle, 2003, 2010; Schmitz et al, 2018; Wardle et al, 2004)
Ecosystem studies of insect herbivory in high-latitude forests have hitherto emphasized the importance of outbreaks (Jepsen et al, 2013; Sandén et al, 2020), especially as some species are increasing their ranges into new areas with climate warming (Jepsen et al, 2008, 2011)
As background insect herbivory (BIH) at high latitudes is generally low—1%–2% of the leaf area (Barrio et al, 2017) compared to the global average of ~8% (Kozlov, et al, 2015)—the rate will likely increase at high latitudes with global warming (Kozlov, et al, 2015), even if insect predation and parasitism increase with temperature (Roslin et al, 2017; Virtanen & Neuvonen, 1999)
Summary
Herbivores alter element cycling in terrestrial ecosystems (Bardgett & Wardle, 2003, 2010; Schmitz et al, 2018; Wardle et al, 2004). Moth outbreaks most often appear close to the treeline in birch forests across subarctic Scandinavia rather than in valley bottoms, probably due to the higher likelihood of winter temperatures below their egg survival limit (
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