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 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.

Meta‐analysis of elevational changes in the intensity of trophic interactions: Similarities and dissimilarities with latitudinal patterns

“…This inconsistency may be explained by the different methods used for data analysis: Galmán et al (2018) combined all data on herbivory from sites located at different mountain ranges, whereas we compared woody and herbaceous plants based on correlations observed within individual elevational gradients. Similar differences between the outcomes of these two types of analysis were previously demonstrated by Kristensen et al (2020): a decrease in herbivory with elevation was significant in individual gradients, but not significant when data from all gradients were combined. Analyses based on individual gradients may be more efficient in detecting general elevational patterns than analyses combing data from different latitudes, because the latter method does not account for variation between individual gradients.…”

Insect herbivory increases from forest to alpine tundra in Arctic mountains

“…Our data from six elevational gradients, in combination with other studies addressing the effects of elevation on plant-feeding insects and insect herbivory at high latitudes (Kristensen et al, 2020;Mjaaseth et al, 2005;Pepi et al, 2017;Ruohomäki et al, 1997), indicate that an increase in herbivory with elevation is more frequent in Arctic mountains than in mountains at lower latitudes (Moreira et al, 2018). This pattern, which contrasts with both theoretical predictions and the results of the analysis of elevational changes in herbivory in tropical and temperate mountains (Galmán et al, 2018), likely emerges due to an inverse elevational gradient in temperatures at the soil surface between closed-canopy forests and open alpine habitats.…”

“…Comparative studies of elevational changes in herbivory across latitudes are hindered by a geographic bias in research toward temperate mountains (accounting for 76% of the gradients reviewed by Moreira et al, 2018), whereas data from elevational gradients in high‐latitude (polar) mountains remain scarce (but see Kristensen et al, 2020). Filling in this gap in knowledge is especially important because ectothermic animals living at high latitudes are closer to their lower thermal limits (Deutsch et al, 2008) and may therefore show greater responses to the same absolute change in temperature compared to their counterparts living at optimum temperatures.…”

Higher vascular plant abundance associated with decreased ecosystem respiration after 20 years of warming in the forest–tundra ecotone

“…In tundra, the field layer is dominated by the dwarf shrubs E. hermaphroditum , V. vitis‐idaea , B. nana , V. uliginosum and the bottom layer by lichens such as Peltigera aphtosa and Nephroma arcticum and bryophytes such as Ptilidium ciliare (Kaarlejärvi et al., 2012). In the area, the vegetation is affected by background herbivory as well as cyclic population outbreaks of insects and rodents (Callaghan et al., 2013; Kristensen et al., 2020; Olofsson et al., 2013). Several rodent and geometric moth outbreaks have occurred while the current experiment has been ongoing and some of them have caused severe damage to the vegetation.…”