DOI: 10.5061/dryad.7m0cfxpw6
title: Insect herbivory increases from forest to alpine tundra in Arctic mountains

description: Current theory holds that the intensity of biotic interactions decreases
 with increases in latitude and elevation; however, empirical data
 demonstrate great variation in the direction, strength, and shape of
 elevational changes in herbivory. The latitudinal position of mountains
 may be one important source of this variation, but the acute shortage of
 data from polar mountains hampers exploration of latitude effects on
 elevational changes in herbivory. Here, we reduce this knowledge gap by
 testing the prediction that a decrease in herbivory occurs with increasing
 elevation from forest to alpine tundra. We examined six elevation
 gradients located in three Arctic mountain ranges. Across the ten most
 abundant evergreen and deciduous woody plant species, relative losses of
 foliage to insect herbivores were 2.2-fold greater at the highest
 elevations (alpine tundra) than in mid-elevation birch woodlands or
 low-elevation coniferous forests. Plant quality for herbivores (quantified
 by specific leaf area) significantly decreased with elevation across all
 studied species, indicating that bottom-up factors were unlikely to shape
 the observed pattern in herbivory. An experiment with open-top chambers
 established at different elevations showed that even a slight increase in
 ambient temperature enhances herbivory in Arctic mountains. Therefore, we
 suggest that the discovered increase in herbivory with elevation is
 explained by higher temperatures at the soil surface in open habitats
 above the treeline compared with forests at lower elevations. This
 explanation is supported by the significant difference in elevational
 changes in herbivory between low and tall plants: herbivory on low shrubs
 increased 4-fold from forest to alpine sites, while herbivory on trees and
 tall shrubs did not change with elevation. We suggest that an increase in
 herbivory with an increase in elevation is typical for high-latitude
 mountains, where inverse temperature gradients, especially at the soil
 surface, are common. Verification of this hypothesis requires further
 studies of elevational patterns in herbivory at high latitudes.
description: The study was replicated at four hierarchical levels:
 three mountain ranges, six gradients, three elevations within each
 gradient corresponding to three vegetation types, and ten plant species.
 We selected our uppermost sites in alpine tundra (high elevation
 hereafter), at the upper distribution limit of our study trees (Scots
 pine, Norway spruce and mountain birch). Our intermediate sites
 (mid-elevation, hereafter) were located in subalpine birch woodland. The
 lowest sites (low elevation, hereafter) were chosen in closed canopy
 coniferous forests at the foot of the respective mountains. The difference
 in elevation between the tundra and forest sites ranged from 170 to 325 m
 (Table S1). We measured herbivory in ten
 plant species: five trees and tall shrubs (evergreen <i>P.
 abies</i>, <i>P. sylvestris</i> and <i>Juniperus
 communis</i> L.; deciduous <i>B. pubescens</i> and
 <i>Salix phylicifolia</i> L.) and five low shrubs (evergreen
 <i>V. vitis-idaea</i> and deciduous <i>V.
 myrtillus</i>, <i>V. uliginosum</i> L., <i>S.
 glauca</i> L. and <i>B. nana</i> L.). Four of these
 species (<i>B. pubescens</i> and three
 <i>Vaccinium</i> species) were present in all sites, whereas
 other species each occurred on 5‒17 plots (Table S2).
 The branches of the study plants were collected at
 the beginning of autumn (13‒17 August 2012, 8‒12 August 2013 and 8‒15
 August 2014), when most insect herbivores had completed their development.
 We haphazardly (on a first found, first sampled basis) selected five
 mature individuals (or patches) of each of our study species present at a
 site while standing at a distance of 5‒10 m away to avoid unconscious
 selection bias. Each selected plant/patch was located at least 10 m apart
 from others of its species, and each collected branch (or group of stems)
 generally contained 100 to 200 leaves. In 2012, at sites in the BG and HI
 gradients, we also collected samples of <i>S. glauca</i>,
 <i>B. nana</i>, <i>V. uliginosum</i>, <i>V.
 myrtillus</i> and <i>V. vitis-idea</i> from the study
 plots enclosed in open-top chambers.
 In the laboratory, each leaf was carefully examined
 for the presence of damage imposed by chewing insect herbivores (both
 miners and defoliators). In conifers, we searched for traces of insect
 feeding in 50 or 100 current-year needles, starting from the tip of the
 branch, and we counted needles that were missing from the shoot. As in the
 previous study (Zvereva et al. 2020), we attributed the loss of entire
 needles on current-year shoots to herbivory, because undamaged needles of
 conifers in our study region persist for several years (Kozlov et al.
 2009). Mechanical damage to leaves and needles rarely occurs in our study
 region, and it can be easily distinguished from herbivory. Both our
 observations and published data on the plant-feeding organisms of Northern
 Europe indicate that all the types of damage recorded in the course of our
 study were imposed by insects.
 Following a widely used methodology (Alliende 1989,
 Kozlov et al. 2015), each leaf/needle (leaf hereafter) was assigned to one
 of the damage classes according to the percentage of the leaf area that
 was consumed or otherwise damaged by insects: 0 (intact leaves), 0.01–1%,
 1–5%, 5–25%, 25–50%, 50–75%, 75–99% and 100%. The last class included
 petioles of fully consumed leaves and missing needles. The foliage lost to
 insects (i.e. the leaf herbivory level) was calculated as follows: the
 numbers of leaves in each damage class were multiplied by the respective
 median values of the damaged leaf area (i.e. 0 for intact leaves, 0.5% for
 the damage class 0.01–1%, 3% for the damage class 1–5%, etc.); the
 obtained values were then summed for all damage classes and divided by the
 total number of leaves (including undamaged ones) in a
 sample. In 2012, we also measured the
 specific leaf area ( SLA ) from all seven species of leaf-bearing plants.
 For this purpose, we made 2‒5 discs 4.5 or 8 mm in diameter (depending on
 leaf size) from five haphazardly selected leaves, dried the discs for 24 h
 at 105ºC and then weighed them to the nearest 0.1 mg. SLA was then
 calculated as total area of the discs (mm<sup>2</sup>) divided
 by dry weight (mg).