DOI: 10.5061/dryad.t1g1jwt4m
title: Climate influence on plant–pollinator interactions in the keystone species Vaccinium myrtillus

description: Background: Climate change is altering the world’s ecosystems through
 direct effects of climate warming and precipitation changes, but also
 indirectly through changes in biotic interactions. For instance,
 climate-driven changes in plant and/or insect communities may alter
 plant-pollinator interactions, thereby influencing plant reproductive
 success and ultimately population dynamics of insect-pollinated plants.
 Methods: To better understand how the importance of insect pollination for
 plant fruit set varies with climate, we experimentally excluded
 pollinators from the partly selfing key-stone species Vaccinium myrtillus
 along elevational gradients in the forest-tundra ecotone in central
 Norway. The study comprised three mountain areas, seven elevational
 gradients spanning from the climatically relatively benign birch forest to
 the colder alpine areas above the tree line, and 180 plots of 1 x 1 m,
 with experimental treatments allocated randomly to plots within sites.
 Within the experimental plots we counted the number of flowers of V.
 myrtillus and counted and weighted all fruits, as well as seeds for a
 selection of fruits. Results: Excluding pollinators resulted in lower
 fruit production, as well as reduced fruit and seed mass of Vaccinium
 myrtillus. In the alpine sites pollinator exclusion resulted in 84 % fewer
 fruits, 50 % lower fruit weight and 50 % lower seed weight compared to
 control conditions. Contrary to our expectations, the negative effect of
 pollinator exclusion was less pronounced in forest compared to alpine
 sites, suggesting that the importance of insect pollination for seed
 production is lower at low elevation. Conclusions: Our findings indicate
 that the keystone species Vaccinium myrtillus is relatively robust to
 changes in the pollinator community in a warmer climate, thereby making it
 less vulnerable to climate-driven changes in plant-pollinator
 interactions.
description: Study area The study was carried out in 2017 and
 2018 in three mountain areas, Forollhogna, Dovrefjell and Grødalen in
 Sunndalsfjella, situated along an east-west gradient in central Norway. In
 each area except Dovrefjell we established two replicate elevational
 gradients from the mountain forest to the alpine tundra, and along each
 gradient we established three experimental sites: one in the mountain
 birch forest, one at the treeline and one in the open alpine tundra. The
 sites were established in heathland vegetation with a high abundance
 of<em> Vaccinium myrtillus</em>. In Dovrefjell we only had one
 gradient due to few <em>V. myrtillus</em> dominated sites. The
 mean distance between elevational levels was app. 160 m, and the
 difference in mean summer temperature was on average 1.0 °C between the
 forest and treeline sites and 0.6 °C between the treeline and alpine
 sites. Experimental
 design In each site we established four blocks,
 approximately 30-100 meters apart (depending on bilberry abundance), each
 with three 1 x 1 m plots with 3‒5 m distance, resulting in a total of 180
 plots. Within each block we applied three
 experimental treatments to examine the importance of pollinators
 for <em>V. myrtillus</em>: control, pollinator reduction, and
 pollinator exclusion. Treatments were randomly assigned to the three plots
 in each block. The pollinator reduction and exclusion treatments were
 achieved by placing dome-shaped cages made of two approximately 2.5 m long
 PVC tubes bent diagonally over the plots. The size of the resulting cages
 (w × l × h) was approximately 1.5 × 1.5 × 1 m. For the reduction treatment
 the cages were covered with berry netting with a mesh size of 1.5 × 1.5 cm, through
 which at least some pollinators, including bumblebees, were able to enter
 and exit the plots, whereas the exclusion cages were covered with insect
 netting with a mesh size of 2 × 2 mm, which no flying insects were
 observed to penetrate. The mesh was fastened to the PVC tubes using
 plastic strips. To prevent non-flying pollinators from accessing the
 plots, the netting was fixed to the ground with n-shaped plugs. Initial
 analyses after one year of treatment suggested that the reduction
 treatment had a very limited effect, most likely because the mesh size was
 too large and therefore did not represent a barrier to pollinators. This
 treatment was therefore discontinued. At peak flowering time, we counted
 the number of flowers in each plot. The timing varied along the
 elevational gradients, with flowering time peaking a week or two earlier
 in the mountain forest compared to the treeline and alpine sites. When the
 majority of the fruits were ripe, all fruits from each plot were
 collected, counted, dried at 60 degrees for 48 hours and weighed. Among
 the fruits collected in 2017, we randomly picked one mature fruit per
 plot, and re-wetted and dissected them before counting the number of seeds
 under a stereo microscope. The seeds were subsequently dried at 60 degrees
 for 48 hours and weighed. The experimental treatments could
 potentially influence e.g. micro-climatic conditions within the mesh cages
 and thereby bias our results. To check for side-effects of the
 exclusion cages, we measured temperature using B-series WatchDog B101 8K
 temperature loggers (Spectrum Technologies Inc., Aurora, USA) and
 illuminance (lux) using a Hagner EC1 digital luxmeter (B. Hagner AB,
 Solna, Sweden) in all control and exclusion plots in 2018.  All
 temperature loggers were placed in white plastic boxes to prevent moisture
 damage, and the boxes were placed at ground level, shaded by the
 vegetation. Temperature was recorded every fourth hour from mid-June to
 mid-August, whereas illuminance was measured once in each plot at peak
 flowering, making sure to measure all plots in one site on the same
 day. More details are found in the
 manuscript.