1. The latitudinal herbivory hypothesis (LHH) predicts that plant losses to herbivores decrease from low to high latitudes. Although the LHH is a community-level hypothesis, it has been rarely tested with data on community-wide herbivory, i.e. the percentage of annual production of foliar biomass consumed by insects from all plant species at a given site. Therefore, we asked whether community-wide leaf herbivory follows the same latitudinal pattern as observed for an unweighted average of herbivory across common plant species.

2. We selected ten study sites in boreal forests from 60°N to 69°N along 1000 km long latitudinal gradient in NW Russia. We measured relative foliar losses to insect herbivores in seven woody plant species (jointly comprising over 95% of the community-wide aboveground biomass) and estimated their current-year foliar biomass. We averaged leaf herbivory for all seven species and calculated community-wide leaf herbivory by weighting the relative foliar losses of each plant species against the contribution of that species to the annual foliar biomass production.

3. Leaf herbivory was five-fold higher in deciduous species than in conifers. Latitudinal patterns in herbivory varied from a significant poleward decrease in all deciduous species to a significant poleward increase in Norway spruce. Herbivory values, averaged across seven plant species, decreased with latitude and followed the pattern observed in deciduous plants due to their higher foliar losses compared with conifers. By contrast, community-wide herbivory did not change with latitude. This discrepancy emerged because the proportion of deciduous plant foliage in the community increased with increasing latitude, and this increase counterbalanced the simultaneous poleward decrease in losses of these species to insects.

4. Synthesis The herbivory measured by averaging relative losses of individual plant species and the community-wide herbivory are likely to show different latitudinal patterns in various plant communities. The contributions of plant species to the total foliar biomass production should be taken into account in studies of spatial patterns of herbivory which test community-level hypotheses. This approach may provide new insight into macroecological research on biotic interactions and improve our understanding of the role of insect herbivores in ecosystem-level processes.

On August 17–22, 2014, when the majority of insect herbivores had completed their feeding, at each site we haphazardly selected five (deciduous plants) to ten (coniferous plants) mature individuals of each of the seven species. Each plant was located at least 10 m apart from others of its species. From each of the deciduous individuals, we collected a haphazardly selected branch with approximately 80–120 leaves. In conifers, we collected a haphazardly selected branch with 120–200 current-year needles. To avoid the impact of unconscious biases on the values of herbivory, the tree branches and bilberry stems were selected while standing at a distance of 5‒10 m away, which does not allow visual evaluation of leaf damage by insects. Leaf herbivory was measured in the laboratory within two days from sampling. In broadleaved species, the leaves on each branch were counted, and each leaf was carefully examined for the presence of damage imposed by chewing insect herbivores (both miners and defoliators). In conifers, we examined 100 current-year needles, starting from the tip of the branch, for traces of insect feeding. Each leaf/needle (leaf hereafter) was assigned to one of the damage classes according to the percentage of the area of the leaf that was consumed or otherwise damaged by insects: 0 (intact leaves), 0.01–1, 1–5, 5–25, 25–50, 50–75 and 75–100%. Petioles of fully consumed leaves were also recorded and attributed to 100% damage. The plant-specific percentage of foliage lost to insects (i.e. 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 summed for all damage classes and divided by the total number of leaves (including undamaged ones) in a sample.

Columns in the data file (space-separated) contain the following information:

Column 1: Latitude of study site (degrees N).

Column 2: Plant name (genus_species).

Column 3: Plant-specific percentage of leaf area consumed by insects.