Abrupt changes in invertebrate herbivory on woody plants at the forest–tundra ecotone

Invertebrate herbivores, insects in particular, play important functional roles in terrestrial ecosystems. At background (non-outbreak) densities, they consume 5–15 % of woody plant foliage in forests across the planet. At the same time, almost nothing is known about the levels of insect herbivory in Arctic tundra. To test the hypothesis that the amount of plant biomass lost to insects in tundra is substantially less than in subarctic forests, in 2013, we explored foliar herbivory in woody plants at three sites in the Arctic tundra and four sites in the subarctic forests of European Russia. A vast majority of foliar damage was imposed by externally feeding defoliators. In forests, defoliators damaged three times more leaves and consumed eight times more leaf area than in the tundra. No miners were found in the tundra, and gallers affected five times less leaf area in the tundra compared with forests. An abrupt decrease in loss of woody plant foliage to insects between subarctic forests and tundra (from 4.34 to 0.56 %) supports the existence of a latitudinal gradient in herbivory in terrestrial ecosystems. More studies are needed to predict how tundra plants, which have been historically exposed to low levels of insect herbivory, will cope with the increased levels of damage that are expected to occur due to climate-driven range expansion and increased abundances of plant-feeding insects.