Insect herbivory increases from forest to alpine tundra in Arctic mountains

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.