Circumpolar variation in anti-browsing defense in tundra dwarf birches

The shrub encroachment (shrubification) currently happening in tundra ecosystems can result in increased greenhouse gas emissions. Shrubification is in turn generally explained to be driven by increased temperature, but there is regional variation in shrub encroachment that cannot be solely explained by climate. Instead, herbivory is proposed as a key factor since browsing has been shown to regulate density of shrubs in the tundra. Furthermore, regional variation in anti-browsing defense, i.e. various deterrent and/or toxic compounds, has been hypothesized to control the herbivory pressure. Dwarf birches are present and often dominant throughout the low arctic. They can be divided into two functional groups based on their anti-browsing defense, i.e. resinous and non-resinous birches. This study investigated the variation in anti-browsing defense within and among different taxa of dwarf birches and the two functional groups. We also examined if these differences in anti-browsing defense affects the level of invertebrate damage in dwarf birch. We found that although there were clear differences in terpenes between resinous and non-resinous shrubs, neither functional groups nor taxa are sufficient to understand the circumpolar variation in defense compounds. Moreover, the variation in chemical anti-browsing defense had no clear effect on the level of invertebrate damage, indicating that many other factors than food quality regulate the abundance and importance of herbivores. This study does, for the first time, reveal the circumpolar variation in anti-browsing defense in dwarf birches, which will be vital for a mechanistic understanding of the greening of the arctic in the future