The Effect of Snow on Plant Chemistry and Invertebrate Herbivory: Experimental Manipulations Along a Natural Snow Gradient

Changing snow conditions have strong effects on northern ecosystems, but these effects are rarely incorporated into ecosystem models and our perception of how the ecosystems will respond to a warmer climate. We investigated the relationships between snow cover, plant phenology, level of invertebrate herbivory and leaf chemical traits in Betula nana in four different habitats located along a natural snow cover gradient. To separate the effect of snow per se from other differences, we manipulated the snow cover with snow fences in three habitats. The experimentally prolonged snow cover delayed plant phenology, but not as much as expected based on the pattern along the natural gradient. The positive effect of the snow treatment on plant nitrogen concentration was also weaker than expected, because plant nitrogen concentration closely followed plant phenology. The level of herbivory by leaf-chewing invertebrates increased in response to an increased snow cover, at least at the end of the growing season. The concentration of phenolic substances varied among habitats, treatments and sampling occasions, indicating that B. nana shrubs were able to retain a mosaic of secondary chemical quality despite altered snow conditions. This study shows that the effect of the snow cover period on leaf nitrogen concentration and level of herbivory can be predicted based on differences between habitats, whereas the effect of a changed plant phenology on plant nitrogen concentration is better explained by temporal trends within habitats. These results have important implications for how northern ecosystems should respond to future climate changes.