Predator density outweighs experimental warming effects on short‐term carbon and nitrogen loss from arctic shrub litter

Rapid climate change in the Arctic is altering biological communities and their subsequent effects on ecosystem functioning. For example, warming‐induced shrub expansion accelerates biogeochemical cycles in part by increasing high‐quality litter inputs. Likewise, warming may enable higher densities of wolf spiders, which are dominant invertebrate predators whose activities indirectly alter plant litter decomposition rates. Although shrubs and wolf spiders are responding to climate change simultaneously, it is unclear how more shrub litter and more spiders together will influence elemental cycling in Arctic ecosystems. To test how warming could influence these processes, we used a fully factorial mesocosm experiment to quantify effects of wolf spiders on litter decomposition of an expanding species of dwarf deciduous shrub Betula nana under ambient and warmed conditions. We found higher densities of wolf spiders were consistently associated with more litter mass loss, and more C and N release regardless of warming treatment, indicating biotic interactions may be a stronger driver of short‐term B. nana litter decomposition than warming when wolf spiders are present. Our findings suggest the combined effects of warming‐induced shifts in plant and arthropod communities may further accelerate C and N cycling, which could cause positive feedbacks on Arctic shrub expansion.